Bandsaws: Why No Max Blade Tension Spec?

[Bill Space]

Hi,

Just curious since ability to tension the blade always comes up with respect to choosing a bandsaw.

E.G.: Can a given saw tension the blade you would like to use?

Why do the manufacturers fail to list such a basic spec? Or do they and am I missing it somehow?

If I were designing a bandsaw I would hope I would include a maximum blade tension spec in the design requirement.

Specifically: Max pull the bandsaw can apply to a blade in pounds or kilograms.

if this number were provided PSI for any blade could be calculated.

Just seems like like this info is missing when the potential purchaser is trying to decide on what bandsaw to purchase.

What is up with this? Do the manufacturers not know? If they do, what do they gain by not sharing?

A simple number showing the maximum tension that can be applied to a blade would be a meaningful number for comparison purposes.

Perhaps this can can be worked backwards from the maximum blade size th manufacturers recommend, but I doubt it, unless the specify the PSI they can apply for that max blade size is specified.

Bill

[Tony Pisano]

I assume, and we know what that can do, that an actual number isn't that important. If a saw is built for up to a 3/4" wide blade, it will have the capacity to tighten the blade enoughfor proper use. If the saw listed a number, we would want it to have an accurate reading of the tension applied. Most have some method of indicating tension for particular width blade to get you in the ballpark, then its a matter of getting a feel for it.

[Geoff Crimmins]

I assume, and we know what that can do, that an actual number isn't that important. If a saw is built for up to a 3/4" wide blade, it will have the capacity to tighten the blade enough for proper use.

Unfortunately, that doesn't always seem to be the case, though it would be nice if it was. The maximum blade-width specification seems to be the maximum width of blade that will physically fit on the saw, not the blade it can actually tension. And carbon steel blades, bimetal blades, and carbide blades have significantly different tension requirements. The old Jet JWBS-18 bandsaw will take up to a 1-1/4" blade, but I expect that the saw would fold itself in half before it would tension a carbide blade of that width. While not everyone would need or want such a blade, it would be nice to be able to look at the saw's specs and know what it was capable of.

--Geoff

[David Kumm]

Unless there is uniform method of measurement specifying the thickness and width of the blade that all must use, we will be faced with more useless info like cfm numbers on dust collectors. A standardized system would be great though. Seems like the only numbers most consumers care about is price. Dave

[Bill Space]

Unless there is uniform method of measurement specifying the thickness and width of the blade that all must use, we will be faced with more useless info like cfm numbers on dust collectors. A standardized system would be great though. Seems like the only numbers most consumers care about is price. Dave

Dave,

If the maximum pounds of pull a bandsaw could put on a blade were provided, this would actually be very useful information, as PSI available for any size blade could be easily calculated.

It it would be a specification similar to wheel size or resaw capability. Not a theoretical number like CFM of a dust collector, which will vary according to the ductwork it is connected to.

Bill

[Steve Demuth]

Dave,

If the maximum pounds of pull a bandsaw could put on a blade were provided, this would actually be very useful information, as PSI available for any size blade could be easily calculated.
Bill

Exactly. This is almost entirely a function of the spring tensioning the upper wheel. It's useful enough to know that I took an old blade and a digital scale and rigged up a measurement of pull for various compressions of the spring. Now I have them, and PSI calculations are trivial - just measure the blade thickness and depth to gullet, and you know the blade PSI tension for any adjustment of spring.

Doesn't help the shopper, but useful enough once you have the machine to be worth the trouble.

[David Kumm]

"There is a difference between the maximum pull a frame can put on a blade and how much frame twist or deflection occurs. I put a heavy spring on an 18" Italian saw and could tension well beyond what the saw could actually operate with. The problem shows up when the blade gets up to speed and little problems become large ones. Frames under high tension can deflect - flutter- as the blade spins, and cause tracking issues that magnify under the force of a cut. I agree that a number at a certain deflection point would help but have little confidence that manufacturers would play fair and the numbers be comparable. The benefit would be something to use when comparing to users real world experience. The point about a number being better than nothing is valid and I'd be happy to see something used. Dave

[Bill Dufour]

As the frame bends the blade is moved in towards the support arm. The guides have to be adjusted to the left and I would think the blades tilt so the rim nearest the table moves back so the wheels care no longer coplaner. At some point the wheels and the guides can no longer control the blade. I would assume this depends on blade thickes and width as well as metal type.

[Bill Space]

Steve said:

"... I took an old blade and a digital scale and rigged up a measurement of pull for various compressions of the spring. Now I have them, and PSI calculations are trivial - just measure the blade thickness and depth to gullet, and you know the blade PSI tension for any adjustment of spring.

Doesn't help the shopper, but useful enough once you have the machine to be worth the trouble."

Steve,

Essentially what you did was measure the spring constant, which is is the force required to compress a spring a certain distance. Your measurement multiplied by two should approximate the spring constant for the spring used in your machine.

You have found this information useful, and I think some shoppers (and users) would also. It is a number that the manufacturers would know, as they have to specify the springs they will use in their bandsaws.

Knowing the spring constant (I believe in the US pounds per inch is used) would help one determine the largest blade that could likely be tensioned on a saw. In other words a 1" blade might fit on a saw, but the spring supplied with that saw might only be strong enough to tension a 1/2" blade. This would be something I would like to know...which should be comparable across machines.

Spring strength does nothing for us with respect to the possible excessive frame bending and twisting as blade tension is increased, which Dave mentioned.

Dave,

I am having trouble seeing why excessive tension would cause problems to be amplified. Perhaps what you experienced was related to some twisting of your saw frame along with some bending? Resulting some kind of harmonic oscillation... In any case you experience illustrates the importance of having the right strength spring for a given frame.

Bill,

Good points about how other things go out of wack when a saw blade is tensioned beyond what the frame can handle. I think the wheels would stay coplaner until the frame began twisting.

Thanks to to all for their input. I have come to the conclusion that if manufacturers published the spring constant and length of the tension springs used in their machines, it would be useful to some shoppers. And it would be something that would be comparable across different brands. This could be provided easily.

A second thing that would be nice to know would be frame deflection for a given amount of spring compression. This would require more effort and I would guess we will never see it.

Still not sure why maximum pounds pull on the blade is not a standard specification listed for bandsaws...but the question brought out a lot of interesting details helpful to me personally!

Bill

[David Kumm]

Bill, I think you are correct about the twist and frame deflection. My guess, and only a guess, is that I tensioned to the point where under speed and due to the additional stress of putting the blade under the cut, the frame moved enough to screw up the tracking of the blade. I just run big cast iron saws now so this was years ago and no way to test anymore. Dave

[Steve Demuth]

Bill,

Nice summary. When I said "not useful to the shopper" I meant that the fact I had to do the measurement with my own instruments after the saw was in my shop, meant I couldn't use it for comparison of different brands and models. I completely agree that if honestly published by manufacturers the maximum force exerted under design deflection would be a great data point when comparing saws.