November 6, 2007
Recognizing heat and vibration in sawing applications is easy. The tough part is finding out why they are happening. By becoming familiar with blade speed and feed rates, blade selection, and blade break-in, fabricators will find that the saw blade is productive for a longer period of time.
Heat and vibration are the biggest enemies of any band saw application. The symptoms are easily defined, but it's much more of a challenge to diagnose why heat and vibration occur. Proper diagnosis through observation is essential to optimize blade performance and life. When the machine is in proper working condition, blades will cut smoother, straighter, and faster and, in turn, lower the cost per cut.
Choosing the right blade for a job is vital for efficient cutting. The first step is to analyze the sawing application. A combination of the machine, the machinability of the material, the width of the material, and whether the material is bundled determines which blade you should use. These factors also determine proper band saw speed, or feet per minute (FPM).
Once these key factors are identified, you must determine the proper teeth per inch (TPI) for the material. The rule of thumb for best cutting performance is for at least three teeth to be in the work, and no more than 24. Six to 12 teeth on the cutting surface proves to be the most efficient TPI.
Once the proper blade has been chosen and installed, you need to set the band speed and feed rate. Band speed refers to the rate at which the blade cuts across the face of the material being worked. A faster band speed achieves a higher, more desirable shear plane angle and, therefore, more efficient cutting. The speed is restricted by the machinability of the material and the amount of heat produced by the cutting action. Using a too high band speed on very hard metals produces excessive heat, resulting in jagged cuts and reduced blade life. The material's properties should be the overall determining factor for setting band speed.
Saws typically have a feed control that determines the rate at which the blade cuts through the material. This feed rate is measured in inches per minute (IPM). The optimal feed rate is determined by the TPI on the blade, the band speed, and the chip load (the average amount of material removed by each tooth). Higher feed rates result in increased productivity, but will usually reduce the life of the blade. Finding the lowest cost per cut typically involves a compromise between productivity and blade life.
Looking at chips is a great way to determine if you are using the correct band speed and feed rate. Check their shape and color. The goal is for chips to be curled and warm to the touch, not hot. The ideal shape is similar to a 6 or 9. If the chips have changed color from silver to brown, too much heat is being generated and the speed or feed rate or both should be reduced. Blue chips indicate extreme heat and will result in premature blade failure. If you choose the proper combination of band speed and feed rate, the color of the chips will be very similar to that of the piece being cut.
Gullet capacity is another factor that affects cutting efficiency. A blade with the proper clearance for the cut allows the chip to curl up uniformly and fall away from the gullet. If too much material is scraped away, the chip will jam into the gullet area, causing more resistance and heat. A deep gullet design allows for increased cutting rates.
Before installing a blade, you must remove it properly from the manufacturer's packaging. Band saw blades typically are folded into a box for shipping. For best results, slowly unfold the blade, and never drop it on the floor to open it. Hitting a hard surface can cause stress fractures and tooth breakage before the blade is even used.
Now that the blade is installed and ready to cut, you may have to run it slowly at first for proper break-in. Starting full speed into the first cut can quickly lead to premature tooth tip breakage. A new band saw blade has razor-sharp tooth tips that are more susceptible to microscopic damage without proper honing. Blade break-in enables the blade to better take on the rigorous strain of cutting.
Manufacturers recommend running the machine at half the normal feed rate during break-in, while maintaining a consistent band saw speed. Gradually increase the feed rate and force over several cuts until the normal cutting rate is reached. This slightly rounds the edge to reduce the chance of breaking. Completing a proper break-in on a new band saw blade dramatically increases the blade life. A rule of thumb to follow is some break-in is better than none. The tooth tips of coated blades are honed and do not need traditional break-in, making them an exception to the rule.
Scheduled maintenance of sawing machines always has been necessary for correct and efficient cutting, but for today's superalloys it is more important than ever. Beyond following the manufacturer's maintenance instructions, attending to the items listed next will help ensure long life and efficient operation.
Many factors can contribute to premature band saw blade failure. To maximize blade life, use the correct blade (tooth form and TPI) for the application. Determine the correct sawing parameters (band speed and feed rate), and regularly maintain the band saw machine itself.. Each job presents its own set of unique circumstances, but if you take the time to look at all of these variables, you will find an economical and practical solution more quickly.
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