Being that I'm old and forgetful, I made a sign to remind me.
Being that I'm old and forgetful, I made a sign to remind me.
Jim Davenport
Reporting from the depths of the Magic Garage
I am not a "detensioner." And, I have never found keeping the blade under tension to be a detriment to the saw. Once my '90s Delta 14" BS is set to saw correctly, nothing is touched until need be. Detensioning causes lots of adjustments to need resetting all over again. Wise or Lazy? You choose!
[/SIGPIC]Necessisity is the Mother of Invention, But If it Ain't Broke don't Fix It !!
When I use my bandsaw regularly, I don't de-tension. When It is going to sit for a while, I will de-tension.
Metalurgy says that as long as the spring is kept within it's elastic region, and not over stretched into it's plastic region, then it really doesn't matter if it stays tensioned or not. So, as long as a manufacturer designs the saw correctly and uses the correct spring for the application (size, material selection, heat treatment), then the spring doesn't care if you leave it under tension or not.
Anyone who has had a tensioning spring failure most likely stretched it beyond it's intended limit, or the manufacturer used the wrong spring for the application because they were cutting manufacturing costs, or because the engineers just didn't care/know better.
As for car tires, older tires would indeed flat-spot if they sat for as little as a week. Newer car tires are made of improved materials and don't have that problem. I have no idea what materials are used for band saw tires across the brands, so this may be a valid reason to de-tension when not in frequent use.
To me, the biggest reason to de-tension (which I don't do) is to avoid stretching the blade. The blade acts as a big spring, so all the metalurgical properties discussed above in relation to the tensioning spring apply here too. It is very easy to stretch thinner blades into their plastic region without knowing it, and once you've crossed that line, you can never go back - permanent damage is done to the blade and it is weaker and longer. This leads the operator to just keep putting it under more and more tension, wondering the whole time why it isn't staying where he set it. The answer is because the blade keeps stretching over the days/weeks/months, and as the blade gets longer, the tension decreases - so the operator cranks it up again until the blade finally breaks. A quality blade is one that is made of the proper material and heat treatment that it is designed to remain in it's elastic region while subjected to the stretching forces capable of being exerted by the saw it is intended for.
I'm in agreement with the guy who said a few posts ago that there are too many adjustments on a bandsaw that need re-adjusting after re-tensioning the blade, so why de-tension in the first place. Then again, maybe it's just because I have a cheap, old, hand-me-down three-wheel saw that should have been scrapped a decade or two ago.
I haven't de tensioned my Delta 14" since 1964. It still works fine,doesn't make flat wheel type noises when I start it up,and it sits for long periods between uses. If I de tensioned it,my wife would be certain to start it up when she uses it(at infrequent intervals),and ruin the blade.
Do I de-tension? Nope.
Never, under any circumstances, consume a laxative and sleeping pill, on the same night
I like to keep thing simple.
For my band saws.
For the dust collection on my planer.
I had not used my delta bandsaw for multiple months, 6 or more, I did ruin the tires leaving it tensioned for that long with a narrow blade
I did take the tires off and reinstalled them inside out, been good for 3 years now
I did not learn to de-tension the blade from this experience...
Carpe Lignum
My 18" low quality bandsaw is 22 years old with the old fashion rubber tires. It's never been de-tensioned (other than blade changes) and may sit idle for weeks since it's used for hobby duty. The tires look fine and the saw runs the same as when new. Re-tensioning while changing blades has not been an issue either.
Jeff
Thank goodness for SMC and wood dough.
I detension but it is a simple lever pull to do so. If I had to spin a knob each time I doubt I would do it.
Joshua, I agree with your elastic and plastic observations pretty much whole heartedly except in practice I have found that springs "sack out" all too often. I used a heavy blade on my Delta BS, it was within the mfgr's spec but as you pointed out their spring choice was not really up to the task and I replaced a few. I used the saw pretty hard.
Having worked in a performance motorcyle shop and having the ability to measure springs I can tell you that they wear out even if they've only been in the elastic region and I'm talking valve trains and suspension. I would imagine that the suspension springs may have taken a few trips into the plastic region but many older bikes were over sprung and still their springs are shot.
It may very well depend upon the quality of the steel in the spring. Too low a carbon content to begin with can be a problem. I know that automobile leaf springs can be too soft because I have sawn them into crossbow bows with carbon steel blades without trouble. If they were 1095,they would soon wear the blades out. I was discussing the spring stuff with a gunsmith some time ago. He remarked that there are quite a few 50 round Thompson machine gun magazines that have been wound up full of ammo for 75 years that still work fine. Many 18th.C. guns still work fine,though their lock springs are always under tension.
Last edited by george wilson; 06-13-2011 at 8:57 AM.
This thread just reminded me that I forgot to detension my bandsaw this weekend. And at the rate I use it, I might not turn it on again in 6 months. Maybe I'll leave it tensioned as an experiment. It'd turn it on a lot more if I didn't have to tension it, as lazy as that sounds, I'd rather just pull a handsaw off a rack, mark a line and cut than tension the bandsaw and screw around with changing blades, etc.
I never detension mine. If I had a 24 hour machine shop, would I have an 8 hour rest period for my bandsaw, where I detensioned it and let it sit?
Springs generally fatigue through cycling, not static load. My guess would be detensioning reduces spring life if anything. I would guess that repeated cycling is probably bad for the blade as well.