Brazed band saws
Bandsaw blades are cut to length by butt welding at the ends to form an endless band.
Tooth sawn ends are precut to length of 1-2 tooth pitches (8 mm on narrow band saw blades and 15 mm on wide band saw blades). Sawing is done manually with a file or with a special grinder. The ends must be smoothly filed and well aligned.
Soldering is done with copper or silver solder and flux (see the Technical Information Sheet on page 54). table. 94).
Three differentive features are characteristic of band saw blades. they are made only from special kinds of tool steel to ensure high strength and durability of the teeth. Russian manufacturers usually use steel of categories 9KhF and B2F, foreign companies give preference to C75 series. The material hardness has to be at least 45 HRS in all cases.
Cutting width of band saw blades is much narrower than with circular saw blades. As mentioned above, this is a fundamental factor in the processing of valuable wood and business wood.
Saws can handle any length and diameter of workpieces without any problems. A properly sharpened band saw blade offers a high production output.
It must be noted that the reliability of the circular saws can be explained by the heterogeneity of the material that is used for the band saw blade.
In particular the body of the saw is usually made of spring steel that has an incredible resistance to tensile and dynamic loads. High tungsten and cobalt steel is used for the cutting edge. Separate parts of the blade are welded together by high-frequency currents. Tools made with this technology can be used for cutting metal.
Design of a band saw for wood
Band saws are similar in design. Professional tools are different in size and weight compared to home saws, designed for domestic use in small workshops, and handheld units. Band saws are structurally made up of several assemblies:
- The frame or frame is the body on which all key components are mounted.
- Work surface, equipped with a ruler and a stop, which helps to adjust the correctness of the cut.
- Engine. drives the saw.
- The band saw blade is placed on a pulley and rotates at high speed.
- Control panel, lubrication system, safety guard. additional components that make the bandsaw safe and comfortable to operate.
Band saw technology: Fast and high-quality cutting of different materials
- almost all band saw materials are difficult to weld;
- The band saw blade or knife is a developed cross-section part from the point of view of contact butt welding;
- the ultimate stresses on the welding seam during band saw operation;
- The influence of the accuracy of the band saw geometry in the area of the weld on the nature and level of the work load.
Resistance welding equipment is structurally simpler than flash-butt welding and thus cheaper, but the use of resistance welding for band saws has considerable limitations. Let’s start with preparatory operations. cutting the blade. Deburring and deformation of band saw blade edge may occur when cutting the blade to size from the coil. It is clear that for cutting a thicker blade, the setting, and the guillotine itself must be somewhat different. the knives must be changed in time and the cut must be strictly perpendicular and have no deformation and burrs. It is possible to observe all these in practice, but it is not always possible. Resistance band saw butt welding is much more sensitive to such defects of the blanking operations.
A greater influence on the welding result has the condition of the contact surface of electrodes and the surface of clamping segments, as well as the adjustment of their position. If in the process of band saw welding by melting an inaccurate setting or insignificant wear of the contact surfaces of the electrodes of the unit primarily affects the subsequent heat treatment (annealing) of the welded blade. then in resistance welding these small defects in the preparation of equipment for work can have a fatal effect on the quality of welding.
Material, composition and condition of the bandsaw blade are generally not disclosed by the manufacturer. In most cases we have to choose modes of butt welding on the basis of approximate data about settings for similar material group. In resistance welding, the relationship between the longitudinal stability of the blade and the degree of softening of the material during heating is of great importance. It would seem. the more alloyed the sheet, the stiffer and more resistant it is and the easier the welding process should be. Practice says no. both a higher secondary transformer voltage and an increase in draft force ahead of stability are required. In addition, when welding bimetal saws, the liquid phase that forms in the joint can sometimes not be completely extruded. After welding, coarse-grained areas are formed in the joint. Often no heat treatment can correct such a defect and the quality of the weld. that is, its strength and fatigue characteristics are at “3 minus” on a scale of 5. Welded saw is serviceable, but life of band saw blade in the area of junction is lower than life of cutting part until complete wear.
Resistance welding of band saw blades as a cheap and simple welding method is quite suitable for welding of most small width band saw blades on wood (up to 50 mm), occasional welding of wider wood blades and small width bimetal saw blades on metal. The limitation in welding of bimetal blades is caused by the fact that cutting of such blades inevitably leads to an accelerated local wear of the guillotine blades. and such wear leaves burrs and deformation zones on the cut edge of the blade which is a totally unacceptable defect in bandsaw welding by resistance.
Fusion welding of band saw blades is free of most of these disadvantages, but the requirements to the preparation of the blades for welding and to the condition of the electrode surfaces remain high as well. There is another subtlety in this process. it is to observe the correct pitch or sequence of the blade tooth when metal is lost along the length. When welding saws in wood with a standard pitch of 22 mm and a standard profile this is solved quite easily (it is recommended to calculate the location of the seam at the bottom of the back of the tooth when trimming the blade to size). It is not recommended to locate seam in a recess. it leads to decrease of useful section of seam and reduces its bearing capacity. as well as to pick up a seam close to tooth tip. In the second case, there is incomplete heating of the part of the weld closer to the tip of the tooth, which may cause incomplete shrinkage of the entire fabric. and if the equipment is worn, the curvature of the fabric on the back in the welding area. The weld should never be positioned in the area from the trough to the leading edge of the tooth. This is the part of the bandsaw blade that is most stressed when the blade is in use.
When positioning the weld on metal saws, which in most cases have a variable pitch, it is necessary in addition to cutting out not a multiple of the teeth to position the weld in the location of the teeth with a smaller pitch (especially important for welding blades with a coarse tooth). This results in a wider weld, a larger area and better strength properties. Also, those teeth that are in the fusion and upsetting area have to be cut before welding to the intended weld cross section. The teeth left behind often interfere with the process, they do not warm up sufficiently and lead to deformation when the blade shrinks “in plan” on the tooth side. It is strictly forbidden to place the seam “anywhere” when welding coarse pitch webs. In this case the cross-section to be welded changes and there is no point in speaking about any precision in observing the welding procedure.
When setting up the machine for resistance welding of band webs, attention should also be paid to the shape of the back profile of the band itself. The fact is that some blades are intentionally produced with a crescent-shaped back, and on some blades this is a defect. Anyway after welding the profile on the back of the blade in the area of welding must not differ. If the blades have a back warp. use a piece of the blade to be welded to adjust the position of the stops on the bandsaw welder. Such distortions are especially troublesome on bandsaw machines.
- Continuous and smooth process of melting from beginning to precipitation. If too much voltage and current is applied, the melting process may be interrupted, because the burnout of the material is faster than the approach of the ends of the fabric being welded. Outwardly, other symptoms (type of weld, the amount and nature of the exit of beads after butt welding) it may not be noticed, but it leads to instability and poor repeatability of welding results. If melting stopped a fraction of a second before upsetting. the weld will be inclusions of the cast structure and possibly oxides with all the ensuing consequences. Still, butt contact welding processes are not fusion welding methods as well as in welding with a semi-automatic or electrode. In the latter, a weld pool is formed and a weld is formed after its crystallization. When performing butt-welding, the weld should be formed in the “solid phase. The melt on the ends of the parts being joined serves only for subsequent extrusion during upsetting together with impurities and introduction into contact of heated and clean atomic surfaces of workpieces.
- the melt should be extruded and the underlying surface should be deformed! This is the first thing to pay attention to when inspecting the weld. On webs it is very well visible from the back side, as well as when scraping the weld from the burr. If there is a burr, but no traces of back surface deformation in the area adjacent to the outgoing burr (tenths of a millimeter). the mode has been selected incorrectly.
- The other extreme is the excessive drafting force. In this case, there is usually a local loss of blade stability on the tooth side and a shift in the plane of the blade surface of the parts being welded one against the other. The same defect may also occur when welding blades at a properly selected mode if the guillotine and electrodes are worn, if the clamping units of the band saw welding machine are incorrectly set.
Brazing band saws
If there is no welding machine the band saw blade is soldered. To do this, degrease it, heat the ends of the blade with a gas torch and perform the following operations. apply flux and melt it with the same torch, connecting by diffusion with a piece of brass:
- Put a piece of brass on the end of the blade, melt it in the flame of the torch. when melted, the saw steel diffuses with the brass melt at the atomic level. As flux we use borax or special flux PV209X.
- Construct a special device, similar to mobile and fixed clamps of the machine for welding saws.
- Secure the ends and apply flux to both ends.
- Align the ends of the band in the overlap by 2 teeth.
- We put a piece of brass between the blades and heat this area with a torch.
- As the melt forms, we join the cloths together.
- We wait for it to cool completely on its own.
- Grind the joint until “that’s enough. it’s straight”.
Brass has a melting point of about 900 degrees. The halves are heated to that temperature. Silver solder with a melting point over 900 degrees can be used instead of brass.
The overlap joint is easier, but it is more difficult to machine the soldering point to the correct thickness. This method is suitable only if it is impossible to use special welding equipment.
Band saw brazing
In the absence of a welding machine, the band of the saw blade is soldered. For this purpose it is degreased, the ends of the blade are heated with a gas torch and the following operations are carried out. flux is applied and melted with the same torch, connecting by diffusion with a piece of brass:
- We put a piece of brass on the end of the blade and melt it in the flame of the torch. when melted, the steel of the saw diffuses with the brass melt at the atomic level. As a flux we use borax or special flux PV209X.
- Construct a special device, similar to mobile and stationary clamps of the band saw welding machine.
- Fasten the ends and apply flux to both ends.
- Align the ends of the tape overlapping by 2 teeth.
- We put a piece of brass between the blades and heat this place with a torch.
- As the melt develops, we join the blades.
- Wait for complete self-cooling.
- Grind the joining area until “that’s enough. it’s already straight”.
Brass has a melting point of approximately 900 degrees Celsius. Warm up the halves to exactly this temperature. Instead of brass you can use silver solder with a melting point over 900 degrees.
Overlap joint is easier, but it is more difficult to process the soldering point to the desired thickness. This method is only suitable if no special welding equipment can be used.
Tape saw soldering
Brazing band saws is also an acceptable way to join the edges of the blade. If you perform this process according to all the rules, you will end up with a high-quality and durable saw, which can safely be used in a band sawing machine or, for example, in a machine for wood.
This method is the most preferred for home use.
Band saw brazing at home is a cheap and affordable way to make cutting tools. The first thing to do before joining is to clean the edges of the blade from any dirt.
Connect the ends of the cloth should be butt to butt. It is important that they are beveled. Get the facets of the desired shape can be obtained on an abrasive wheel. The shape of the bevel is a rectangular triangle, the length of one of the cathetuses of which is determined by the thickness of the metal.
Beveled surfaces must be as even as possible and adjoin perfectly to each other. Do not forget about the tooth pitch. When shaping the bevel, care must be taken to maintain the pitch.
There are many ways to heat the metal to the desired temperature. However, the most common and common is the use of a gas torch. Its flame, if possible, should be slightly oxygen deficient or neutral.
Depending on the brand of solder chosen, the temperature is set. It is important not to overheat. At certain temperatures unwanted structural-phase transformations can occur in the product.
It is important to have a band saw welding machine at your disposal that allows you to place the ends of the blade back to back. Such a device can be made by yourself according to one of the drawings available on the Internet.
Welding machine for band saw welding.
By and large, soldering this tool is very similar to the same process of working with high-temperature solder of other products.
The beveled edges of the strip are covered with flux and clamped in the machine so that the edges are close to each other. It is important to make sure that only the edges of the tool are covered with flux. If you do this procedure incorrectly, the flux will flow where it is not needed. Later you will have to spend extra time to remove it.
Then the connection is heated with a torch to the desired temperature. Solder is fed into the joint. Through the action of capillary forces, the solder flows between the edges. The result is a stripe around the joint.
It is very important that the solder is not melted by the burner flame, but by the temperature transmitted from the heated metal.
Allow a few minutes for the workpiece to cool before removing it from the machine. If the sequence of operations is performed correctly, there should be no solder buildup. If they are, the excess should be carefully removed.
This method is great for home use, such as making homemade tools and using it as a wood saw.
Band saw welding machines and electrode material
The material for the electrodes must necessarily be of the hard type, and also conduct electricity well. Copper is soft, so it is not recommended. In most cases, bronze is used for this purpose, but it begins to move on the surface of the electrode at high temperatures, and its properties and conductivity change. When more than 25 welds are made, it becomes unusable.
Automotive brass is often used in electrodes, but zinc and lead often change resistance. Therefore, the electrode quickly becomes unsuitable for further use.
Bronze with chromium makes the metal drift on the surface and their setting slower. They can withstand more than half a thousand welds.
Annealing of welds
Low-alloyed fabrics must be annealed at 420 to 450 degrees F, alloyed fabrics at 630 degrees F or higher.
The manufacturer usually indicates the recommended temperature for annealing, and there are several reasons for this:
- Annealing is not a separate process, but is included in the welding procedure;
- the time interval of change of the metal and its structure is in direct dependence on the temperature regime, so that the section is heated evenly, it is recommended to use low temperatures with a longer process;
- the most optimal is considered step annealing, that is, the first heating is sustained and equalizes the temperature across the section, and then there is a rapid heating and cooling;
- the heat treatment mode is adjusted after the mode for welding is debugged;
- it is not necessary to rush and release the clamps immediately after welding, it is necessary to fully cool the material on the electrodes;
- do not immediately pull out the saw after welding to clean up, because this part is quite fragile, so it can cause cracks;
- after the blowing by compressed air has been carried out, you need to pull out the product, blast all the necessary places once again and then fix the product for annealing.
There is an opinion that the fanning of the electrodes wears out the linear guides faster. However, the quality units provide for sealing the bearings with seals, so this does not affect the subsequent use of the products.