Metal Fatigue, Resharpening, and Bandsaw Blade Life
Question (WOODWEB Member) :
From Contributor P:
About the fatigue: I think the breakage is caused by to hard work before the sharpening procedure, (it must not be caused by many sharpenings). If the sharpening is performed correct it will remove all micro cracks in the bottom of the gullet. That means the blade is nearly new (from fatigue point of view) after every sharpening. I have slowed down the feed speed in my grinder to about 12 teeth/min. That way I have better surface finish, lower temp and a sharper blade. I could not believe that the steel in a blade would need rest to recover from fatigue. I asked a friend, stress and fatigue "gury" in the aircraft industry. He confirmed that a blade will last a lot longer if it is used (running and tensioned) about one to two hours at the time, and then get rest, hanging free on a hook some hours. In my case I like to re-sharpen with the same period of time.
Blade tracking: Assume the blade is running in idle, with the middle of the body on the top of the crown. ("Middle" is 50% from the bottom of the gullet to the back). The blade tension is now equally spread in the steel in the back and in the front gullet bottoms. (The tension is about 10 % of the force as would break a new blade, it prolongs the blade about 1mm/meter). If you now start to cut the blade it moves back 1/8" and will be pushed by the guide roller flanges. More of the stress from blade tension will now be distributed to the steel in the gullets bottom. It is not good from a stress/fatigue point. It is good because the blade will cut straighter. If you let the blade run so far as 1/4" behind the idle position, you will, in my opinion, shorten the blade life. It makes it much harder for the blade.
Blade thickness and bandwheel diameter are also factors. Thicker blades on smaller wheels will fatigue more quickly. I've had good luck using .042 thick blades on 19" bandwheels.
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