Tips for common applications on band saw blades

By Thomas L. Fahringer

It has been said that resistance welding is a function of current, pressure, and time. As it is true in other forms of resistance welding, it is also true in the flash-butt welding of band saw blades. Gases or coatings are not used to shield the weld area as they are in gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW), and no filler material is added during welding. Rather, the ends of the blades or strips are butted together, heated until they are molten, and then pushed together forcefully to form a welded joint. This is called resistance butt welding, and there are two different types - - butt welding and flash-butt welding.

Butt Welding

All blade welding machines using resistance welding have two sides into which the blade is clamped - - a fixed and electrically insulated side, which is generally the left side, and a movable and electrically grounded side, which is generally the right side. Once the ends of the blade are butted together and firmly clamped, pressure, which can be air, spring, or hydraulic, is applied immediately. When the start button is pushed, current begins to flow across the blade ends, causing them to heat up and become molten. The applied pressure forces the ends of the blades together as soon as they become plastic and then molten, destroying the molten region at each blade end nearly as fast as it is created. This traps impurities in the weld making the weld brittle and weak. Flash welding addresses this problem by allowing the material to stay molten for a much longer period of time before forging.

Flash Welding

Not only is the flash-butt welding process different from butt welding, but so are the machines used. Flash-butt welding is not a new process. This author owns one of the first 20 flash welding machines built by Ryals and Stone in the 1920s. It is a flash welding machine but has no motor-driven cam, and the movable carriage must be advanced by pushing it with a long bar that is positioned on a pivot. Operating this machine required great skill and nerves of steel. The operator could not flinch when the arc was struck, and had to gauge the rate of burnoff at the blade ends by observing how much metal was being blown away, how yellow or white (cold or hot) the sparks seemed to be, and the loudness and regularity of the arc.

The addition of the motor-driven cam to these welding machines removed the human judgment factor needed for operating previous flash-butt welding equipment. Controlling the advancement rate of the right-hand carriage allowed current to flow at a constant rate. This created wide molten regions at each blade tip, allowing the impurities to float, sink, or get squeezed out of the weld during forging. Without a motor-driven cam, advancing the right-hand carriage at an even rate was almost impossible, and the flashing was quite irregular. Because the strength of the weld depends on an uninterrupted flow of current, the addition of the motor-driven cam was a significant change that greatly increased the percentage of good welds.

Spring pressure application is another difference between flash and butt welding. With butt welding, spring pressure is applied before current begins to flow, destroying the molten zones as they are formed. In flash welding, spring pressure is applied at the end of the weld cycle, after the flowing current has had time to heat up the blade ends. The net effect of this extra time in the molten state is a metallurgically strong weld that is relatively free of harmful impurities. After learning the basic dynamics of flash welding, and why it is important to have wide molten regions at the blade ends during welding, some other factors must be understood that cause the molten zones to be too small or too cold. Most of these factors are operator-controlled. However, the ones that are not operator-controlled are some of the most common causes of weak and brittle welds.

Inadequate Wiring and Fusing

When the weld start button is pushed on a flash welding machine, an instantaneous current spike occurs. Adequate fusing and wiring is essential to accommodate this spike. If the amperage rating of the circuit breaker or the size of the wire running from that breaker to the welding equipment is insufficient, the current spike may be cut off and limited, resulting in cold welds. Machine operators should consult the operator's manual or call the manufacturer to learn the proper wire sizes and circuit breaker amperage rating for the equipment they are using.

Weak Circuit Breaker

A weak circuit breaker is not a common cause of bad welds, and, when it does occur, it often goes unnoticed. A failing circuit breaker can rob the weld of current flow and cause bad welds. This failure is hard to detect because it is common practice to use a voltmeter to determine if the circuit breaker is operating properly. Unfortunately, the voltmeter will not reveal a breaker that cannot carry the required amperage load. Operators should consult the equipment manufacturer to determine the amperage characteristics of their machines. A qualified electrician should then be able to determine whether or not the circuit breaker is causing a current flow problem.

Insufficient Power

While it is not a frequent cause of bad welds, insufficient incoming power is a problem that may exist. Before a flash welding machine can be used effectively, an adequate power supply that provides uninterrupted current and voltage must be established. The operator should consult the machine's manual or contact the manufacturer to discuss all aspects of this requirement. Insufficient power can cause hesitation in the flashing. In this case, the operator will hear the flashing cycle begin, stop for a fraction of a second, and then start again. If this happens, the primary power (line side) may be insufficient.

The Dreaded R-Factors

In all types of resistance welding, one of the biggest - if not the biggest - enemy of adequate current flow is a point of resistance to that flow somewhere in the electrical circuit. In the average welding machine, there may be many points of resistance, each of which is referred to as an R-factor. It is important for welding machine operators and owners to find and eliminate as many R-factors as possible. Allowing R-factors limits the flow of current to the blade ends. When the volume of current is diminished sufficiently, cold and brittle welds always will result.

Examples of R-factors are pitted jaws, dirty jaws, poor clamping pressure, jaws that are not flat, worn or pitted switch contacts, and loose electrical connections.

Pitted Jaws

As the machine is used on a regular basis, pieces of flashing, as well as dirt and grit on the band saw blade create holes in the clamping surfaces, causing a condition known as pitting. Obviously, these holes do not conduct electrical current and eventually will accumulate to a point that will affect weld quality. At this point, the jaws should be removed and reground or simply replaced.

Dirty Jaws

Dirty jaws are as dangerous as pitted jaws because current flow is diminished as it tries to pass through a dirty surface. Jaws should be cleaned periodically to prevent this resistive buildup and to maintain weld quality.

Uneven Clamping Pressure

Another source of resistance to current flow is poor or uneven clamping pressure. Current flows most readily through areas where clamping pressure is the greatest and flows the least where clamping pressure is diminished. Therefore, if clamping pressure is uneven, the weld will be hot at one point and cold at another, often causing sinks at the tooth edge or at the back edge of the weld. It is important to check periodically that clamping pressure is flat and even over the entire area of the clamping surfaces. A simple check is to place a piece of carbon paper face down on a white sheet of paper and insert and clamp them in the machine as would be done to clamp a blade. Examining the paper will reveal areas of strongest and weakest clamping. Any differences should be adjusted immediately.

Jaws That Are Not Flat

Clamping surfaces should be flat and parallel within a tolerance no greater than 0.001 inch. An out-of-flat condition greater than this will cause uneven clamping pressure and bad welds. Jaws should never be sanded or filed but should always be surface-ground to ensure the required flatness.

Worn or Pitted Switch Contacts

Switch contacts that are pitted or worn can restrict current flow and cause cold welds. Regular maintenance includes checking all contacts in the welding machine and replacing or refurbishing them as necessary.

Loose Electrical Connections

Loose electrical connections resist the flow of electrical current and must be tightened. Soldering should be performed whenever possible to provide an electrical connection that will not come loose. Crimped connectors on a solid wire are a very common cause of cold welds. Because the wire tends to straighten out in use, these connections commonly work themselves loose over time. A single severely loose or several mildly loose connections single-handedly can cause weak and brittle welds. If the machine is producing welds that break, loose electrical connections may be a contributing factor.

Annealing

After the weld is made, it is in a very brittle state and must be reheated at a low temperature to relieve the internal stresses caused by the welding process. The methodology for annealing blades varies greatly from blade manufacturer to blade manufacturer. Some blade makers recommend annealing the band once, letting it cool, and annealing it again, while others believe annealing twice is unnecessary. Some recommend that the operator anneal the blade very hot in the beginning and slowly taper it to nothing. Still others suggest bringing the temperature up slowly, holding it for a designated period of time, and slowly bringing it back down. This author has tried to compile a typical methodology for annealing from all of the companies visited through the years. The best advice for annealing most of the band saw blades in the marketplace today is to bring the anneal heat up to the desired temperature or redness, hold it at that temperature for a certain length of time (usually around 2-5 seconds), then shut the heat off and let the blade cool naturally. The actual temperature at which most annealing takes place varies from blade to blade, but the average is from approximately 1,050 to 1,275 degrees Fahrenheit. If a welding machine is not equipped with an optical pyrometer and a temperature readout, the operator can bring the temperature up slowly until redness is just visible and then begin to taper the temperature down until it is no longer visible. The manufacturers of the band saw blades can provide recommended annealing temperatures and practices for their blades. Some of the newer blades have a high degree of silicon and may require hotter temperatures during annealing. While the annealing is taking place, the operator should observe how the anneal pattern travels across the blade. The heat given off during annealing causes the band to change color for a small distance on each side of the weld. The width of the heat-affected zone generally is just smaller than the width of the jaws during annealing, and it should be parallel to the weld area from the tooth edge to the back edge. If it is not parallel to the weld on both sides of the weld, there is a current flow problem that must be corrected. Uneven clamping could be causing this problem, and immediate action should be taken to correct it.

Conclusion

Thanks to equipment improvements through the years, flash-butt welding today is an international business. While there are different methods of welding band saw blade material, such as overlap brazing, GMAW, and electron beam welding, end-to-end coil joining still is accomplished most efficiently by the flash-butt method.

As with any piece of mechanical equipment, flash welding machines require ongoing maintenance and attention to keep them in peak welding condition. Armed with an understanding of the function of a flash welding machine and an in-depth, ongoing maintenance program, these machines can become valuable welding tools.