Remembering to de-tension your bandsaw Blade

[Harvey Pascoe]

Well, I usually de-tension mine but occasionally forget. I figure its no big deal. But guess what? After three years I wore out the threads in the damn tensioner even though I lube them. Screw that, not gonna do that no more.

Chris, I have some education in metallurgy. Castings do not warp all by themselves, if one ever should then it is defective and I've never heard of such a thing. Iron castings should not even warp under heat and stress or they wouldn't make car engines from iron. The modulus of elasticity of both iron and cast or extruded aluminum is sufficiently great to withstand the rather low loads of a tensioned blade.THE AMOUNT OF STRESS IS INSIGNIFICANT. Cast iron has nearly zero elasticity and breaks before it bends. That is why engine blocks crack with even minute heat distortion. As for the tires, just depends on the quality of the material. As for the springs, any long time saw owner knows the answer to that one, but they weaken whether you relieve them or not due to changing load. They solved that problem with car springs long ago so poor quality springs is the problem with saw springs.

[Dan Hintz]

25k psi sounds like a lot of stress to put on something Just sayin'...

[Harvey Pascoe]

So the bottom line is that there are too many variables, particularly quality, right materials to draw any general conclusions?

As usual, do what works best for you . . .

[John Coloccia]

Well, I usually de-tension mine but occasionally forget. I figure its no big deal. But guess what? After three years I wore out the threads in the damn tensioner even though I lube them. Screw that, not gonna do that no more.

Chris, I have some education in metallurgy. Castings do not warp all by themselves, if one ever should then it is defective and I've never heard of such a thing. Iron castings should not even warp under heat and stress or they wouldn't make car engines from iron. The modulus of elasticity of both iron and cast or extruded aluminum is sufficiently great to withstand the rather low loads of a tensioned blade.THE AMOUNT OF STRESS IS INSIGNIFICANT. Cast iron has nearly zero elasticity and breaks before it bends. That is why engine blocks crack with even minute heat distortion. As for the tires, just depends on the quality of the material. As for the springs, any long time saw owner knows the answer to that one, but they weaken whether you relieve them or not due to changing load. They solved that problem with car springs long ago so poor quality springs is the problem with saw springs.

My bandsaw table is cast. Without the alignment pin, I can easily bend the table by quite a bit. That's just an example and I don't want to belabor this point, but stiff and brittle isn't the first thing that comes to mind when I think about cast iron.

[Harvey Pascoe]

My bandsaw table is cast. Without the alignment pin, I can easily bend the table by quite a bit. That's just an example and I don't want to belabor this point, but stiff and brittle isn't the first thing that comes to mind when I think about cast iron.

John, then they must made it of maleable iron because that's not normal. What's the point of using iron if it bends? I don't even bother with the pin since mine doesn't. Of course, they can cast iron with hundreds of different alloys for various properties and perhaps the standards used for our types of machines aren't so great. But I'd suggest that you never drop a thin casting on concrete floor if you don't think its brittle.

[Chris Fournier]

Perhaps warp is a poor choice of words. While I am not a metalurgist I have taken related courses via my welding/machining activities. I don't think that castings are as stable as you state Harvey due to the very nature of the casting process. The molten casting cools at different rates in different areas where thickness varies. A larger casting can even start to cool in places when its being poured still. All of this creates internal stresses which will create movement in a casting, iron, aluminum etc.. Of course good post casting heat treating is designed to deal with this stress but just as poor castings are designed, there is poor heat treating too and we then have castings that "move".

Cast iron certainly isn't super bendy stuff but bend it does when under load. A classic example if the Bridgeport Milling machine with a 9" by 42" horizontal working surface. On older machines this table droops at the unsupported ends just from its own weight.

How does all of this affect us and our bandsaws? I don't really know but I see no harm in de-tensioning while I see the potential harm in not de-tensioning. I have chosen to take the 20 seconds to de-tension my machine at the end of the day.

[Ken Fitzgerald]

A lot of the major brands have the alignment pin to which John is referring. My Italian made bandsaw has one.

That being said, as Harvey points out, I have dropped cast iron parts onto a concrete floor and had them break.

I don't see any harm in detensioning. In my shop I detension. You don't want to detension, in your shop you can do what you want.

I certainly don't think there is a strong enough argument against it to warrant aguing about it.

[Billy Trinh]

I'd rather replace a tire on purpose than a good blade on accident when forgetting to re-tension .

[Marvin McConoughey]

You have a valid point. Cars stress their suspension springs for the life of the car and very few have noticeable spring collapse. I have observed some flat spotting on a few brands of tires, but it goes away after a few minutes of driving. Has any long term research been conducted? Anyone?

[Mel Fulks]

In different shops I've only seen one bandsaw that was de tensioned . That was a Wadkin resaw that the boss said was factory specified. The other bandsaws in the same shop were not de tensioned. So now you have ALL the industry facts!

[Bruce Page]

In different shops I've only seen one bandsaw that was de tensioned . That was a Wadkin resaw that the boss said was factory specified. The other bandsaws in the same shop were not de tensioned. So now you have ALL the industry facts!

In almost 40 years of working in various machine shops, I never saw a bandsaw de-tensioned. That said, I do de-tension my MiniMax and my small Rikon.

[Jim Andrew]

My farm tractors get flat spots on their tires, when I first pull them out and go down the road towards the field, sometimes they bounce for several minutes. And I would never leave a full load on a truck for extended periods.

[Jeff Duncan]

Wow, talk about bringing back an old thread!!!

Might as well ask people what's better chocolate or vanilla, you'll probably get a similar split

JeffD

[Ole Anderson]

I just cut a 4x4 square of 1/4" ply and ran a BS kerf half way through, used a marker to say simply "TENSION" on both sides. I leave it in place as though I just cut the kerf. Impossible to miss.

[Bill McNiel]

Well, now that I have a firm understanding of detensioning I think I'll go back to the Shop and get to work.