Electrical resistance and bandsaw blade tension

[Ed62]

Someone made a post on how to properly tension a bandsaw blade, with use of an ohm meter. I don't remember who it was, or which thread it was in. But I can't find it, even with a search. Does this work effectively? Does anyone know where to find the post?
Edit: Can anybody help?

Ed

[tkarlmann]

Bandsaw Tensioning

Don't know about an ohmmeter, but here is a method that only requires: 1) a 6" x 3/4" x 1/2" hardwood stick sawn into 2 parts -- one 5" long and one 1" long, 2) 2 nails, and 3) a feeler gauge set. Oh, and 4) some smarts:

http://www.finewoodworking.com/subsc...e.aspx?id=5295

Now this link may not work for you if you are not a FWW member, but you can get a 15-day free trial. It's a short video that tells you all you need to know to tension your bandsaw blade quite nicely and inexpensively. Please note that this article appeared in FWW magazine a few years ago. When I read the article, I did not get what was going on -- the video referenced above clarified everything such that you probably don't need the article.

Basically the stick is in two parts, total length is 6". One nail is short and fixed and driven into the long block. On the same face of the other block the longer nail is fixed into the short block and sanded until you get a sliding fit into a hole in the longer block. These 2 nails are separated on the face (end grain) of the blocks. Now you have a 6" block assembly that has a bottom part that can slide -- making it about 1.5" longer. You clamp this block assembly to your bandsaw blade with just enough tension to keep the blade from falling off the saw -- use tiny "C" clamps spaced 5" apart; clamp the assy on the blade so that a .005" feeler gauge blade has nice sliding fit between the short nail and the short block.

Now you crank up the tension until an .008" feeler gauge blade just fits between the 2 blocks under the short nail. Watch to see that you are not cranking too much -- the video notes that not all tension springs are able to get to .008". If you cannot get to .008" -- get a new spring for your saw. DO NOT continue applying tension until your spring coils are crushed together!!! Now back off your tension slightly and your blade is tensioned to 15,000 lbs. You're done! Oh, just don't forget to unclamp your gauge before you saw, lol.

Remember: a .001" of stretch in your blade over a 5" span is about 6,000 lbs of tension.

[Ed62]

Thanks for the reply, Thom. That looks somewhat similar to an older thread about tension on Woodnet. Only they were using a dial.

Ed

[RAFlorida]

Ed, you might pm

Loring about your quest. I think he's an EE and very well could be knowledgeable to that.

[LCHIEN]

i can't imagine how you might easily use a ohm-meter to measure bandsaw tension.
I liked the idea referenced above by Thom or the even simpler one of opening the jaws of a 6" digitl caliper to about 5 inches and clamping them to a untensioned BS blade on the saw 5" apart. Then increase the tension - each .001" of stretch over the 5" = 6000 psi of blade strain, about 2.7 mils is needed for a nominal recomended 15,000 psi. Makes sense to me.

But then I read the threads linked to sawmill creek and associated threads including responses by Mark Duginske and I'm totally confused. Mark for instance didn't seem to think much of that methodology.

I think I'll mosey on out to my shop and see what stretch reading my normal setting gives...

[Ed62]

I must have seen it on another site because nobody remembers it, and I don't find it with a search. I didn't really read the post because something came up, but I did make a mental note of seeing it. Just not a very deep mental note.

Ed

[AlanWS]

Duginske had a good point, but it was a bit obscured by using the wrong terminology. His complaint about strain meters was that they are not sufficiently precise, that is, they don't give readings that distinguish well between similar values. That's important because the entire range of tensions from 0 to 5000 psi would fall in a range of 0.006" on the calipers, if the above cited method were used (and if I remember the numbers correctly). He refers to precision as accuracy, but that really means how close it is to the actual value, not how fine a distinction can be made, and is really the wrong word for what he is measuring. (Though to be fair, he does use it correctly when he says the gauges he used were inaccurate because they gave wildly different results, so they can't be close to the correct value. I believe the gauges were not properly used to get those inaccurate measurements though, since any real measurement must include a reproducibility check.)

He prefers the built in gauges, which are definitely more precise, but not necessarily more accurate. That's because, in contrast to the tension gauges, the gauges built in to the saws (and the separate gauge he prefers) do not measure tension at all. They measure force. Tension is the force per cross sectional area of the blade, so you need to take into account both the width of the blade behind the gullets, and the thickness of the band. Because the "tension" gauges actually measure strain, how much the steel stretches, and the Young's modulus (ratio of stress over strain) of steels used for bands are very similar, the tension gauges actually read out tension, but not very precisely. The force gauges are more precise, but the reading is not proportional to tension.

I've tried these methods, and don't think they are quite as bad as Duginske says: I can get repeatable readings when I use a dial indicator and hardwood blocks, though I was not able to get reproducible readings with my calipers clamped to the blade. It is critical exactly how they are clamped on. And reproducible here only means within about 1000 psi of the same value. Once you have calibrated your saw, its built in gauge is definitely more precise for getting you back to the same tension you had before. It's just not as good at telling you what tension that happens to be.

To cut to the chase, Duginske and others really claim that there's no real benefit to using a tension gauge, and that you will get fine results if you simply use the gauge on your saw. That's because tension is one of the less important parameters on bandsaw setup. I think he is absolutely right about that.

If you look up the source of tension recommendations, they appear to stem from a rather conservative estimate of what the band will stand up to, combined with the assumption that increased tension will always improve the cut. In other words, they are pretty arbitrary.

[tkarlmann]

Bandsaw tension AGAIN.

To cut to the chase, Duginske and others really claim that there's no real benefit to using a tension gauge, and that you will get fine results if you simply use the gauge on your saw. That's because tension is one of the less important parameters on bandsaw setup. I think he is absolutely right about that.

If you look up the source of tension recommendations, they appear to stem from a rather conservative estimate of what the band will stand up to, combined with the assumption that increased tension will always improve the cut. In other words, they are pretty arbitrary.

I must partially disagree with you here. I'm sure Mark knows how to set up a bandsaw! There are certainly other factors involved, but to get proper spring tension from a fixed scale on your saw? I don't think so.

What proper tension is supposed to do is to minimize the teeth from wandering as you make a cut. I'm presuming here, that everyone has their saw set up properly, tracking correctly on the tire, guides behind the gullets, and have taken a stone to the back of the blade, etc, etc.? If not, we need some remedial refresher thinking before we think that making a tension adjustment is going to solve our bandsaw problems!

I think we know that springs will change over time -- it's just a fact. To say that any saw's built-in gauge -- with no actual measurement for any specific saw -- is the most accurate seems conservative and simplistic.

I liked the idea of clamping a calipers to the blade, but then someone said it didn't work -- perhaps to much oomph required to move the caliper? Certainly the dial indicator seems better than using feeler gauges -- I guess the author was going for simple tools anyone would have around the shop.

Let's not get all bollixed up about what Mark Duginski said or may have said. Does anyone but me remember Jim Cummins? He was another FWW editor, now deceased -- but his tape on tips for the small shop lives on -- he also describes using a gauge after talking with some fancy-sounding-bandsaw-blade-making folks about how bandsaw blades are supposed to be tensioned.

I'll agree this topic is not well covered in this industry! Folks who are making bandsaws aren't talking. Bottom Line -- use whatever it takes to get straight, reliable cuts from your own saw.

[LCHIEN]

the guys who sell them (Iturra, etc) say tension gauges are great.
Guys who don't make them (Duginske) say they're not any good nor are they needed.

Oh, who to believe?

I guess if I haven't had a problem really, I don't need one... maybe.
Of course it would be nice to have one made from shop tools and gauges lying around, just to confirm I've got the right tensions.

[headhunter636]

I do something very similar to what tkarlmann does. I put the blade on the wheels and tighten it until the little metal gauge on the back looks about inline with the blade thickness I am using.

Don't know about an ohmmeter, but here is a method that only requires: 1) a 6" x 3/4" x 1/2" hardwood stick sawn into 2 parts -- one 5" long and one 1" long, 2) 2 nails, and 3) a feeler gauge set. Oh, and 4) some smarts:

http://www.finewoodworking.com/subsc...e.aspx?id=5295

Now this link may not work for you if you are not a FWW member, but you can get a 15-day free trial. It's a short video that tells you all you need to know to tension your bandsaw blade quite nicely and inexpensively. Please note that this article appeared in FWW magazine a few years ago. When I read the article, I did not get what was going on -- the video referenced above clarified everything such that you probably don't need the article.

Basically the stick is in two parts, total length is 6". One nail is short and fixed and driven into the long block. On the same face of the other block the longer nail is fixed into the short block and sanded until you get a sliding fit into a hole in the longer block. These 2 nails are separated on the face (end grain) of the blocks. Now you have a 6" block assembly that has a bottom part that can slide -- making it about 1.5" longer. You clamp this block assembly to your bandsaw blade with just enough tension to keep the blade from falling off the saw -- use tiny "C" clamps spaced 5" apart; clamp the assy on the blade so that a .005" feeler gauge blade has nice sliding fit between the short nail and the short block.

Now you crank up the tension until an .008" feeler gauge blade just fits between the 2 blocks under the short nail. Watch to see that you are not cranking too much -- the video notes that not all tension springs are able to get to .008". If you cannot get to .008" -- get a new spring for your saw. DO NOT continue applying tension until your spring coils are crushed together!!! Now back off your tension slightly and your blade is tensioned to 15,000 lbs. You're done! Oh, just don't forget to unclamp your gauge before you saw, lol.

Remember: a .001" of stretch in your blade over a 5" span is about 6,000 lbs of tension.

[LCHIEN]

I do something very similar to what tkarlmann does. I put the blade on the wheels and tighten it until the little metal gauge on the back looks about inline with the blade thickness I am using.

using the manufacturer's scale and pointer on the back is what guru Duginske recommends but esteemed late bandsaw guru Cummins condemned many years ago. Cummins said the gauges he bought all said the scale on the back of commercial saws was way off.

Now Duginske claims the commercial gauges were all wrong and the scales are right and cummins (who can't defend himself) is all wrong, and you can't believe everything you read on the internet.

[cgallery]

I think Duginske's problem w/ the gauges was that the results were not repeatable.

I still think my method of determining the frequency of a plucked blade has merit. One of these days I need to actually compare it to a saw w/ a DRO.

[cgallery]

using the manufacturer's scale and pointer on the back is what guru Duginske recommends but esteemed late bandsaw guru Cummins condemned many years ago. Cummins said the gauges he bought all said the scale on the back of commercial saws was way off.

Now Duginske claims the commercial gauges were all wrong and the scales are right.

The Duginske tests were actually performed by a degreed engineer (I think w/ a masters, I think he is a materials engineer). He was comparing results to the Carter DRO system.

The way it seemed to boil down was that the clamp-on gauges were not repeatable. The built-in gauges were at least repeatable.

[tkarlmann]

I think Duginske's problem w/ the gauges was that the results were not repeatable.

I still think my method of determining the frequency of a plucked blade has merit. One of these days I need to actually compare it to a saw w/ a DRO.

Actually, Jim Cummins also described this method in his tape for those of us unwilling to spend $300 for the gauge -- I guess he was also into music.

[just started]

i can't imagine how you might easily use a ohm-meter to measure bandsaw tension.
I liked the idea referenced above by Thom or the even simpler one of opening the jaws of a 6" digitl caliper to about 5 inches and clamping them to a untensioned BS blade on the saw 5" apart. Then increase the tension - each .001" of stretch over the 5" = 6000 psi of blade strain, about 2.7 mils is needed for a nominal recomended 15,000 psi. Makes sense to me.

But then I read the threads linked to sawmill creek and associated threads including responses by Mark Duginske and I'm totally confused. Mark for instance didn't seem to think much of that methodology.

I think I'll mosey on out to my shop and see what stretch reading my normal setting gives...

Won't the amount of stretch differ with different metals at the same psi?