22.05.2015
Purpose and types of band saws
Band saws are the cutting tool of band saws: carpentry, dividing and log saws. The saws used in these machines differ only in size, tooth profile and are divided into three types: carpentry (narrow), dividing (medium) and log saw (wide). The first two types are produced in accordance with GOST 6532-53, and log saws - in accordance with GOST 10670-63 "Band saws for sawing logs and beams." Band saws are used for curved and longitudinal sawing of boards, beams, logs and board wood materials.
Design of band saws
The design of band saws is characterized by the dimensions of the blade (width B of the band, including teeth, thickness 5, length L), profile and dimensions of the teeth of the cutting edge. The dimensions of the band saw blade mainly depend on the design of the band saw machines: the distance between the axes of the saw pulleys k, their diameter D and width.
Band saw length can be determined by the formula
Since the tape is supplied by the manufacturer in rolls, when cutting the estimated length, it is necessary to take into account the soldering allowance, and maintain the total tooth pitch at the soldering point.
Band saw thickness depends on the diameter of the saw pulley and must satisfy the dependencies
The value of bending stresses depends on the ratio of the thickness of the saw and the diameter of the pulley, which in the general balance of stresses have a large specific value. The magnitude of the stress from the bending of the saw
The magnitude of stresses from bending at s/D=0.001 will be
The tensile strength at the soldering point does not exceed 70-80 kgf/mm2. Therefore, with a minimum margin of safety equal to 2, the stresses in a working saw should be less than 35-40 kgf / mm2. In this regard, tend to use the smallest possible thickness of the saw and large diameters of the saw pulleys.
The width of the band saw blade depends on the width of the saw pulleys and can exceed the latter only by the height of the teeth. When choosing the width of joiner's band saws when sawing out curved parts, it is additionally necessary to take into account the radius of curvature R mm of the cut and the widening of the teeth on the side Δs mm. Then the saw width
Wider saws will flex in cross section, causing them to gouge and even slip off the pulleys.
For sawing parts with a very small radius of curvature, jigsaw machines are used, in which jigsaws are used as a cutting tool. Dimensions of jigsaws L = 130/140 mm, B = 2.3/8 mm, s = 0.26/0.5 mm, t = 0.6/1.5 mm. Angular parameters of a tooth with a straight back face: α = 5/10°, β = 40/45°. The dimensions of carpentry, dividing and log saw band saws are given in Table. 25.
Each type of GOST saw has its own tooth profile. For example, for dividing saws there are two: profile I - with an elongated cavity and profile II - with a straight back edge (Fig. 41.6). Reciprocating saws with tooth profile I are used when sawing hard and frozen soft woods, with tooth profile II - when sawing soft woods. The dimensions of the teeth of band saws depend on their thickness, width and sawing conditions.
For joiner's band saws with a width of 10-60 mm, the dimensions of the teeth are determined by the following approximate expressions (mm):
For reciprocating and log saw band saws, the tooth dimensions are (mm):
For saws with a set tooth, the pitch is reduced by 25-30%. The angular values of the tooth profiles provided by GOST are shown in fig. 41. The front angle of the teeth must be made as large as possible, since in this case the cutting power decreases and the pressing force in the horizontal plane decreases, which shifts the saw from the pulley. However, with an increase in the front angle γ, it is necessary to take into account the properties of the material being sawn and the strength of the tooth, which depends on its dimensions and the angle of sharpening β. The angle γ should be kept within 20-35°.
Band saw soldering
Welding of band saws is carried out in case of preparation of new saws from a rolled band, repair of saws in the presence of significant cracks (over 0.12V) or breakage. It includes the following operations: marking, trimming, chamfering, soldering, heat treatment of the soldered seam, its cleaning and straightening. Proper soldering requires that the seam is half the distance between the tops of adjacent teeth of the soldered saw. To do this, before cutting, the saw is marked with a ruler, square and scriber.
Marking and cutting out defective places (when repairing a saw) must be done according to the scheme shown in Fig. 42. The width of the seam B depends on the thickness of the saw s and is approximately taken equal to 105. After marking, the saw is cut along the marked lines ab and cd with scissors or a chisel. The cut ends are straightened with a hammer on the anvil and cleaned with a file. The ends of the saw are soldered with an overlap. To keep the thickness of the soldered seam equal to the thickness of the saw, its ends are beveled (chamfered) within the marked strip. Chamfering is performed manually with a file on a special device, or on milling or grinding machines. The honed ends are carefully cleaned with sandpaper and degreased.
The ends of the saws are soldered in special presses with soldering bars, electric soldering machines, or the flame of a blowtorch. Presses with soldering bars are heated up to 830-1000°C in PM-6 muffle furnaces. The ends of the saw to be soldered are installed in a soldering press, and solder is laid between them in the form of a plate with a thickness of 0.075-0.15 mm, together with a flux - dehydrated borax. The flux is necessary to protect the soldered surfaces from oxidation, as well as their better wetting. Then, heated soldering bars are inserted into the press and tightly pressed with screws to the place of soldering. After melting the solder and cooling the bars to a dark red color, they are removed, and the place of soldering is cooled in the cold section of the press. After some time, the saw is tempered for 1-2 minutes using the same bars, but already heated to a temperature of 650-700 ° C. Silver solders P-Sr-45, P-Sr-65 or brass L62 with a temperature of melting point 605-905 ° C. After cooling, the soldering site is cleaned of scale and filed on both sides with a personal file to a thickness equal to the thickness of the saw blade. Then the soldering place is rolled.
To connect the ends of the tape, you can use the method of butt welding using ASLP-1 devices. The ends of the saws are cut at an angle of 90 ° C, fixed in the clamps of the welding machine, brought into contact and the current is turned on. As soon as the ends of the saws are heated to a plastic state, the current is turned off, and the ends of the saws are shifted even more tightly by moving the clamps until they are welded. This method has not yet been widely used, since it requires special equipment.
Straightening and rolling of band saws
At editing band saws detect and repair defects in the same way as when editing frame saws. Given the large length of the saws and their small thickness, defects should be eliminated with great care, mainly with the help of a rolling machine. The less the band saw is dressed with straightening hammers, the longer its service life. Therefore, dressing should be resorted to in case of emergency, replacing it with rolling if possible.
Rolling band saws is performed in two ways.
The first way: symmetrical rolling is performed similar to rolling of frame saws and consists in lengthening the middle part of the blade (Fig. 43, a). Rolling starts from the middle part of the blade and ends, not reaching 10-15 mm on one side to the line of tooth troughs, on the other - to the rear edge of the saw. Rolling, straightening and quality control of saw preparation are carried out on special tables equipped with a rolling machine, anvil, test plate and supporting rollers for moving the saw. The degree of rolling is determined by the arrow of transverse curvature when the saw is bent using a short test ruler. The sag should be approximately 0.2-0.4 mm for straight rim pulleys and 0.3-0.5 mm for convex ones. Larger deflection values apply to wider and thinner bandsaws. An accurate check of the cross-curvature of the saw can be made with a convex template with a radius of curvature corresponding to a correctly machined saw. In addition, the straightness of the rear edge of the saw is checked by placing it on a test flat plate and applying a long straight ruler to the edge.
The second method of rolling band saws is called cone rolling (Fig. 43, b). Rolling starts at a distance of 15-20 mm from the line of tooth cavities. Closer to the rear edge of the saw, the pressure of the rollers is increased. The last track of the rolling rollers must be located no further than 10 mm from the rear edge. As a result of this, the cutting edge is shorter than the rear and, when pulled, receives greater stresses than the rest of the saw. The back edge of a saw that has been tapered and placed on a flat proofing plate will be located in an arc of a circle with the center placed towards the teeth. The value of the convexity of this edge over a length of 1 m serves as a characteristic of the degree of rolling. The bulge arrow is determined by a test ruler with an indicator along the entire length of the saw. The convexity of the boom should be 0.3-0.5 mm over a length of 1 m, with larger values referring to wider saws. If a part of the blade along the length of the saw has a convexity greater than required, this place must be rolled with an increase in the pressure of the rollers from the rear edge to the cutting one. On the contrary, if the convexity is small, roll with increasing pressure of the rollers from the cutting edge to the rear. The second rolling method is the best for wide band saws, especially when heating is uneven across the width of the blade.
Installing band saws in the machine
The normal operation of a band saw depends not only on the quality of the preparation of the saw, but also on its correct tension and installation in the machine. For this, the following conditions must be met:
1. The band saw during installation and operation should be placed on the saw pulleys so that the cutting edge protrudes beyond the edge of the pulleys by at least half the height of the tooth, but not more than its height.
2. The tension of the saw should be sufficient to ensure its rigidity in the lateral direction and, on average, be at least 5-6 kgf/mm2.
3. Guide vanes for the saw must be fitted and adjusted to the saw with a gap not exceeding 0.1-0.15 mm.
In order to exclude the displacement of the saw blade along the pulley due to the horizontal components of the cutting forces, the discrepancy between the resultant tension of the saw and its center line, heating the saw, etc., a number of preventive measures are used. Saw pulleys are made with convex rims, while the bulge is not located in their middle, but closer to 25-40 mm to the cutting edge. Flat pulleys to prevent slipping of the tape are tilted (on the worker) relative to the horizontal axis at an angle of 10-15 ", and the saws are rolled into a cone. In addition, most modern machines allow you to turn the upper pulley around its vertical axis due to the lateral displacement of the front support of the pulley shaft. Such a turn (working branch outward) allows you to hold the saw with strong heating and cutting to the inside.Guiding vanes make it possible to protect the saw from strong lateral bends, slipping from the pulleys and dampen resonant vibrations.In the process of work, you must carefully monitor the cleanliness of the surfaces of the saw pulleys, timely cleaning them from dust, sawdust, resin, etc.
Technical requirements for band saws
Limit deviations in the dimensions of band saws should not exceed the values \u200b\u200bspecified in Table. 26.
The roughness of the side surfaces of the saws must be at least class 7 according to GOST 2283-57. Cracks, delaminations, scratches, burns from grinding are not allowed. The canvas must be straightened and rolled evenly. On the saw, every 10 m there should be a stamp indicating the type of saw, its dimensions, GOST. For example, for log band saws according to the scheme: “Saw PLB Bхsхt GOST 10670-63”.
The presence of a complete set of equipment, fixtures and control and measuring tools is the first condition necessary for rolling band saws.
It is also necessary to assess the technical condition of the equipment used, that is, to identify defects and, if necessary, bring it into a condition that meets the accuracy standards.
When visiting enterprises, the author often recorded the absence of some devices and measuring instruments or the unsatisfactory condition of the equipment used and, accordingly, the unsuccessful practice of rolling band saws.
For example, one "craftsman" tried to roll band saws in the absence of a pressure gauge on the rolling machine. At another enterprise, radial and axial play of the rolling roller was discovered. The reason is the wear of the bronze bushing due to the lack of lubrication. The two cases mentioned above refer to obvious violations, but there are also hidden defects in the technology used, which are quite difficult to detect.
Equipment, fixtures and instrumentation for rolling wide band saws
The kit for rolling wide band saws includes the following:
- unit (workbench) for the preparation of band saws (preferably double-sided), including a rolling machine, a calibration plate with a length of at least 1.5 m, a flat anvil, a block of lifting rollers and a device for grinding the trailing edge of the band saw blade;
- a set of straightening hammers, including a hammer with a round head, a hammer with a cross arrangement of longitudinal dies and a hammer with an oblique arrangement of longitudinal dies;
- a set of rulers to control both the flatness of the blade and the straightness (bulge) of the trailing edge of the band saw blade;
- a set of templates and rulers, including those with an indicator, to control the stress state (degree of rolling) of the web.
It is appropriate to recall once again that the absence of any of the above elements will not allow high-quality rolling.
The standard equipment of the machine for preparing band saws does not include the supply of a block of lifting rollers and a device for grinding the trailing edge. Therefore, they must be manufactured and mounted independently.
The lifting roller block is installed in close proximity behind the rolling machine and serves to eliminate the transverse bend (chute) of the band saw blade.
The trailing edge grinder is an electric motor with a grinding wheel mounted on a slide in a horizontal plane behind the surface plate. With this device, the trailing edge is leveled, that is, its local waviness is eliminated. This is a very important operation preceding the rolling of the blade, which will ensure in the future high-quality rolling of the blade into a cone and sharpening of the teeth.
Identification of defects and assessment of the technical condition of equipment for rolling wide band saws
First of all, it is required to assess the technical condition of the rolling machine, even if it is new. Practitioners say: "Why evaluate the technical condition of a new rolling machine?". The author met new rolling machines, in which the relative position of the rolling rollers was unsatisfactory. Therefore, even a new rolling machine should be checked.
Methods for assessing the technical condition of the rolling machine and its installation on the unit for the preparation of band saws are indicated in the table.
| No. p / p | Controlled indicator | Permissible deviation, mm |
Tool and method of control |
| 1 | Roller diameter (upper and lower) | 0,02 | Micrometer. The difference in diameters of the rolling rollers is controlled |
| 2 | Parallelism of the axes of the rolling rollers in the horizontal plane | 2 (on a length of 1000 mm) | Arrows 500 mm long are fixed on the shafts of the rolling rollers. In the right position, the arrows should close, then the arrows turn to the left. The resulting gap is measured |
| 3 | Parallelism of the axes of the rolling rollers in the vertical plane | 2 (on a length of 1000 mm) | The arrows according to paragraph 2 are used, which are installed alternately vertically, first in the upper position, then in the lower position. Plumb deviation is measured |
| 4 | Radial runout of the working surface of the upper and lower rolling rollers | 0,01 | Magnetic stand with dial indicator |
| 5 | The hardness of the working surface of the upper and lower rolling rollers | 2HRC | Hardness tester |
| 6 | The size of the major axis of the ellipses formed as an imprint on the plate from the top and bottom sides | 5,0 |
Between the rollers of the rolling machine, a copper or aluminum plate is compressed. The pressure is removed and the large axes of the ellipses of the resulting prints (impressions) are measured with a caliper |
| 7 | Parallelism of the plate clamped in the rolling rollers to the surface of the table plate on which the rolling machine is installed | 2 (on a length of 200 mm) | Between the rollers a flat plate 300x200 mm from the saw blade is engaged. The height gauge measures the difference in distance between the edges of the plate and the surface of the table plate on which the rolling machine is installed. |
| 8 | Working surface radius of rolling rollers in axial section | Radius Pattern | |
| 9 | Assessment of the state of the working surface of the rolling rollers | Visual inspection with a magnifying glass for dents, chips and other damage | |
| 10 | Installation of the working surfaces of the surface plate, anvil and lower rolling roller in the same horizontal plane | 0,1 | For control and installation, a 2 m long straightedge is used |
I would like to draw your attention to the fact that the assessment according to indicators 6 and 7 is an indirect control of the state of the rolling rollers and their relative position, but it is quite sufficient for the practice of rolling band saws.
To perform a check on indicator 8, a radius template is made, which is a part of a ring machined on a lathe. The gap between the working surface of the rolling roller and the template indicates wear or plastic deformation of the working surface of the rolling roller and the need for its regrinding.
When assessing the condition of the working surface of the rolling roller (indicator 9), it is necessary to pay attention to risks, dents, chips and other damage - they are not acceptable.
An integral (general) assessment of the good technical condition of the rolling machine and its correct installation on the unit for the preparation of band saw blades is the flatness of the band saw blade, that is, the absence of residual deformation of the blade after rolling, leading to the loss of flatness of the band saw blade.
Of course, identifying defects in equipment for rolling band saws is a rather difficult job and requires certain skills and practice.
I hope that the material presented in the article will help technical specialists in this work. If necessary, you can contact the author for advice and technical assistance.
11. Preparation of band saws: rolling, sharpening of teeth, repair of saws.
Rolling is an operation, as a result of which useful internal stresses are first created in the saw blade (before installation in the machine), increasing the transverse rigidity of the saw, created by its tension on the machine pulleys. With symmetrical rolling, three to five tracks are rolled in width from the middle to the edges: the extreme tracks are at a distance of 10 ... 15 mm from the tooth cavity line and from the rear (rear) edge of the saw. The correctness of rolling is controlled by transverse curvature (deflection arrow at the width of the tape) with a special template. The deflection of a normally rolled saw is 0.15 ... 0.3 for cylindrical pulleys of the machine, 0.4 ... 0.6 mm for barrel-shaped pulleys (higher values \u200b\u200bare related to thinner and wider saws).
The sharpening of the teeth of band saws is carried out on TchL machines with corundum wheels of a flat 45 ° conical profile (ZP) with a grain size of 40 ... .25 on a bakelite (B) bond with a hardness of C1 - ST1. The wheel thickness is 0.2,..0.33 tooth pitches. Good quality provides the following mode of sharpening: circumferential speed of rotation of the circle
20.. .25 m/s; infeed in one pass 0.02.. .0.06 (up to 0.1) mm; number of passes 4.. .6 (up to 7).
The finishing of the teeth consists in grinding the front and back faces with a fine-grained whetstone, fixed in a special holder. The durability of a saw with adjusted teeth increases by 15...20%. Finishing is performed using a manual device.
Repair of band saw blades includes localization of cracks, cutting out defective zones of the blade and preparation of segments of inserts. Localization is subject to single cracks with a length of not more than 10-15% of the width of the saw, but not more than 15 mm. Localization is carried out by drilling holes with a diameter of 2 ... 2.5 mm at the end of the crack.
In the presence of single long cracks, as well as group cracks (4–5 pieces over a length of 400–500 mm) and two teeth broken in a row, the defective place is cut out. The length of the cut should be at least 500 mm in order to avoid difficulties when straightening.
12. Installation of band saws: tension methods, guides, adjustment of pulleys.
1. The cutting edge of the saw should protrude beyond the edge of the saw pulley by a tooth height.
2. Displacement of the tape along the pulleys is prevented by adjusting the position of the axis of the upper pulley (together with the pulley) in the vertical (tilt forward - backward) and horizontal (turn left - right) planes. The angle of inclination of the pulley forward (on the worker) is 0.2–0.3 °.
3. The tension force of the saw, N, total for both branches of the tape, is set depending on the width and thickness of the saw:
where σ is the tensile stress in the section of the tape (50–60 MPa); a, b- width and thickness of the tape, mm.
4. The gap between the guides and the saw should be 0.1–0.15 mm. The contact of the saw with the guide devices is only allowed when sawing out curved parts.
5. During operation, the band saw should not have wave-like movements and make vibrating sounds. The tension force should be minimal to exclude these phenomena.
6. Do not allow the saw to run idle for a long time. During long stops of the machine, the tension from the saw should be removed.
7. It is necessary to regularly clean the saw blade from resin, dirt, sticking sawdust.
8. In a number of cases, good results are obtained by lubricating saws with water, soapy water, a mixture of diesel fuel and machine oil by dropping or spraying.
