Bandsaw Tension?

[crokett]

I was tuning up my bandsaw this weekend and decided to test it on a project. Simple freehand radius cuts in SYP. My jigsaw could make the cuts easily. The bandsaw has a 1/4" wide blade on it, not sure of the tooth count. When I tried to cut the radius the blade would twist instead of turning to follow the line. The cuts were clean, no burning, tearout, etc and looked very smooth so I am not sure it is a dull blade. I had the guides adjusted down to just above the thickness of the wood. I am wondering if I don't have the blade tight enough. Edited to add that the feed rate didn't seem to make a difference.

This article:
http://www.finewoodworking.com/pages/w00013.asp

says more or less on most smaller bandsaws and no tension meter to compress the spring all the way. I don't have a meter. I know I could compress mine a lot more. It is a Delta, can't recall the model # off-hand.

 

[ehpoole]

One trick to tell if you are at least in the ballpark of an appropriate tension is to 'pluck' (think guitar) the band (do so at the rear column so that the blade guides don't interfere).

If you get a clean tone (albeit very low, depending on the length of your blade) then you are close. If you get a higher frequency tone then you have likely overtensioned your blade. If the tone is very muddy then you don't have nearly enough tension present. Just be aware that bandsaws have very long blades so the tones tend to be pretty deep so you will want a quiet shop if you go this route. For beginners, get to a point where you just get a reliably clear tone then add a bit more for good measure. From there let cut quality determine whether you need to dial the tension up a few more notches, or not.

The other time-tested trick is to check for the amount of blade deflection when a moderate lateral force is applied (not at all unlike testing for belt tension). Raise your blade guides to full height and look for 1/4" or so of give.

Just beware that all of these are acquired through practice and experience and none of them is a hard and fast rule. Ultimately your cuts will suffer if there is inadequate tension (e.g. barrel or wandering cuts) and if the tension is set much too high then you will suffer more blade breakage (from metal fatigue, esp. on bandsaws <18") than is expected. From this you will learn to dial in the proper tension for your saw.

FWIW, when scrolling it is not uncommon for the blade to twist a bit due to the forces being applied to it (especially if the blade guides are set too loosely or too far back -- position them just behind the tooth gullets and with just a paper's gap between them and the blade -- some like to fold a dollar bill around the blade to set the gap).

If you use an aftermarket blade guide, like the Carter Stabilizer (which is intended for blades between 1/8" and 1/4"), they are acually designed to provide the blade minimal support (mostly just rear support with minimal side stablization) so as to intentionally allow the blade to twist a bit in tight curves, which allows for tighter turn radiuses than would otherwise be possible with a rigidly guided blade.

HTH

 

[eyekode]

All good stuff. I have heard of one other way as well which is to watch for blade flutter. The idea being the blade will run true under ideal tension. Here are the details: http://www.suffolkmachinery.com/six_rules.asp
Salem

 

[crokett]

Thanks for the suggestions. I am a little leery of backing off the tension while the saw is running though. I will take another look at it this weekend when the weather warms up a bit.

 

[crokett]

Well after checking everything else. I replaced the blade and the bandsaw cuts beautifully now. Any errors in the cutting are now the fault of the operator. :wsmile: So it was a dull blade.