Fixture clamping devices (wedge and lever clamps). Clamping devices for machine tools - machine tools Clamping devices for machining fixtures

Clamping elements must ensure reliable contact of the workpiece with the setting elements and prevent its disruption under the action of the forces arising during processing, fast and uniform clamping of all parts and not cause deformation and damage to the parts to be fixed.

Clamping elements are divided into:

By design - on screw, wedge, eccentric, lever, lever-articulated (combined clamping elements are also used - screw-lever, eccentric-lever, etc.).

According to the degree of mechanization - on manual and mechanized with hydraulic, pneumatic, electric or vacuum drive.

Clamping fur-we can be automated.

Screw terminals used for direct clamping or clamping through clamping bars, or tacks of one or more parts. Their disadvantage is that that it takes a lot of time to fix and detach the part.

Eccentric and wedge clamps, as well as screw ones, they allow you to fix the part directly or through clamping bars and levers.

The most widespread are circular eccentric clamps. An eccentric clamp is a special case of a wedge clamp, and to ensure self-braking, the wedge angle should not exceed 6-8 degrees. Eccentric clamps are made from high carbon or hardened steel and heat treated to a hardness of HRC55-60. Eccentric clamps are classified as quick clamps, because for clamping rotate the eccentric at an angle of 60-120 degrees.

Lever hinge elements are used as driving and reinforcing links of clamping mechanisms. By design, they are divided into single-lever, double-lever (single-sided and double-acting - self-centering and multi-link). Lever mechanisms do not have self-braking properties. The simplest example of lever-articulated mechs is clamping bars of devices, levers of pneumatic cartridges, etc.

Spring clamps used to clamp products with little effort that occurs when the spring is compressed.

To create constant and large clamping forces, reduce clamping time, remote control of the clamps, pneumatic, hydraulic and other drives.

The most common pneumatic drives are piston pneumatic cylinders and pneumatic chambers with an elastic diaphragm, stationary, rotating and oscillating.

Pneumatic actuators are actuated compressed air at a pressure of 4-6 kg / cm² If it is necessary to use small drives and create large clamping forces, hydraulic drives are used, the operating oil pressure in which. reaches 80 kg/cm².

The force on the rod of a pneumatic or hydraulic cylinder is equal to the product of the working area of the piston in square cm and the pressure of the air or working fluid. In this case, it is necessary to take into account the friction losses between the piston and the cylinder walls, between the rod and the guide bushings and seals.

Electromagnetic clamping devices performed in the form of plates and faceplates. They are designed for clamping steel and cast iron workpieces with a flat base surface when grinding or fine turning.

Magnetic clamping devices can be made in the form of prisms that serve to fix cylindrical blanks. Plates appeared, in which ferrites are used as permanent magnets. These plates are characterized by high holding force and smaller distance between poles.

Clamping devices for machine tools

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Metal cutting machines

Clamping devices for machine tools

The process of supplying automatic machine tools with blanks is carried out with the close interaction of loading devices and automatic clamping devices. In many cases, automatic clamping devices are a structural element of the machine or an integral part of it. Therefore, despite the existence of special literature on clamping devices, it seems necessary to briefly dwell on some characteristic designs,

The movable elements of automatic clamping devices receive movement from the corresponding controlled drives, which can be mechanical controlled drives that receive movement from the main drive of the working body or from an independent electric motor, cam drives, hydraulic, pneumatic and pneumohydraulic drives. Separate movable elements of clamping devices can receive movement both from a common drive and from several independent drives.

Consideration of the designs of special fixtures, which are mainly determined by the configuration and dimensions of a particular workpiece, is beyond the scope of this work, and we will limit ourselves to familiarization with some wide-purpose clamping fixtures.

Clamping chucks. There are a large number of designs of self-centering chucks, in most cases with a piston hydraulic and pneumatic drive, which are used on lathes, turrets and grinders. These chucks, providing reliable clamping and good centering of the workpiece, have a small consumption of cams, which is why when switching from processing one batch of parts to another, the chuck must be rebuilt and, to ensure high centering accuracy, machine the centering surfaces of the cams in place; at the same time, hardened cams are ground, and raw cams are turned or bored.

One common chuck design with a pneumatic piston drive is shown in fig. 1. The pneumatic cylinder is fixed with an intermediate flange at the end of the spindle. The air supply to the pneumatic cylinder is carried out through the axle box, which sits on rolling bearings on the shank of the cylinder cover. The piston of the cylinder is connected by a rod to the clamping mechanism of the cartridge. The pneumatic chuck is attached to a flange mounted on the front end of the spindle. The head, mounted on the end of the rod, has inclined grooves, which include L-shaped protrusions of the cams. When moving the head along with the stem forward, the cams approach each other, when moving backwards, they diverge.

On the main jaws with T-slots, overhead jaws are fixed, which are installed in accordance with the diameter of the clamping surface of the workpiece.

Due to the small number of intermediate links that transmit movement to the cams, and the significant size of the rubbing surfaces, the cartridges of the described design have a relatively high rigidity and durability.

Rice. 1. Pneumatic chuck.

A number of pneumatic chuck designs use linkages. Such cartridges have less rigidity and, due to the presence of a number of swivel joints, wear out faster.

Instead of a pneumatic cylinder, a pneumatic diaphragm actuator or a hydraulic cylinder can be used. Cylinders rotating with the spindle, especially at high spindle speeds, require careful balancing, which is a disadvantage of this design option.

The piston drive can be fixedly mounted coaxially with the spindle, and the cylinder rod is connected to the clamping rod by a coupling that ensures free rotation of the clamping rod together with the spindle. The rod of the stationary cylinder can also be connected to the clamping rod by a system of intermediate mechanical gears. Such schemes are applicable in the presence of self-braking mechanisms in the clamping device drive, since otherwise the spindle bearings will be loaded with significant axial forces.

Along with self-centering chucks, two-jaw chucks with special jaws driven by the drives mentioned above and special chucks are also used.

Similar drives are used when fixing parts on various expanding mandrels.

Collet clamping devices. Collet clamping devices are a design element of turret machines and automatic lathes designed for the manufacture of parts from a bar. However, they are widely used in special clamping fixtures.

Rice. 2. Collet clamping devices.

In practice, there are three types of collet clamping devices.

The collet, which has several longitudinal cuts, is centered with a rear cylindrical tail in the spindle hole, and a front conical tail in the cap hole. When clamping, the pipe moves the collet forward and its front conical part enters the conical hole of the spindle cap. In this case, the collet is compressed and clamps the bar or workpiece. A clamping device of this type has a number of significant disadvantages.

The accuracy of centering the workpiece is largely determined by the coaxiality of the conical surface of the cap and the axis of rotation of the spindle. To do this, it is necessary to achieve the coaxiality of the conical hole of the cap and its cylindrical centering surface, the coaxiality of the centering shoulder and the axis of rotation of the spindle and the minimum gap between the centering surfaces of the cap and the spindle.

Since the fulfillment of these conditions presents significant difficulties, collet devices of this type do not provide good centering.

In addition, during the clamping process, the collet, moving forward, captures the bar, which moves along with the collet, which can

lead to a change in the dimensions of the workpieces along the length and to the appearance of large pressures on the stop. In practice, there are cases when a rotating bar, pressed with great force against the stop, is welded to the latter.

The advantage of this design is the possibility of using a small diameter spindle. However, since the diameter of the spindle is largely determined by other considerations and primarily by its rigidity, this circumstance in most cases is not significant.

Due to these disadvantages, this variant of the collet clamping device is of limited use.

The collet has a reverse taper, and when the material is clamped, the pipe pulls the collet into the spindle. This design provides good centering, as the centering cone is located directly in the spindle. The disadvantage of the design is the movement of the material along with the collet during the clamping process, which leads to a change in the dimensions of the workpiece, but does not cause any axial loads on the stop. Some disadvantage is also the weakness of the section at the threaded connection. The spindle diameter increases slightly compared to the previous version.

Due to the noted advantages and simplicity of design, this option is widely used on turret machines and multi-spindle automatic lathes, the spindles of which must have a minimum diameter.

The option shown in fig. 2, c, differs from the previous one in that during the clamping process, the collet, which abuts the front end surface against the cap, remains stationary, and the sleeve moves under the action of the pipe. The conical surface of the sleeve is pushed onto the outer conical surface of the collet, and the latter is compressed. Since the collet remains stationary during the clamping process, this design does not cause displacement of the processed bar. The sleeve has good centering in the spindle, and ensuring the alignment of the inner conical and outer centering surfaces of the sleeve does not present technological difficulties, due to which this design provides a fairly good centering of the processed bar.

When the collet is released, the pipe is retracted to the left and the sleeve moves under the action of a spring.

In order for the friction forces arising during the clamping process on the end surface of the collet petals not to reduce the clamping force, the end surface is given a conical shape with an angle slightly greater than the friction angle.

This design is more complicated than the previous one and requires an increase in the diameter of the spindle. However, due to the noted advantages, it is widely used on single-spindle machines, where an increase in the diameter of the spindle is not significant, and on a number of models of turret machines.

The dimensions of the most common collets are standardized by the corresponding GOST. Collets of large sizes are made with replaceable jaws, which allows you to reduce the number of collets in the set and replace them with new ones when the jaws are worn out.

The surface of the jaws of collets working under heavy loads has a notch, which ensures the transmission of high forces of the clamped part.

Clamping collets are made of steels U8A, U10A, 65G, 9XC. The working part of the collet is hardened to a hardness of HRC 58-62. Tail

the part is tempered to a hardness of HRC 38-40. For the manufacture of collets, case-hardened steels are also used, in particular steel 12ХНЗА.

The pipe that moves the clamping collet itself receives movement from one of the listed types of drives through one or another system of intermediate gears. Some designs of intermediate gears for moving the clamping tube are shown in fig. IV. 3.

The clamping tube receives movement from crackers, which are part of the sleeve with a protrusion that enters the groove of the spindle. The crackers rest on the tail lugs of the clamping tube, which hold them in position. Crackers receive movement from the levers, the L-shaped ends of which go into the end groove of the sleeve 6, sitting on the spindle. When clamping the collet, the sleeve moves to the left and, acting on the ends of the levers with its inner conical surface, turns them. The rotation occurs relative to the points of contact of the L-shaped protrusions of the levers with the undercut of the bushing. At the same time, the heels of the levers press on crackers. In the drawing, the mechanisms are shown in the position corresponding to the end of the clamp. In this position, the mechanism is closed, and the sleeve is unloaded from axial forces.

Rice. 3. Clamping tube movement mechanism.

The clamping force is regulated by nuts, with the help of which the bushing is moved. To avoid the need to increase the diameter of the spindle, a threaded ring is planted on it, which abuts against the half rings that go into the groove of the spindle.

Depending on the diameter of the clamping surface, which can vary within tolerance, the clamping tube will take a different position in the axial direction. Deviations in the position of the pipe are compensated by the deformation of the levers. In other designs, special spring compensators are introduced.

This option is widely used on single-spindle automatic lathes. There are numerous design modifications that differ in the shape of the levers.

In a number of designs, the levers are replaced by wedge balls or rollers. A flange sits on the threaded end of the clamping tube. When clamping the collet, the flange moves to the left along with the pipe. The flange receives movement from the sleeve acting through the roller on the disk. When the sleeve is moved to the left, its inner conical surface causes the barrel rollers to move towards the center. In this case, the rollers, moving along the conical surface of the washer, are displaced to the left, moving the disk and the flange with the clamping tube in the same direction. All parts are mounted on a sleeve mounted on the end of the spindle. The clamping force is adjusted by screwing the flange onto the pipe. In the required position, the flange is locked with a lock. The mechanism can be equipped with an elastic compensator in the form of Belleville springs, which allows it to be used for clamping bars with large diameter tolerances.

The movable sleeves that carry out the clamping receive movement from the cam mechanisms of automatic lathes or from piston drives. The clamping tube can also be connected directly to the piston drive.

Drives of clamping devices of multi-position machines. Each of the clamping devices of a multi-position machine may have its own, usually a piston drive, or the moving elements of the clamping device may receive movement from a drive installed in the loading position. In the latter case, the fixture mechanisms that enter the loading position are linked to the drive mechanisms. At the end of the clamp, this connection is terminated.

The latter option is widely used on multi-spindle automatic lathes. In the position in which the feed and clamping of the bar takes place, a slider with a ledge is installed. When turning the spindle unit, the protrusion enters the annular groove of the movable sleeve of the clamping mechanism and at the appropriate moments moves the sleeve in the axial direction.

A similar principle can in some cases be used to move the moving elements of clamping devices installed on multi-position tables and drums. The earring is clamped between the fixed and movable prisms of the clamping device installed on the multi-position table. The prism receives its movement from a slider with a wedge bevel. When clamping, the plunger, on which the gear rack is cut, moves to the right. Through the toothed gear, the movement is transmitted to the slider, which moves the prism to the prism with a wedge bevel. When the clamped part is released, the plunger moves to the right, which is also connected to the slider by a gear.

The plungers may be driven by piston drives mounted in the loading position or by appropriate cam links. Clamping and release of the workpiece can also be carried out during the rotation of the table. When clamping, the plunger, equipped with a roller, runs into a fixed fist installed between the loading and first working positions. When released, the plunger runs into a fist located between the last working and loading positions. The plungers are located in different planes. To compensate for deviations in the dimensions of the clamped part, elastic compensators are introduced.

It should be noted that such simple solutions are not sufficiently used in the design of clamping devices for multi-position machines when processing medium-sized parts.

Rice. 4. Clamping device of a multi-position machine, powered by a drive installed in the loading position.

If there are individual piston motors for each of the clamping devices of a multi-position machine, compressed air or pressurized oil must be supplied to the turntable or drum. The device for supplying compressed air or oil is similar to the rotary cylinder device described above. The use of rolling bearings in this case is unnecessary, since the rotation speed is low.

Each of the fixtures can have an individual control valve or spool, or a common switchgear can be used for all clamping devices.

Rice. 5. Switchgear for piston drives of clamping devices of a multi-position table.

Individual cranes or switchgears are switched by auxiliary drives installed in the loading position.

The common switchgear connects the piston drives of the fixtures in series as the table or drum rotates. An exemplary design of such a switchgear is shown in Fig. 5. The housing of the switchgear, installed coaxially with the axis of rotation of the table or drum, rotates with the latter, and the spools remain motionless along with the axis. The spool controls the supply of compressed air to the cavity, and the spool controls the cavity of the clamping cylinders.

Compressed air enters through the channel into the space between the spools and is directed with the help of the latter into the corresponding cavities of the clamping cylinders. The exhaust air escapes into the atmosphere through the holes.

Compressed air enters the cavity through the hole, arcuate groove and holes. As long as the holes of the corresponding cylinders coincide with the arcuate groove, compressed air enters the cylinder cavities. When, at the next turn of the table, the hole of one of the cylinders is aligned with the hole, the cavity of this cylinder will be connected with the atmosphere through the annular groove, channel, annular groove and channel.

The cavities of those cylinders, in the cavities of which compressed air enters, must be connected with the atmosphere. The cavities are connected to the atmosphere through the channels, the arcuate groove, the channels, the annular groove and the hole.

The cavity of the cylinder, which is in the loading position, must be supplied with compressed air, which is supplied through the hole and channels.

Thus, when the multi-position table is rotated, the compressed air flows are automatically switched.

A similar principle is used to control the flow of oil supplied to the fixtures of multi-position machines.

It should be noted that similar distribution devices are also used on machines for continuous processing with rotating tables or drums.

Principles for determining the forces acting in clamping devices. Clamping devices are usually designed in such a way that the forces generated during the cutting process are perceived by the fixed elements of the devices. If certain forces arising in the process of cutting are perceived by moving elements, then the magnitude of these forces is determined on the basis of the equations of friction statics.

The method for determining the forces acting in the lever mechanisms of collet clamping devices is similar to the method used to determine the engagement forces of friction clutches with lever mechanisms.

CONTENT

Page

INTRODUCTION………………….…………………………………………..…….....2

GENERAL INFORMATION ABOUT DEVICES…………………………... …3

MAIN ELEMENTS OF DEVICES……………….…………...6

Clamping elements of fixtures……………………………….……. …..6
1 Purpose of clamping elements………………………………………...6
2 Types of clamping elements………………………………………….…..…. .7
REFERENCES…………………………………………………………..17

INTRODUCTION

The main group of technological equipment is made up of fixtures for mechanical assembly production. Devices in mechanical engineering are called auxiliary devices for technological equipment used in the performance of processing, assembly and control operations.
The use of devices allows you to: eliminate the marking of workpieces before processing, increase its accuracy, increase labor productivity in operations, reduce product costs, facilitate working conditions and ensure its safety, expand the technological capabilities of equipment, organize multi-machine maintenance, apply technically sound time standards, reduce the number of workers required for production.
The frequent change of production facilities, associated with an increase in the pace of technological progress in the era of the scientific and technological revolution, requires technological science and practice to create structures and systems of devices, methods for their calculation, design and manufacture, ensuring a reduction in production preparation time. In mass production, it is necessary to use specialized quick-adjustable and reversible fixture systems. In small-scale and single-piece production, the system of universally prefabricated (USP) fixtures is increasingly being used.
The new requirements for fixtures are determined by the expansion of the CNC machine tool fleet, the changeover of which for processing a new workpiece comes down to changing the program (which takes very little time) and to replacing or readjusting the device for locating and fixing the workpiece (which should also take little time) .
The study of the regularities of the influence of the device on the accuracy and productivity of the operations performed will allow us to design devices that intensify production and increase its accuracy. The work on the unification and standardization of fixture elements creates the basis for the automated design of fixtures using electronic computers and automatic machines for graphic representation. This speeds up the technological preparation of production.

GENERAL INFORMATION ABOUT DEVICES.
TYPES OF DEVICES

In mechanical engineering, a variety of technological equipment is widely used, which includes fixtures, auxiliary, cutting and measuring tools.
Devices are called additional devices used for machining, assembly and control of parts, assembly units and products. According to the purpose, the devices are divided into the following types:
1. Machine fixtures used to install and fix workpieces on machine tools. Depending on the type of machining, these devices, in turn, are divided into devices for drilling, milling, boring, turning, grinding machines, etc. Machine tools make up 80 ... 90% of the total stock of technological equipment.
The use of devices provides:
a) increasing labor productivity by reducing the time for setting and fixing workpieces with partial or complete overlap of auxiliary time with machine time and reducing the latter through multi-site processing, combining technological transitions and increasing cutting conditions;
b) increasing the accuracy of processing due to the elimination of alignment during installation and associated errors;
c) facilitating the working conditions of machine operators;
d) expansion of technological capabilities of equipment;
e) improving work safety.
2. Devices for installing and fixing a working tool that communicate between the tool and the machine, while the first type connects the workpiece with the machine. With the help of devices of the first and second types, the technological system is adjusted.
3. Assembly devices for connecting mating parts into assembly units and products. They are used for fastening the base parts or assembly units of the assembled product, ensuring the correct installation of the connected elements of the product, pre-assembly of elastic elements (springs, split rings, etc.), as well as for making connections with an interference fit.
4. Control devices for intermediate and final control of parts, as well as for control of assembled parts of machines.
5. Devices for gripping, moving and turning over workpieces and assembly units used in the processing and assembly of heavy parts and products.
According to the operational characteristics, machine tools are divided into universal ones, designed for processing a variety of workpieces (machine vice, chucks, dividing heads, rotary tables, etc.); specialized, designed for processing workpieces of a certain type and representing interchangeable devices (special vice jaws, shaped cams for chucks, etc.), and special ones, designed to perform certain machining operations of a given part. Universal devices are used in conditions of single or small-scale production, and specialized and special devices are used in conditions of large-scale and mass production.
With a unified system of technological preparation for production, machine tools are classified according to certain criteria (Fig. 1).
Universal prefabricated fixtures (USP) are assembled from prefabricated standard elements, parts and high-precision assembly units. They are used as special short-term devices for a specific operation, after which they are disassembled, and the delivering elements are subsequently reused in new layouts and combinations. Further development of USP is associated with the creation of aggregates, blocks, individual special parts and assembly units, providing the layout of not only special, but also specialized and universal adjustment devices of short duration,
Collapsible fixtures (PSA) are also assembled from standard elements, but less accurate, allowing local refinement of the seats. These devices are used as special long-term devices. After disassembly, new layouts can be created from the elements.

Rice. 1 - Classification of machine tools

Non-separable special devices (NSP) are assembled from standard parts and assembly units for general purposes, as irreversible long-term devices. The structural elements of the layouts that are part of the system, as a rule, are operated until they are completely worn out and are not reused. The layout can also be made by building a device from two main parts: a unified base part (UB) and a replaceable setup (SN). This design of the NSP makes it resistant to changes in the design of the workpieces being processed and to adjustments in technological processes. In these cases, only the interchangeable adjustment is replaced in the fixture.
Universal non-adjustment devices (UBD) for general purposes are most common in mass production. They are used for fixing blanks from shaped rolled products and piece blanks. UBP are universal adjustable housings with permanent (non-removable) basic elements (cartridges, vise, etc.) included in the machine kit when it is delivered.
Specialized adjustment devices (SNP) are used to equip operations for processing parts grouped according to design features and basing schemes; the layout according to the aggregation scheme is a basic housing design with interchangeable settings for groups of parts.
Universal adjustment devices (UNP), as well as SNP, have permanent (body) and replaceable parts. However, the replacement part is suitable for only one machining operation on one part only. When switching from one operation to another, the devices of the UNP system are equipped with new replaceable parts (adjustments).
Aggregate means of clamping mechanization (AMZ) is a complex of universal power devices made in the form of separate units, which, in combination with devices, allow mechanizing and automating the process of clamping workpieces.
The choice of fixture design largely depends on the nature of production. So, in mass production, relatively simple fixtures are used, designed mainly to achieve a given accuracy in processing a workpiece. In mass production, high demands are placed on fixtures in terms of productivity. Therefore, such devices, equipped with quick clamps, are more complex designs. However, the use of even the most expensive devices is economically justified.

MAIN ELEMENTS OF DEVICES

There are the following fixtures:
adjusting - to determine the position of the workpiece surface to be machined relative to the cutting tool;
clamping - for fixing the workpiece;
guides - to give the required direction to the movement of the cutting tool relative to the surface being machined;
body of devices - the main part on which all elements of devices are placed;
fasteners - for connecting individual elements to each other;
dividing or rotary, - to accurately change the position of the workpiece surface to be machined relative to the cutting tool;
mechanized drives - to create clamping force. In some devices, the installation and clamping of the workpiece being processed is performed by one mechanism, called the installation-clamping mechanism.

Clamping fixtures

1 Purpose of clamping elements
The main purpose of clamping devices is to ensure reliable contact of the workpiece with the mounting elements and prevent its displacement relative to them and vibration during processing. The introduction of additional clamping devices increases the rigidity of the technological system and thereby achieves an increase in the accuracy and productivity of processing, and a decrease in surface roughness. On fig. Figure 2 shows a diagram of the installation of workpiece 1, which, in addition to the two main clamps Q1, is fixed with an additional device Q2, which imparts greater rigidity to the system. Support 2 is self-aligning.

Rice. 2 - Scheme of setting the workpiece

Clamping devices are in some cases used to ensure correct installation and centering of the workpiece. In this case, they perform the function of mounting and clamping devices. These include self-centering chucks, collets, etc.
Clamping devices are not used when processing heavy, stable workpieces, in comparison with the mass of which the forces arising during the cutting process are relatively small and are applied in such a way that they cannot disturb the installation of the workpiece.
Clamping devices of fixtures must be reliable in operation, simple in design and easy to maintain; they should not cause deformations of the fixed workpiece and damage to its surface, they should not shift the workpiece in the process of its fixing. The machine operator must spend a minimum of time and effort on fixing and detaching workpieces. To simplify the repair, it is advisable to make the most wear parts of the clamping devices replaceable. When fixing workpieces in multi-place fixtures, they are clamped evenly; with limited movement of the clamping element (wedge, eccentric), its stroke must be greater than the tolerance for the size of the workpiece from the mounting base to the place where the clamping force is applied.
Clamping devices are designed taking into account safety requirements.
The place of application of the clamping force is chosen according to the condition of the greatest rigidity and stability of the fastening and the minimum deformation of the workpiece. When increasing the accuracy of processing, it is necessary to observe the conditions for a constant value of the clamping force, the direction of which must be recognized with the arrangement of the supports.

2 Types of clamping elements
Clamping elements are mechanisms directly used to clamp workpieces, or intermediate links in more complex clamping systems.
The simplest type of universal clamps are clamping screws, which are driven by keys, handles or handwheels mounted on them.
To prevent movement of the clamped workpiece and the formation of dents on it from the screw, as well as to reduce the bending of the screw when pressing on a surface not perpendicular to its axis, rocking shoes are placed on the ends of the screws (Fig. 3, a).
Combinations of screw devices with levers or wedges are called combined clamps, a variation of which are screw clamps (Fig. 3, b). The clamping device allows you to move or rotate them so that you can more conveniently install the workpiece in the fixture.

Rice. 3 - Schemes of screw clamps

On fig. 4 shows some designs of quick clamps. For small clamping forces, a bayonet device is used (Fig. 4, a), and for significant forces, a plunger device (Fig. 4, b). These devices allow the clamping element to be retracted a long distance from the workpiece; fastening occurs as a result of the rotation of the rod through a certain angle. An example of a clamp with a folding stop is shown in fig. 4, c. Having loosened the nut-handle 2, the stop 3 is retracted, rotating it around the axis. After that, the clamping rod 1 is retracted to the right at a distance h. On fig. 4, d shows a diagram of a high-speed lever-type device. When the handle 4 is turned, the pin 5 slides along the bar 6 with an oblique cut, and the pin 2 slides along the workpiece 1, pressing it against the stops located below. Spherical washer 3 serves as a hinge.

Rice. 4 - Constructions of quick clamps

The time consuming and significant forces required to clamp workpieces limit the application of screw clamps and in most cases make quick-acting eccentric clamps preferable. On fig. 5 shows disk (a), cylindrical with L-shaped clamp (b) and conical floating (c) clamps.

Rice. 5 - Various clamp designs
Eccentrics are round, involute and spiral (according to Archimedes' spiral). In clamping devices, two types of eccentrics are used: round and curved.
Round eccentrics (Fig. 6) are a disk or roller with an axis of rotation shifted by the size of the eccentricity e; the self-braking condition is ensured at the ratio D/e ? 4.

Rice. 6 - Diagram of a round eccentric

The advantage of round eccentrics lies in the ease of their manufacture; the main disadvantage is the inconsistency of the angle of elevation a and the clamping forces Q. Curvilinear eccentrics, the working profile of which is performed along the involute or Archimedes spiral, have a constant angle of elevation a, and, therefore, ensure the constancy of the force Q when clamping any point of the profile.
The wedge mechanism is used as an intermediate link in complex clamping systems. It is easy to manufacture, easily placed in the device, allows you to increase and change the direction of the transmitted force. At certain angles, the wedge mechanism has self-braking properties. For a single-bevel wedge (Fig. 7, a) when transferring forces at a right angle, the following dependence can be adopted (for j1=j2=j3=j, where j1...j3 are friction angles):
P=Qtg(a±2j),

Where P - axial force;
Q - clamping force.
Self-braking will take place at a For a double-beveled wedge (Fig. 7, b), when forces are transmitted at an angle b> 90 °, the relationship between Р and Q at a constant friction angle (j1=j2=j3=j) is expressed by the following formula

P \u003d Q sin (a + 2j / cos (90 ° + a-b + 2j).

Lever clamps are used in combination with other elementary clamps, forming more complex clamping systems. Using the lever, you can change the magnitude and direction of the transmitted force, as well as carry out simultaneous and uniform clamping of the workpiece in two places.

Fig. 7 - Schemes of a single-sided wedge (a) and a double-sided wedge (b)

Figure 8 shows the diagrams of the action of forces in one-arm and two-arm straight and curved clamps. The equilibrium equations for these lever mechanisms are as follows:
for one-shoulder clamp (Fig. 8, a)
,
for a straight two-shoulder clamp (Fig. 8, b)
,
for two-arm curved clamp (for l1 ,
where r is the angle of friction;
f is the coefficient of friction.

Rice. 8 - Schemes of the action of forces in one-arm and two-arm straight and curved clamps

Centering clamping elements are used as mounting elements for the outer or inner surfaces of bodies of revolution: collets, expanding mandrels, clamping sleeves with hydroplastic, and also membrane cartridges.
The collets are split spring sleeves, the design variations of which are shown in fig. 9 (a - with a tension tube; b - with a spacer tube; c - vertical type). They are made of high-carbon steels, for example, U10A, and are heat-treated to a hardness of HRC 58...62 in the clamping and to a hardness of HRC 40...44 in the tail parts. Collet cone angle a=30. . .40°. At smaller angles, collet jamming is possible. The taper angle of the compression sleeve is made 1° less or greater than the taper angle of the collet. Collets provide installation eccentricity (runout) no more than 0.02...0.05 mm. The base surface of the workpiece should be machined according to the 9th...7th grade of accuracy.
Expanding mandrels of various designs (including designs using hydroplastic) are classified as clamping fixtures.
Membrane cartridges are used for precise centering of workpieces on the outer or inner cylindrical surface. The cartridge (Fig. 10) consists of a round membrane 1 screwed to the faceplate of the machine in the form of a plate with symmetrically located protrusions-cams 2, the number of which is chosen in the range of 6 ... 12. A rod of 4 pneumatic cylinders passes inside the spindle. When the pneumatics are turned on, the membrane flexes, pushing the cams apart. When the rod moves back, the membrane, trying to return to its original position, compresses the workpiece 3 with its cams.

Rice. 10 - Scheme of the membrane cartridge

The rack-to-lever clamp (Fig. 11) consists of a rack 3, a gear wheel 5 sitting on a shaft 4, and a handle lever 6. By turning the handle counterclockwise, the rack is lowered and the workpiece 1 is fixed with clamp 2. The clamping force Q depends on the value force P applied to the handle. The device is equipped with a lock, which, jamming the system, prevents the wheel from turning back. The most common types of locks are:

Rice. 11 - Rack and pinion clamp

The roller lock (Fig. 12, a) consists of a driving ring 3 with a cutout for the roller 1, which is in contact with the cut plane of the shaft 2 of the gear. The driving ring 3 is fastened to the handle of the clamping device. Rotating the handle in the direction of the arrow, the rotation is transmitted to the gear shaft through the roller 1. The roller is wedged between the bore surface of the housing 4 and the cut plane of the roller 2 and prevents reverse rotation.

Rice. 12 - Schemes of various designs of locks

A roller lock with direct transmission of torque from the driver to the roller is shown in fig. 12b. The rotation from the handle through the leash is transmitted directly to the shaft 6 of the wheel. Roller 3 is pressed through pin 4 by a weak spring 5. Since the gaps at the points of contact of the roller with ring 1 and shaft 6 are chosen, the system instantly wedges when the force is removed from handle 2. By turning the handle in the opposite direction, the roller wedges and rotates the shaft clockwise .
The conical lock (Fig. 12, c) has a conical sleeve 1 and a shaft 2 with a cone 3 and a handle 4. The spiral teeth on the middle neck of the shaft are engaged with the rail 5. The latter is connected to the actuating clamping mechanism. When the angle of inclination of the teeth is 45°, the axial force on the shaft 2 is equal (excluding friction) to the clamping force.
An eccentric lock (Fig. 12, d) consists of a wheel shaft 2, on which an eccentric 3 is wedged. The shaft is driven by a ring 1 fastened to the lock handle; the ring rotates in the body bore 4, the axis of which is offset from the shaft axis by a distance e. When the handle is rotated backwards, the transmission to the shaft occurs through the pin 5. In the process of fixing, the ring 1 is wedged between the eccentric and the body.
Combined clamping devices are a combination of elementary clamps of various types. They are used to increase the clamping force and reduce the dimensions of the device, as well as to create the greatest ease of management. Combined clamping devices can also provide simultaneous clamping of the workpiece in several places. Types of combined clamps are shown in fig. thirteen.
The combination of a curved lever and a screw (Fig. 13, a) allows you to simultaneously fix the workpiece in two places, evenly increasing the clamping forces to a predetermined value. The usual rotary clamp (Fig. 13, b) is a combination of lever and screw clamps. The swing axis of the lever 2 is aligned with the center of the spherical surface of the washer 1, which unloads the pin 3 from bending forces. Shown in fig. 13, an eccentric clamp is an example of a quick combination clamp. With a certain lever arm ratio, the clamping force or stroke of the clamping end of the lever can be increased.

Rice. 13 - Types of combination clamps

On fig. 13, d shows a device for fixing a cylindrical workpiece in a prism by means of a cap lever, and in fig. 13, e - scheme of a quick-acting combined clamp (lever and eccentric), which provides lateral and vertical pressing of the workpiece to the fixture supports, since the clamping force is applied at an angle. A similar condition is provided by the device shown in Fig. 13, e.
Toggle clamps (Fig. 13, g, h, and) are examples of quick-acting clamping devices driven by turning the handle. To prevent self-detachment, the handle is moved through the dead position until it stops 2. The clamping force depends on the deformation of the system and its rigidity. The desired deformation of the system is set by adjusting the pressure screw 1. However, the presence of a tolerance for size H (Fig. 13, g) does not ensure the constancy of the clamping force for all workpieces of a given batch.
Combined clamping devices are operated manually or from power units.
Clamping mechanisms for multiple fixtures must provide the same clamping force in all positions. The simplest multi-place device is a mandrel, on which a package of blanks (rings, disks) is mounted, fixed along the end planes with one nut (sequential clamping force transmission scheme). On fig. 14a shows an example of a clamping device operating on the principle of parallel clamping force distribution.
If it is necessary to ensure the concentricity of the base and workpiece surfaces and prevent deformation of the workpiece, elastic clamping devices are used, where the clamping force is uniformly transmitted by means of a filler or other intermediate body to the clamping element of the fixture (within the limits of elastic deformations).

Rice. 14 - Clamping mechanisms for multiple fixtures

Conventional springs, rubber or hydroplastic are used as an intermediate body. A parallel action clamping device using hydraulic plastic is shown in fig. 14b. On fig. 14, c shows a device of mixed (parallel-sequential) action.
On continuous machines (drum-milling, special multi-spindle drilling), workpieces are installed and removed without interrupting the feed movement. If the auxiliary time overlaps with the machine time, then various types of clamping devices can be used to secure the workpieces.
In order to mechanize production processes, it is advisable to use clamping devices of an automated type (continuous action), driven by the feed mechanism of the machine. On fig. 15, a shows a diagram of a device with a flexible closed element 1 (cable, chain) for fixing cylindrical workpieces 2 on a drum-milling machine when processing end surfaces, and in fig. 15, b - diagram of a device for fixing piston blanks on a multi-spindle horizontal drilling machine. In both devices, operators only install and remove the workpiece, and the clamping of the workpiece occurs automatically.

Rice. 15 - Automated clamping devices

An effective clamping device for holding thin sheet workpieces during their finishing or finishing is a vacuum clamp. The clamping force is determined by the formula

Q=ap,
where A is the active area of the cavity of the device, limited by the seal;
p=10 5 Pa - the difference between atmospheric pressure and pressure in the cavity of the device from which air is removed.
Electromagnetic clamping devices are used to clamp workpieces made of steel and cast iron with a flat base surface. Clamping devices are usually made in the form of plates and cartridges, in the design of which the dimensions and configuration of the workpiece in plan, its thickness, material and the required holding force are taken as initial data. The holding force of the electromagnetic device largely depends on the thickness of the workpiece; at small thicknesses, not all of the magnetic flux passes through the cross section of the part, and part of the magnetic flux lines is scattered into the surrounding space. Parts processed on electromagnetic plates or cartridges acquire residual magnetic properties - they are demagnetized by passing them through a solenoid powered by alternating current.
In magnetic clamping devices, the main elements are permanent magnets, isolated from each other by non-magnetic spacers and fastened into a common block, and the workpiece is an anchor through which the magnetic power flow is closed. To detach the finished part, the block is shifted using an eccentric or crank mechanism, while the magnetic force flow closes to the device body, bypassing the part.

BIBLIOGRAPHY

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The clamping elements hold the workpiece workpiece from displacement and vibrations arising under the action of cutting forces.

Classification of clamping elements

Clamping elements of fixtures are divided into simple and combined, i.e. consisting of two, three or more interlocked elements.

The simple ones include wedge, screw, eccentric, lever, lever-articulated, etc. - they are called clamps.

Combined mechanisms are usually performed as screw-
lever, eccentric-lever, etc. and are called tacks.
When using simple or combined
mechanisms in layouts with a mechanized drive

(pneumatic or otherwise) they are called mechanisms - amplifiers. According to the number of driven links, the mechanisms are divided: 1. single-link - clamping the workpiece at one point;

2. two-link - clamping two workpieces or one workpiece at two points;

3. multi-link - clamping one workpiece at many points or several workpieces simultaneously with equal efforts. By degree of automation:

1. manual - working with a screw, wedge and others
devices;

2. mechanized, in
subdivided into

a) hydraulic

b) pneumatic,

c) pneumohydraulic,

d) mechanohydraulic,

e) electrical,

e) magnetic,

g) electromagnetic,

h) vacuum.

3. automated, controlled from the working bodies of the machine. They are driven by the machine table, caliper, spindle and centrifugal forces of rotating masses.

Example: centrifugal-energy chucks for semi-automatic lathes.

Requirements for clamping devices

They must be reliable in operation, simple in design and easy to maintain; should not cause deformation of the fixed workpieces and damage to their surfaces; fastening and unfastening of workpieces should be carried out with a minimum expenditure of effort and working time, especially when fixing several workpieces in multi-place fixtures, in addition, clamping devices should not move the workpiece during its fastening. Cutting forces should, if possible, not be taken up by the clamping devices. They should be perceived by more rigid installation elements of devices. To improve the accuracy of processing, devices that provide a constant value of clamping forces are preferred.

Let's make a small excursion into theoretical mechanics. What is the coefficient of friction?

If a body weighing Q moves along a plane with a force P, then the reaction to the force P will be the force P 1 directed in the opposite direction, that is

slip.

Friction coefficient

Example: if f = 0.1; Q = 10 kg, then P = 1 kg.

The coefficient of friction varies with surface roughness.

Method for calculating clamping forces

First case

Second case

The cutting force P z and the clamping force Q are directed to one

In this case Q => O

The cutting force P g and the clamping force Q are directed in opposite directions, then Q \u003d k * P z

where k - safety factor k = 1.5 finishing k = 2.5 roughing.

Third case

The forces are directed mutually perpendicular. Cutting force P, counteracting the friction force on the support (installation) Qf 2 and the friction force at the clamping point Q * f 1, then Qf 1 + Qf 2 \u003d k * P z

G
de f, and f 2 - coefficients of sliding friction Fourth case

The workpiece is processed in a three-jaw chuck

In this direction, P, tends to move the workpiece relative to the cams.

Calculation of threaded clamping mechanisms First case

Clamping with a flat head screw From the equilibrium condition

where P is the force on the handle, kg; Q - clamping force of the part, kg; R cp - average thread radius, mm;

R is the radius of the support end;

Helix angle of the thread;

Friction angle in threaded connection 6; - self-braking condition; f is the coefficient of friction of the bolt on the part;

0.6 - coefficient taking into account the friction of the entire surface of the butt. The moment P*L overcomes the moment of the clamping force Q, taking into account the frictional forces in the screw pair and at the end of the bolt.

Second case

■ Clamping with a spherical bolt

With an increase in the angles α and φ, the force P increases, because in this case, the direction of the force goes up the inclined plane of the thread.

Third case

This clamping method is used when processing bushings or discs on mandrels: lathes, dividing heads or rotary tables on milling machines, slotting machines or other machines, gear hobbing, gear shaping, radial drilling machines, etc. Some information from the guide:

Screw Ml6 with a spherical end with a handle length L = 190 mm and a force P = 8 kg, develops a force Q = 950 kg
Clamping screw M = 24 with a flat end at L = 310mm; P = 15kg; Q=1550mm
Clamp with hexagon nut Ml 6 wrench L = 190mm; P = 10kg; Q = 700kg.

Eccentric clamps

Eccentric clamps are easy to manufacture for this reason, they are widely used in machine tools. The use of eccentric clamps can significantly reduce the time for clamping the workpiece, but the clamping force is inferior to threaded clamps.

Eccentric clamps are available in combination with clamps and without them.

Consider an eccentric clamp with a clamp.

Eccentric clamps cannot work with large tolerance deviations (±δ) of the workpiece. With large tolerance deviations, the clamp requires constant adjustment with screw 1.

Calculation of the eccentric

the material used for the manufacture of the eccentric are U7A, U8A with heat treatment up to HR from 50....55 units, steel 20X with carburizing to a depth of 0.8... 1.2 With hardening HR c 55...60 units.

Consider the scheme of the eccentric. Line KN divides the eccentric into two? symmetrical halves consisting, as it were, of 2 X wedges screwed onto the "initial circle".

The axis of rotation of the eccentric is displaced relative to its geometric axis by the amount of eccentricity "e".

For clamping, the section Nm of the lower wedge is usually used.

Considering the mechanism as a combined one consisting of a lever L and a wedge with friction on two surfaces on the axis and the “m” point (clamping point), we obtain a force dependence for calculating the clamping force.

where Q is the clamping force

P - force on the handle

L - handle arm

r - distance from the axis of rotation of the eccentric to the point of contact with

blank

α - slope angle of the curve

α 1 - angle of friction between the eccentric and the workpiece

α 2 - angle of friction on the axis of the eccentric

To prevent the eccentric from moving away during operation, it is necessary to observe the condition of self-braking of the eccentric

The condition of self-braking of the eccentric. = 12R

about someone with an expentoic

de α - sliding friction angle at the workpiece contact point ø - coefficient of friction

For approximate calculations Q - 12P Let's consider the scheme of a double-sided clamp with an eccentric

Wedge clamps

Wedge clamping devices are widely used in machine tools. Their main element is one, two and three-beveled wedges. The use of such elements is due to the simplicity and compactness of designs, speed of action and reliability in operation, the possibility of using them as a clamping element acting directly on the workpiece to be fixed, and as an intermediate link, for example, an amplifier link in other clamping devices. Usually self-braking wedges are used. The self-braking condition of a single-sided wedge is expressed by the dependence

α >2ρ

where α - wedge angle

ρ - the angle of friction on the surfaces Г and Н of the contact of the wedge with the mating parts.

Self-braking is provided at an angle α = 12°, however, in order to prevent vibrations and load fluctuations during the use of the clamp from weakening the fastening of the workpiece, wedges with an angle α are often used.

Due to the fact that a decrease in the angle leads to an increase in

self-braking properties of the wedge, it is necessary, when designing the drive to the wedge mechanism, to provide devices that facilitate the removal of the wedge from the working state, since it is more difficult to release the loaded wedge than to put it into working condition.

This can be achieved by connecting the actuator stem to the wedge. When the rod 1 moves to the left, it passes the path "1" to idle, and then hitting the pin 2, pressed into the wedge 3, pushes the latter. During the reverse stroke of the rod, it also pushes the wedge into the working position with a blow to the pin. This should be taken into account in cases where the wedge mechanism is driven by a pneumatic or hydraulic actuator. Then, to ensure the reliability of the mechanism, it is necessary to create different pressures of liquid or compressed air from different sides of the drive piston. This difference when using pneumatic actuators can be achieved by using a pressure reducing valve in one of the tubes supplying air or fluid to the cylinder. In cases where self-braking is not required, it is advisable to use rollers on the contact surfaces of the wedge with the mating parts of the device, thereby facilitating the introduction of the wedge into its original position. In these cases, the locking of the wedge is mandatory.

Consider the scheme of the action of forces in a single-bevel, most commonly used in fixtures, wedge mechanism

Let's build a force polygon.

When transferring forces at a right angle, we have the following relationship

+ pinning, - pinning

Self-braking takes place at α

Collets

The collet clamping mechanism has been known for a long time. Clamping workpieces with collets has proved to be very convenient in the creation of automated machines because only one translational movement of the clamped collet is required to secure the workpiece.

When operating collet mechanisms, the following requirements must be met.

The clamping forces must be provided in accordance with the emerging cutting forces and not allow the workpiece or tool to move during the cutting process.
The clamping process in the overall machining cycle is an auxiliary movement, therefore, the time of operation of the collet should be minimal.
The dimensions of the clamping mechanism links should be determined from the conditions of their normal operation when clamping workpieces of both the largest and smallest dimensions.
The error of locating the fixed workpieces or tools should be minimal.
The design of the clamping mechanism should provide the least elastic compression during the processing of workpieces and have high vibration resistance.
The parts of the collet, and especially the collet, must have a high wear resistance.
The design of the clamping device should allow its quick change and convenient adjustment.
The design of the mechanism must provide for the protection of collets from chips.

Collet clamps work in a wide range of sizes.
Practically the minimum allowable size for fastening is 0.5 mm. On the
multi-spindle bar machines, bar diameters, and

consequently, the holes of the collets reach 100 mm. Collets with a large hole diameter are used to fasten thin-walled pipes, because. relative uniform fastening over the entire surface does not cause large pipe deformations.

The collet clamping mechanism allows clamping workpieces of various cross-sectional shapes.

The resistance of collet clamping mechanisms varies widely and depends on the design and correctness of technological processes in the manufacture of mechanism parts. As a rule, clamping collets come out earlier than others. In this case, the number of fastenings with collets ranges from one (collet breakage) to half a million or more (jaw wear). The work of the collet is considered satisfactory if it is able to hold at least 100,000 workpieces.

Collet classification

All collets can be divided into three types:

1. Collets of the first type have a "straight" cone, the top of which is turned away from the machine spindle.

For fastening, it is necessary to create a force that pulls the collet into the nut screwed onto the spindle. The positive qualities of this type of collets are that they are structurally quite simple and work well in compression (hardened steel has a large allowable stress in compression than in tension. Despite this, collets of the first type are currently of limited use due to disadvantages. What are these disadvantages:

a) the axial force acting on the collet tends to unlock it,

b) when feeding the bar, premature locking of the collet is possible,

c) when fixing with such a collet, a harmful effect on

d) there is an unsatisfactory centering of the collet in
spindle, since the head is centered in the nut, the position of which is on
the spindle is not stable due to the threads.

Collets of the second type have a "reverse" cone, the top of which is facing the spindle. For fastening, it is necessary to create a force that draws the collet into the conical hole of the machine spindle.

Collets of this type provide good centering of the workpieces to be clamped, since the cone for the collet is located directly in the spindle;

jamming occurs, the axial working forces do not open the collet, but lock it, increasing the clamping force.

At the same time, a number of significant drawbacks reduces the efficiency of collets of this type. Since numerous contacts with the collet, the conical bore of the spindle wears out relatively quickly, the thread on the collets often fails, not providing a stable position of the bar along the axis when fastened - it moves away from the stop. Nevertheless, collets of the second type are widely used in machine tools.

The main purpose of the clamping devices of fixtures is to ensure reliable contact (continuity) of the workpiece or the part to be assembled with the setting elements, to prevent its displacement during processing or assembly.

Lever clamps. Lever clamps (figure 2.16) are used in combination with other elementary clamps, forming more complex clamping systems. They allow you to change the magnitude and direction of the transmitted force.

wedge mechanism. The wedge is very widely used in clamping mechanisms of fixtures, this ensures simplicity and compactness of the design, reliability in operation. The wedge can be either a simple clamping element acting directly on the workpiece, or it can be combined with any other element that is simple when creating combined mechanisms. The use of a wedge in the clamping mechanism provides: an increase in the initial force of the drive, a change in the direction of the initial force, self-braking of the mechanism (the ability to maintain the clamping force when the force generated by the drive stops). If the wedge mechanism is used to change the direction of the clamping force, then the wedge angle is usually 45 °, and if to increase the clamping force or increase reliability, then the wedge angle is taken equal to 6 ... 15 ° (self-braking angles).

o Mechanisms with a flat single-sided wedge (

o multi-wedge (multi-plunger) mechanisms;

o eccentrics (mechanisms with a curved wedge);

o face cams (mechanisms with a cylindrical wedge).

11. The action of cutting forces, clamps and their moments on the workpiece

During processing, the cutting tool makes certain movements relative to the workpiece. Therefore, the required arrangement of the surfaces of the part can only be ensured in the following cases:

1) if the workpiece occupies a certain position in the working area of the machine;

2) if the position of the workpiece in the working area is determined before the start of processing, on the basis of this it is possible to correct the movements of shaping.

The exact position of the workpiece in the working area of the machine is achieved in the process of installing it in the fixture. The installation process includes basing (i.e., giving the workpiece the required position relative to the selected coordinate system) and fixing (i.e., applying forces and pairs of forces to the workpiece to ensure the constancy and invariance of its position achieved during basing).

The actual position of the workpiece installed in the working area of the machine differs from the required one, which is due to the deviation of the position of the workpiece (in the direction of the holding dimension) during the installation process. This deviation is called the installation error, which consists of the basing error and the fixing error.

The surfaces belonging to the workpiece and used in its basing are called technological bases, and those used for its measurements are called measuring bases.

To install the workpiece in the fixture, several bases are usually used. Simplistically, it is believed that the workpiece is in contact with the fixture at points called reference points. The layout of reference points is called the basing scheme. Each reference point determines the connection of the workpiece with the selected coordinate system in which the workpiece is processed.

1. With high demands on machining accuracy, a precisely machined workpiece surface should be used as a technological base and such a basing scheme should be adopted that provides the smallest installation error.

2. One of the easiest ways to improve the accuracy of basing is to follow the principle of base alignment.

3. To improve the accuracy of processing, the principle of constancy of bases should be observed. If this is not possible for some reason, then it is necessary that the new databases be processed more accurately than the previous ones.

4. As bases, simple surfaces (flat, cylindrical and conical) should be used, from which, if necessary, a set of bases can be created. In cases where the surfaces of the workpiece do not meet the requirements for bases (i.e., in terms of their size, shape and location, they cannot provide the specified accuracy, stability and ease of processing), artificial bases are created on the workpiece (center holes, technological holes , plates, grooves, etc.).

The main requirements for fixing workpieces in fixtures are as follows.

1. The fastening should ensure reliable contact of the workpiece with the supports of the fixtures and guarantee the invariance of the position of the workpiece relative to the tooling during processing or when the power is turned off.

2. Workpiece clamping should be used only in cases where the processing force or other forces can displace the workpiece (for example, when pulling a keyway, the workpiece is not clamped).

3. The fixing forces should not cause large deformations and collapse of the base.

4. Securing and releasing the workpiece must be carried out with a minimum expenditure of time and effort on the part of the worker. The smallest fixing error is provided by clamping devices that create

constant clamping force (for example, devices with pneumatic or hydraulic drive).

5. To reduce the fixing error, base surfaces with low roughness should be used; to use devices with a drive; place workpieces on flat head supports or precision-machined base plates.

Ticket 13

Clamping mechanisms of fixtures Clamping mechanisms are called mechanisms that eliminate the possibility of vibration or displacement of the workpiece relative to the setting elements under the action of its own weight and forces arising in the process of processing (assembly). The main purpose of clamping devices is to ensure reliable contact of the workpiece with the setting elements, to prevent its displacement and vibration during processing, as well as to ensure the correct installation and centering of the workpiece.

Calculation of clamping forces

The calculation of the clamping forces can be reduced to solving the problem of statics for the equilibrium of a rigid body (workpiece) under the action of a system of external forces.

On the one hand, the workpiece is subjected to the force of gravity and the forces arising in the process of processing, on the other hand, the required clamping forces - the reactions of the supports. Under the action of these forces, the workpiece must maintain balance.

Example 1. The clamping force presses the workpiece against the supports of the fixture, and the cutting force arising during the processing of parts (Figure 2.12, a) tends to move the workpiece along the reference plane.

Forces act on the workpiece: on the upper plane, the clamping force and the friction force, which prevents the workpiece from shifting; along the lower plane, the reaction forces of the supports (not shown in the figure) are equal to the clamping force and the friction force between the workpiece and the supports. Then the workpiece equilibrium equation will be

where is the safety factor;

– coefficient of friction between the workpiece and the clamping mechanism;

is the coefficient of friction between the workpiece and fixture supports.

Where

Figure 2.12 - Schemes for calculating clamping forces

Example 2. The cutting force is directed at an angle to the fixing force (Figure 2.12, b).

Then the workpiece equilibrium equation will be

From figure 2.12, b we find the components of the cutting force

Substituting, we get

Example 3. The workpiece is processed on a lathe and fixed in a three-jaw chuck. The cutting forces create a torque tending to rotate the workpiece in the cams. Friction forces that occur at the points of contact of the cams with the workpiece create a friction moment that prevents the workpiece from turning. Then the equilibrium condition for the workpiece will be

The cutting moment is determined by the value of the vertical component of the cutting force

Friction moment

Elementary clamping mechanisms

Elementary clamping devices include the simplest mechanisms used to secure workpieces or act as intermediate links in complex clamping systems:

screw;

wedge;

eccentric;

lever;

centering;

rack-and-lever.

Screw clamps. Screw mechanisms (Figure 2.13) are widely used in fixtures with manual clamping of workpieces, with a mechanized drive, as well as on automatic lines when using satellite fixtures. Their advantage is the simplicity of design, low cost and high reliability in operation.

Screw mechanisms are used both for direct clamping and in combination with other mechanisms. The force on the handle required to generate the clamping force can be calculated using the formula:

where is the average thread radius, mm;

– key outreach, mm;

- the angle of the thread;

Angle of friction in a threaded pair.

wedge mechanism. The wedge is very widely used in clamping mechanisms of fixtures, this ensures simplicity and compactness of the design, reliability in operation. The wedge can be either a simple clamping element acting directly on the workpiece, or it can be combined with any other element that is simple when creating combined mechanisms. The use of a wedge in the clamping mechanism provides: an increase in the initial force of the drive, a change in the direction of the initial force, self-braking of the mechanism (the ability to maintain the clamping force when the force generated by the drive stops). If the wedge mechanism is used to change the direction of the clamping force, then the wedge angle is usually 45 °, and if to increase the clamping force or increase reliability, then the wedge angle is taken equal to 6 ... 15 ° (self-braking angles).

The wedge is used in the following design options for clamps:

mechanisms with a flat single-sided wedge (Figure 2.14, b);

multi-wedge (multi-plunger) mechanisms;

eccentrics (mechanisms with a curvilinear wedge);

face cams (mechanisms with a cylindrical wedge).

Figure 2.14, a shows a diagram of a two-angled wedge.

When the workpiece is clamped, the wedge moves to the left under the action of a force. When the wedge moves, normal forces and friction forces arise on its planes and (Figure 2.14, b).

A significant drawback of the considered mechanism is the low coefficient of performance (COP) due to friction losses.

An example of using a wedge in a fixture is shown in
Figure 2.14,d.

To increase the efficiency of the wedge mechanism, sliding friction on the surfaces of the wedge is replaced by rolling friction using support rollers (Figure 2.14, c).

Multi-wedge mechanisms come with one, two or more plungers. Single and double plungers are used as clamping; multi-plunger are used as self-centering mechanisms.

Eccentric clamps. The eccentric is a connection in one part of two elements - a round disk (Figure 2.15, e) and a flat one-sided wedge. When the eccentric rotates around the axis of rotation of the disk, the wedge enters the gap between the disk and the workpiece and develops a clamping force.

The working surface of the eccentrics can be a circle (circular) or a spiral (curvilinear).

Eccentric clamps are the fastest of all manual clamping mechanisms. In terms of speed, they are comparable to pneumatic clamps.

The disadvantages of eccentric clamps are:

small working stroke;

limited by the eccentricity;

increased fatigue of the worker, since when detaching the workpiece, the worker needs to apply force due to the self-braking property of the eccentric;

unreliability of the clamp when the tool is operated with shocks or vibrations, as this can lead to self-detachment of the workpiece.

Despite these shortcomings, eccentric clamps are widely used in fixtures (Figure 2.15, b), especially in small-scale and medium-scale production.

To achieve the required fixing force, we determine the largest moment on the eccentric handle

where is the force on the handle,

- length of the handle;

- angle of rotation of the eccentric;

- angles of friction.

Lever clamps. Lever clamps (figure 2.16) are used in combination with other elementary clamps, forming more complex clamping systems. They allow you to change the magnitude and direction of the transmitted force.

There are many constructive varieties of lever clamps, however, they all come down to three power circuits shown in Figure 2.16, which also shows the formulas for calculating the required force to create a workpiece clamping force for ideal mechanisms (excluding friction forces). This force is determined from the condition that the moments of all forces relative to the point of rotation of the lever are equal to zero. Figure 2.17 shows the structural diagrams of lever clamps.

When performing a number of machining operations, the rigidity of the cutting tool and the entire technological system as a whole is insufficient. Various guiding elements are used to eliminate depressions and deformations of the tool. The main requirements for such elements are: accuracy, wear resistance, changeability. Such devices are called conductors or conductor bushings and are used in drilling and boring work .

The designs and dimensions of the drill bushings for drilling are standardized (Fig. 11.10). Bushings are permanent (Fig. 11.10 a) and replaceable

Rice. 11.10. Conductor bushing designs: a) permanent;

b) interchangeable; c) quick change with a lock

(Fig. 11.10 b). Permanent bushings are used in single-piece production when machining with one tool. Replaceable bushings are used in serial and mass production. Quick-change bushings with a lock (Fig. 11.10 c) are used when machining holes with several successively replaceable tools.

With a hole diameter of up to 25 mm, the bushings are made of U10A steel, hardened to 60 ... 65. With a hole diameter of more than 25 mm, the bushings are made of steel 20 (20X), followed by carburizing and hardening to the same hardness.

If the tools are guided in the sleeve not by the working part, but by cylindrical centering sections, then special sleeves are used (Fig. 11.11). On fig. 11.11 a shows a sleeve for drilling holes on the slope

15. Adjusting elements of devices.

-Tuning elements (height and angle settings) are used to control the position of the tool when setting up the machine.)

- Tuning elements , ensuring the correct position of the cutting tool when setting up (tuning) the machine to obtain the specified dimensions. These elements are high-rise and angular installations of milling fixtures used to control the position of the cutter during setup and re-adjustment of the machine. Their use facilitates and speeds up the setup of the machine when processing workpieces by automatically obtaining the specified dimensions

Tuning elements perform the following functions : 1) Prevent tool withdrawal during operation. 2) Give the tool an exact position relative to the fixture, these include settings (dimensions), copiers. 3) Perform both functions above, these include conductor bushings, guide bushings. Conductor bushings are used when making holes with drills, countersinks, reamers. Conductor bushings are: permanent, quick-change and replaceable. Linen with a shoulder and without prim-Xia when the hole is processed with one tool. They are pressed into the part of the casing - H7/n6 jig plate. Replaceable sleeves are used when processing with one tool, but taking into account replacement due to wear. Quick-change applications when a hole is machined sequentially with several tools in an operation. They differ from replaceable ones by a through groove in the collar. Special conductor bushings are also used, having a design corresponding to the characteristics of the workpiece and operation. Elongated bushing Bushing with inclined end Guide bushings, which perform only the function of preventing tool withdrawal, are made permanent. For example, on turret machines, it is installed in the spindle bore and rotates with it. The hole in the guide bushings is made according to H7. Copiers are used to accurately position the tool relative to the fixture when machining curved surfaces. Copiers are overhead and built-in. Overheads are superimposed on the workpiece and fixed with it. The guide part of the tool has continuous contact with the Copier, and the cutting part fulfills the required profile. Built-in copiers are installed on the body of the device. A copier finger is guided along the copier, which, through a specially built-in device, transmits to the machine the corresponding movement for processing a curved profile to the spindle with the tool. Installations are standard and special, high-rise and corner. High-altitude settings orient the tool in one direction, angular in 2 directions. Coordination of the tool according to the settings is carried out with the help of standard flat probes with a thickness of 1.3.5 mm or cylindrical ones with a diameter of 3 or 5 mm. The fixtures are located on the body of the fixture away from the workpiece, taking into account the insertion of the tool, and are fixed with screws and fixed with pins. The probe used to set up the tool for installation on the assembly drawing of the device is indicated in the technical requirements, it is also allowed graphically.

To set (adjust) the position of the machine table together with the fixture relative to the cutting tool, special templates are used, made in the form of plates, prisms and squares of various shapes. The installations are fixed on the body of the device; their reference surfaces should be located below the workpiece surfaces to be machined so as not to interfere with the passage of the cutting tool. Most often, setups are used when processing on milling machines configured to automatically obtain dimensions of a given accuracy.

Distinguish between high-rise and corner installations. The first ones serve for the correct location of the part relative to the cutter in height, the second - both in height and in the lateral direction. They are made of steel 20X, carburized to a depth of 0.8 - 1.2 mm, followed by hardening to a hardness of HRC 55 ... 60 units.

Setting elements for cutting tool (example)

A comprehensive production study of the accuracy of the operation of existing automatic lines, experimental research and theoretical analysis should provide answers to the following basic questions of designing technological processes for the production of body parts on automatic lines accuracy requirements b) establishing the optimal degree of concentration of transitions in one position, based on the loading conditions and the required accuracy of processing c) the choice of methods and installation schemes when designing the installation elements of devices for automatic lines to ensure the accuracy of processing d) recommendations for the use and design of nodes of automatic lines, providing direction and fixation of cutting tools in connection with the requirements of machining accuracy e) the choice of methods for setting up machines for the required r dimensions and selection of control means for reliable maintenance of the setting size processing, as well as the establishment of standard values for the calculation of allowances for processing h) the identification and formation of methodological provisions for accurate calculations in the design of automatic lines.

16. Pneumatic drives. Purpose and requirements for them.

Pneumatic drive (pneumatic drive)- a set of devices designed to set in motion parts of machines and mechanisms by means of compressed air energy.

A pneumatic drive, like a hydraulic drive, is a kind of “pneumatic insert” between the drive motor and the load (machine or mechanism) and performs the same functions as a mechanical transmission (reducer, belt drive, crank mechanism, etc.). The main purpose of the pneumatic actuator , as well as a mechanical transmission, - transformation of the mechanical characteristics of the drive motor in accordance with the requirements of the load (transformation of the type of movement of the output link of the engine, its parameters, as well as regulation, overload protection, etc.). Mandatory elements of the pneumatic drive are a compressor (pneumatic energy generator) and an air motor

Depending on the nature of the movement of the output link of the pneumatic motor (pneumatic motor shaft or pneumatic cylinder rod), and accordingly, the nature of the movement of the working body, the pneumatic actuator can be rotary or translational. Pneumatic actuators with translational motion are most widely used in technology.

The principle of operation of pneumatic machines

In general terms, the energy transfer in a pneumatic actuator occurs as follows:

1. The drive motor transmits torque to the compressor shaft, which imparts energy to the working gas.

2. The working gas after special preparation through pneumatic lines through the control equipment enters the pneumatic motor, where the pneumatic energy is converted into mechanical energy.

3. After that, the working gas is released into the environment, in contrast to the hydraulic drive, in which the working fluid returns through the hydraulic lines either to the hydraulic tank or directly to the pump.

Many pneumatic machines have their structural counterparts among volumetric hydraulic machines. In particular, axial-piston pneumatic motors and compressors, gear and vane pneumatic motors, pneumatic cylinders are widely used ...

Typical diagram of a pneumatic actuator

Typical diagram of a pneumatic actuator: 1 - air intake; 2 - filter; 3 - compressor; 4 - heat exchanger (refrigerator); 5 - moisture separator; 6 - air collector (receiver); 7 - safety valve; 8- Throttle; 9 - oil sprayer; 10 - pressure reducing valve; 11 - throttle; 12 - distributor; 13 pneumomotor; M - manometer.

Air enters the pneumatic system through the air intake.

The filter cleans the air in order to prevent damage to the drive elements and reduce their wear.

The compressor compresses the air.

Since, according to Charles's law, the air compressed in the compressor has a high temperature, before air is supplied to consumers (usually air motors), the air is cooled in a heat exchanger (in a refrigerator).

To prevent icing of pneumatic motors due to the expansion of air in them, as well as to reduce corrosion of parts, a dehumidifier is installed in the pneumatic system.

The receiver serves to create a supply of compressed air, as well as to smooth out pressure pulsations in the pneumatic system. These pulsations are due to the principle of operation of volumetric compressors (for example, piston compressors), which supply air to the system in portions.

Lubrication is added to the compressed air in the oil sprayer, which reduces friction between the moving parts of the pneumatic drive and prevents them from jamming.

A pressure reducing valve must be installed in the pneumatic actuator, which ensures the supply of compressed air to the pneumatic motors at a constant pressure.

The distributor controls the movement of the output links of the air motor.

In a pneumatic motor (pneumatic motor or pneumatic cylinder), the energy of compressed air is converted into mechanical energy.

Pneumatic actuators are equipped with:

1. stationary devices fixed on the tables of milling, drilling and other machines;

2. rotating devices - cartridges, mandrels, etc.

3) devices installed on rotating and dividing tables for continuous and positional processing.

As a working body, pneumatic chambers of single-sided and double-sided action are used.

With double-acting, the piston is moved in both directions by compressed air.

With a one-sided action, the piston is moved by compressed air during fixing of the workpiece, and by a spring during unfastening.

To increase the fixing force, two and three piston cylinders or two and three chamber pneumatic chambers are used. At the same time, the clamping force increases by 2... .3 times

An increase in the fixing force can be achieved by integrating amplifier levers into the pneumatic drive.

It is necessary to note some advantages of pneumatic drives of devices.

Compared to the hydraulic drive, it is clean, it is not necessary to have a hydraulic station for each device if the machine on which the device is installed is not equipped with a hydraulic station.

Pneumatic drive is characterized by speed of action, it surpasses not only manual, but many mechanized drives. If, for example, the flow rate of oil under pressure in the pipeline of a hydraulic device is 2.5 ... 4.5 m / s, the maximum possible is 9 m / s, then air, being at a pressure of 4 ... 5 MPa, propagates through pipelines at a speed of up to 180 m/s and more. Therefore, within 1 hour it is possible to carry out up to 2500 actuations of the pneumatic actuator.

The advantages of the pneumatic drive include the fact that its performance does not depend on fluctuations in ambient temperature. The great advantage is that the pneumatic drive provides a continuous clamping force, as a result of which this force can be significantly less than with a manual drive. This circumstance is very important when processing thin-walled workpieces that are prone to deformation during fixing.

Advantages

· in contrast to the hydraulic drive - no need to return the working fluid (air) back to the compressor;

Less weight of the working fluid compared to the hydraulic drive (important for rocket science);

Less weight of actuating devices in comparison with electric ones;

the ability to simplify the system by using a compressed gas cylinder as an energy source, such systems are sometimes used instead of squibs, there are systems where the pressure in the cylinder reaches 500 MPa;

simplicity and economy due to the cheapness of the working gas;

quick response and high rotational speeds of pneumatic motors (up to several tens of thousands of revolutions per minute);

fire safety and neutrality of the working environment, providing the possibility of using the pneumatic actuator in mines and chemical industries;

· in comparison with a hydraulic drive - the ability to transmit pneumatic energy over long distances (up to several kilometers), which makes it possible to use a pneumatic drive as a main drive in mines and mines;

Unlike a hydraulic drive, a pneumatic drive is less sensitive to changes in ambient temperature due to a lesser dependence of efficiency on leakage of the working medium (working gas), therefore, changes in the gaps between the parts of pneumatic equipment and the viscosity of the working medium do not seriously affect the operating parameters of the pneumatic drive; this makes the pneumatic drive suitable for use in hot shops of metallurgical enterprises.

disadvantages

heating and cooling of the working gas during compression in compressors and expansion in pneumatic motors; this drawback is due to the laws of thermodynamics, and leads to the following problems:

Possibility of freezing of pneumatic systems;

· condensation of water vapor from the working gas, and in connection with this, the need to dry it;

· high cost of pneumatic energy compared to electrical energy (about 3-4 times), which is important, for example, when using a pneumatic drive in mines;

Even lower efficiency than hydraulic drive;

low accuracy and smooth running;

the possibility of an explosive rupture of pipelines or industrial injuries, due to which small pressures of the working gas are used in an industrial pneumatic actuator (usually the pressure in pneumatic systems does not exceed 1 MPa, although pneumatic systems with a working pressure of up to 7 MPa are known - for example, at nuclear power plants), and, as a result, the efforts on the working bodies are much smaller in comparison with the hydraulic drive). Where there is no such problem (on rockets and aircraft) or the systems are small, pressures can reach 20 MPa or even higher.

· to control the amount of rotation of the drive rod, it is necessary to use expensive devices - positioners.