Bandsaw Tensioning with a Digital Caliper

[Wood_workur]

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

http://nanopedia.case.edu/image/stre...0curve%204.jpg

That image is a stress strain curve. The linear portion (The "elastic region") on the left for most steels is pretty much the same. By using the calipers to measure how much the blade has stretched when you tighten it (the strain, or elongation), you can then figure how much tension has been put on the blade (the stress)

[Cochese]

I get what we are testing, I don't get the how.

Pics I see (sideways) show a caliper attached to the blade. Beyond that I'm lost.

[Dal300]

Cochise,
We aren't testing, we are measuring the elasticity of the steel, which in effect gives us the strain or tension reading we are looking for.
The caliper is attached to the loose blade. (Loose enough that there is little or no appreciable strain on the blade).
Once the caliper is attached it is zeroed.
Once zeroed, we can proceed to apply tension to the blade via the tension adjusting knob.
Since we know that stretching the blade .001" takes approximately 6000 pounds per square inch of tension, we can assume that the relationship between stretch and tension will be linear, so .003" would equal 18000 pounds per square inch of tension, which puts us right near the ball park of what most manufacturers suggest for blade tensioning.

This is the exact same thing that the digital and dial tensioning gauges from the different manufacturers measure, therefore, there is no real reason to pay high dollars for a gauge that does the same thing as a tool that is already on hand.

Addendum:
As for the 5" distance between the jaws of the caliper, that is only for convenience. It doesn't matter what the distance is because you aren't measuring just the space between the jaws, but the entire blade.

[Cochese]

I couldn't tell what was being measured or how it was being measured. Now I do.

[pelligrini]

Addendum:
As for the 5" distance between the jaws of the caliper, that is only for convenience. It doesn't matter what the distance is because you aren't measuring just the space between the jaws, but the entire blade.

That doesn't sound right. I would think that the initial distance of the measurement would be a factor.

If I started with a 1/2" distance on the calipers and tensioned the blade to increase that measurement .001 would the tension still be 6000 PSI?
Or just for the sake of argument, if my initial distance was able to be .001 and I increased it .001 that steel would be under a lot more tension than 6K.

[pelligrini]

Woodgears has some more info on this type of tensioning.
http://woodgears.ca/bandsaw/tension.html

[Dal300]

You are still only measuring the distance that the jaws move so it doesn't matter where they start.
The total stretch remember isn't just the one section of the blade, but the entire circumference because you aren't using the caliper to stretch the blade, you are using the tensioning bolt which reacts on the whole blade, not just the section between the jaws of the caliper.
Maybe this will help:

http://www.engineeringtoolbox.com/yo...lus-d_773.html

and here:

http://www.engineeringtoolbox.com/yo...lus-d_417.html

That doesn't sound right. I would think that the initial distance of the measurement would be a factor.

If I started with a 1/2" distance on the calipers and tensioned the blade to increase that measurement .001 would the tension still be 6000 PSI?
Or just for the sake of argument, if my initial distance was able to be .001 and I increased it .001 that steel would be under a lot more tension than 6K.

[jdon]

Here's another link that seems to explain the relationships pretty clearly: http://woodgears.ca/bandsaw/tension.html

As lchien alluded to above, the tension is the ratio of increased length (stretch, 0.003", say) divided by the length that is stretched (5", say), and multiplied by 29,000,000 (Young's modulus for steel). A length of 0.5" would stretch 0.0003" under the same tension- not readily measurable, at least with a HF caliper! The assumption, which to my mind is reasonable, is that tension is exerted evenly over the entire blade, so the "sampling" of 5" (or whatever your initial test length is) is representative of the tension exerted on the entire blade.

[pelligrini]

I found another article from an old issue of Fine Woodworking.
http://dc105.4shared.com/doc/VJtINh-4/preview.html

[LCHIEN]

That doesn't sound right. I would think that the initial distance of the measurement would be a factor.

If I started with a 1/2" distance on the calipers and tensioned the blade to increase that measurement .001 would the tension still be 6000 PSI?
Or just for the sake of argument, if my initial distance was able to be .001 and I increased it .001 that steel would be under a lot more tension than 6K.

Actually think of a rubber band.
If you pulled 1 pound on a 5" rubber band section and it stretched 1 inch
then 10" rubber band with the same pull would stretch 2 inches
a 2.5" rubber band would stretch .5"

Assuming that we want 18000 PSI and we know that .001" stretch over 5" = 6000 PSI, we need .003" stretch over 5" to get 18,000.

If we measure over only 2.5" then we'll need 0.0015" stretch. That's so small a part of the caliper's measuring range the accuracy will be in the +/-40% range.

Most optimally we need a longer length so we can measure a longer stretch.
It would be nice to measure stretch over a span of 10" instead of 5", then we would be looking for .006" stretch for more accuracy.
But, most BS won't have a 10" straight length accessible for measuring and most of us don't have a a 10" capacity caliper, 6" being cheapest and most commonly available.

[pelligrini]

I'm sure I have a good grasp of the concept. I just think Dal300's last statement about the distance not mattering might be confusing to some folks if they don't recalculate for the change in the initial length measured. For steel, a delta change of .001" over a 5" initial distance roughly equals 6000 PSI. If one were using different starting measurement length they would need to recalculate the delta change needed since .001 would no longer equal 6000 PSI.

[Dal300]

You could be right. Since I'm not a mathematician or an engineer, I just took for granted that the distance between the two points didn't matter because the caliper, although being mounted to the blade is not interacting with the blade other than to measure the amount of stretch.
I'll have to cogitate on thisun and wrap my little mind around it somehow, maybe with a cold adult beverage or 6, 7, 8....

Just a thought here, not trying to be obstreperous, since we are actually measuring the distance between the two, (or 3), wheels, wouldn't the amount of distance of the base measurement be the same whether we start to measure 5" apart or 5mm apart? The blade still stretches the same amount to achieve the required tension? I'll have to come up with some way to do the measurement using feeler gauges like as Woodgears.com did, just to see what the difference is.
Another thought would be to use a medium other than steel with an elasticity modulus that is more easily measured and then extrapolate the findings.

If worst comes to worst, maybe I'll ask a couple of friends who actually can add 2 numbers together and come up with the same answer at least 3 out of 7 times.

This has turned in to a very interesting thread, lets keep it going until we reach a consensus.

I'm sure I have a good grasp of the concept. I just think Dal300's last statement about the distance not mattering might be confusing to some folks if they don't recalculate for the change in the initial length measured. For steel, a delta change of .001" over a 5" initial distance roughly equals 6000 PSI. If one were using different starting measurement length they would need to recalculate the delta change needed since .001 would no longer equal 6000 PSI.

[LCHIEN]

the 5" distance absolutely matters. You're measuring the stretch between those two points. Not the movement of the wheels or anything else.

For a given tension the stretch is longer for a longer span.

[MykRian]

After reading this entire thread. that says it with the fewest words.

the 5" distance absolutely matters. You're measuring the stretch between those two points. Not the movement of the wheels or anything else.

For a given tension the stretch is longer for a longer span.

[eccentrictinkerer]

If you could attach another identical setup (caliper, clamps, etc.) on the other leg of the sawblade, wouldn't it stretch the same .001"?

Or at any other point on the blade?

Just askin'.