Bandsaw Tensioning with a Digital Caliper

[Dal300]

In another thread I was asked about my method of tensioning a bandsaw blade with digital caliper. I was also asked for pictures on my method so here they are. They are mostly self explanatory.

To view the actual webpage, go here: http://busconversionstuff.com/galler...s.php?album=12

[cork58]

Wow, just tried it and it works! Just saved myself $200.00!

Thanks!

[toolguy1000]

is that .001 = 6k psi the same across all blade widths (1/8", 1/4" 3/8" 1/2", etc) and blade thicknesses?

[Dal300]

yes, the formula is pretty much universal. It doesn't matter if it's one micron or one mile, for it to stretch .001" it will have 6000 pounds pressure per square inch of steel.

[LCHIEN]

is that .001 = 6k psi the same across all blade widths (1/8", 1/4" 3/8" 1/2", etc) and blade thicknesses?

Think about it this way:
It takes 6000 psi to stretch a given length .001"
Thats some tension force F divided by the cross section area of the blade A, or F/A = 6000 psi.

If you make the blade twice as thick that makes the cross section 2A. Since its thicker you have to pull twice as hard, 2F, to stretch it the same length. So the load 2F/2A = F/A = 6000 PSI.

Similarly you can double the width from 1/4 to 1/2; that will also double the area but again make it harder to stretch causing the force to increase by the same factor to maintain 6000 PSI. The tension F will have to double to get the same stretch, but you'll still have the recommended 6000 PSI.

[Skaning]

I think I need to challenge this a bit before it goes too far

The idea of using a digital caliper to measure the stretch between the teeth is outstanding BUT one cannot simply use .001 in deflection = 6000psi regardless of the length between the initial measurement. The cross sectional area of the blade does indeed drop out of the equation so it works for any thickness or width of blade. The Young's modulus for all steels is pretty much the same whether hardened or not so that is not a variable. The initial length between the teeth you are measuring does not however drop out of consideration.

If you start out with 1 inch between the teeth you will stretch the blade to one value to get the desired stress. If you measure between teeth that are three inches apart then the deflection you want will be three times more than for one inch.

[Cochese]

Am I the only one who doesn't understand the process here?

[pelligrini]

If you start out with 1 inch between the teeth you will stretch the blade to one value to get the desired stress. If you measure between teeth that are three inches apart then the deflection you want will be three times more than for one inch.

I believe the .001=6000 PSI is based on an initial 5" length. If one were to use a shorter inital distance on the calipers a change of .001 would be more than 6000 PSI.

*edit* corrected inital length to 5"

[jeepman71]

Skaning, I believe the idea is to measure the strech of the blade not the increase in distance between the teeth of the blade. Notice the caliper is clamped to the blade but not in line with the teeth. The distance does not appear to matter.

Quote from thread: I spread the caliper as far apart as will fit in the throat of the saw.


Jim

[tommyt654]

I'm goin to comment on this as well and don't think by any means that I have all the answers ,but it occurs to me that regardless of the length you will be measuring it has to be perfectly straight in order to get the correct and accurate reading this way. I think thats gonna be hard to do on an untensioned blade in the beginning. How will you know unless you put it up against a straight edge maybe?

[leehljp]

As mentioned, it is not about the stretch between the teeth, but the tension. This thread is about tensioning and how to achieve or measure reasonably reliable tension per blade. Instead of using a tensioning meter that costs hundreds of dollars, use calipers which do the same thing. When tightening the blade on then BS, the Calipers can measure the "stretch" over a given "span" (5 inches, 4 inches, etc) which can be used to calculate the tension.

While blade "deflection" (pushing sideways on the blade) is often used subjectively for measuring tension, the "deflection" is not the issue here, but tension stretch is.

I started all of this on another thread when I mentioned that I could not get my bandsaw blade to the right tension by the method recommended by the manufacturer. Grizzly basically suggested using the flutter method and then adding tension until it quits fluttering. My bandsaw blades never flutter even down to almost no tension, so I don't have any "starting point" from which to measure.

The gauge on the back of the bandsaw is an approximation, and from experience, many users know that the tension gauge is not consistent enough to be reliable in resaw situations and in some very hardwood situations. So, the search for more reliable measurements have brought us to this.

And I give my appreciation to Loring and others who also posted on this several years ago, and to DAL now for adding to it. I missed it the first time around.

[jeepman71]

I can accept this as an relatively accurate method of tensioning a blade. The question would be what is the required tension for say a Timber Wolf blade verses other materials?

Jim

[LCHIEN]

The measure is between a specific span, e.g. 5" so the stretch is in inches per inch which is in this case .001" per inch for 6000 PSI.

The idea is to get recommended tension is to be 18,000 psi so you go for .003" stretch over the 5".

That is for steel bandsaw blades... for most alloys of steel, the primary element will be steel with small (5 or 10%) additions of other materials so the value will not change significantly for blades between different manufacturers, such as Timberwolf/Suffolk.

You do have to realize that the resolution of most of these calipers is .0005" (half a thousandth) so that in measuring .003" you may have an least an error of 1 part in six or more so when someone states this is accurate, we're only talking about +/-20% or so...

While that is probably not what most people would call accurate and repeatable, it is probably accurate and repeatable enough for tensioning a band saw blade. Moreso than the marked gauge on the back of the bandsaw. That gauge on the bandsaw depends upon the blade being exactly a certain length. Since blades are cut to length and welded together in would be hard to imagine an accuracy of better than say 1/2" in length (that's 0.5% for a 100 inch BS blade) - then the scale indicator on the back of the bandsaw would have a 1/4" of uncertainty around it.

Someone else asked how do you get it straight at zero tension when it may have a bow in it. well, the answer is that you actually call it zero when you have just enough tension to make it straight...maybe 10% is OK since the total accuracy as i pointed out can't be much better than +/-20%. Just don't let it be so loose as to flop when calling it zero.

[wardprobst]

In another life, I ran a bandsaw 8-10 hours a day. I learned to tension the blades by plucking them and listening as I tightened them. It's another way to get the job done. I was working on a machine much larger than the MM20 in our current shop and the blades were about $350-$400 bucks 30 years ago. The thread just brought back some memories because we had tension devices and such but couldn't stop a run too often for very long. Tension by ear made it possible to keep the production going. I still do it that way today. Pitch varies and some blades seem to prefer lower tension than others. Just another view.
DP

[JoeyGee]

Am I the only one who doesn't understand the process here?

No, you're not. I'm lost, too, and getting more lost the more everyone gets into the explanation .