Turning Archive 2005

Bandsaw blades and tension

Russ Fairfield
>I don't have much access to the Internet while we are traveling around the country, but I couldn't let this topic get past me without saying something about it.

I have always heard that not relieving the tension spring causes flat spots on the rubber tires. But, like the flat spots on a car tire from sitting overnight, they will be round again after a few minutes of running.

The real "downside" from not releiving the tension is that the steel shaft for the top wheel will deform the sliding pot-metal block assembly that it is attached to. This is a greater problem for woodturners because they usually crank in, and leave, more tension in the blade than other woodworkers. This was a problem with the first Rockwell saw of 50 years ago, and one that has never been corrected by Delta or any of the other saws that have copied the design since then.

As this metal creep progresses, it becomes necessary to keep advancing the tilt screw to keep the band running on the top wheel. Do this enough times and the pivot arm on the tilt machanism will jamb up against the back of the top wheel cover. When that happens, the top wheel becomes fixed and it cannot move to allow the tension spring to compensate for the the change in blade length as it gets hot from sawing the wood. The result is a detensioned blade that wants to wander, bow, and do other strange things in the cut.

There are a couple cures. One is to cut away some of the metal in the back of the top wheel guard to allow the wheel to move. Another is to replace the sliding top wheel block and tension assembly after it has been damaged. As a last resort, we can relieve the tension when we aren't using the saw so the problem never happens.

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