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JamieOriginally Posted by Jamie Buxton
The vernier method is measuring the "stretch" of a blade when tensioned.
The value read from the vernier is then put into a formula utilizing Young's modulus for steel.2.9 E07 psi.
John Steven s did a really nice post detail on it here on the forum about 10 years ago.
I'm on my iPhone right now in a horse barn, and am working from memory.
I'll try to find the thread when I get to my real computer. Bigger screen😉
Last edited by Mike Cutler; 01-01-2017 at 7:42 PM.
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Several of you have referred to the manufacturer's suggested tension. I cannot find that on either Lenox or Laguna's web sites; they're the most popular carbide blades in small shops. Can any of you point to a spec?
Contributor
Sure. Here's the setup:
Note the blade guides are removed or at least backed away from the blade. Clamp the vernier with about a 5" gage length so that each jaw is on the flat of the blade, between two teeth. On really narrow blades it's best to face the vernier towards the back of the saw in order to avoid the teeth altogether.
The basic calculation is Young's Modulus = Stress/Strain, where:
1) The Young's Modulus of steel is about 30 x 10^6 psi.
2) Stress, the value you are after - the tension in the blade in psi.
3) Strain = Deflection / gage length. The vernier measures deflection, and the gage length is the starting distance between the jaws under zero load.
So, you rewrite the equation as Stress = Young's Modulus x Strain = 30 X 10^6 x deflection/gage length
The gage length I started with was 4.768", so when I measured a deflection of 0.003" that was equal to a blade tension of 30 X 10^ x 0.003/4.768 = 18,876 psi.
My little Delta was more than maxed out with the 1/2" blade I was using. The Iturra spring was nearly fully compressed, the frame was distorted by nearly 0.010" and that pulled the upper guide completely out of alignment. The data showed that I can't really run more than about 12K psi on a 1/2" blade. But with a 1/4" blade I can run at nearly 25K psi w/o over stressing the saw. The 1/2" blade will still cut OK at 12K psi, but I have to run more slowly than I would if I could apply higher tension to avoid blade deflection.
I hope that helps. And I hope it shows you can easily measure blade tension for essentially no cost. I think I paid a couple of dollars for those two little c-clamps.
John
Member
I can recount the numbers but just get 'em on the phone, the Lenox guys are VERY helpful either on the phone or in person at a show. The Laguna guys are less helpful because you generally talk to salesmen and they are either less forthcoming or simply don't know the answer.
To those looking for the cheap but effective approach besides the older threads here there is a good "how to" on woodgears https://woodgears.ca/bandsaw/tension.html
Of all the laws Brandolini's may be the most universally true.
Deep thought for the day:
Your bandsaw weighs more when you leave the spring compressed instead of relieving the tension.
Contributor
From the Lennox website:
LENOX blades are designed to handle up to 30 000PSI. Please consult your machine manual for proper setting on your machine. Over-tensioning blades can cause premature machine failure.
Note they say over tensioning can cause premature MACHINE failure, not pre-mature blade failure. My measurements on my 14" Delta showed how easily that could happen if I insisted on trying to run high tension on a 1/2" blade.
John
Member
You are the named but unindicted co-conspirator in the Duginski camp. I do not mean that in a pejorative manner BTW. I have always wanted to be in the same place as you because I would love to pick your brain beyond what you wrote here years ago and what Mark recounted, here, there and in his book. Beyond your overall thoughts and what exactly your role was I have always been curious what your reply/opinion of the Louis Ittura's response to Mark's conclusions are/were.
Alas the load cell based Carter is out of production, I have a couple of them I picked up for 75% off on clearance. I would love to build one for my saws since they work in real time and allow one to monitor and adjust tension on the fly to compensate for increases in length due to temperature. The Carter version with the automatic power switch to cut off the saw when tension was lost due to a broken blade was very cool.
Of all the laws Brandolini's may be the most universally true.
Deep thought for the day:
Your bandsaw weighs more when you leave the spring compressed instead of relieving the tension.
Contributor
Yes, that's the only comment on tension I could find on the Lenox web site. Notice that it only provides a maximum psi. It does not provide an ideal psi. In fact, Lenox is passing the buck to the saw manufacturer.
Moderator
I actually bent the tension support bracket on my 14" delta by trying to tension blades that were too big. A new bracket and spring from Iturra and I was in business again and a little bit wiser.
JKJ
Contributor
Van, if you are motivated to do that there are plenty of miniature load cells available that could be placed between the spring and frame of the saw to provide a real time readout of load. But unless you are running a production saw I don't see what real benefit it would have. The spring's job is to provide load to the band; over small changes in length its force is pretty constant, dependent upon the spring constant. It would take a lot of expansion of the band for the load to fall off appreciably. Remember, the frame will heat up during use, too, and that will add tension. It might be an interesting exercise to see how the load changes during use, but I doubt it would lead to any meaningful operational changes for the hobbiest user.
For example, I measured the spring constant of the Iturra spring to be about 435 lbs/inch. Assuming no expansion/contraction of the frame, a change in blade length of 0.010" would result in a change in spring force of only 2 lbs., and that changes the tension in the blade by only about 100 psi.
John
Member
A load cell based tension meter is on my to-do list and I agree it is of little real use or value to a hobbyist woodworker but I am as much a bandsaw hobbyist as a woodworker so it is really more about increasing my understanding of band dynamics than producing any objective increase in the quality of my work. To paraphrase Jay Z, I got 99 projects and a cell ain't first. In fact I am half-heartedly looking for a complete early art deco Delta 14" saw since I kinda want one in my collection and it would allow me to use the Carter gauge I have to facilitate gathering data. The math indicates tension would mainly be impacted significantly on a high speed bandmills with very long bands but I am still interested in the real world usage on small saws.
Of all the laws Brandolini's may be the most universally true.
Deep thought for the day:
Your bandsaw weighs more when you leave the spring compressed instead of relieving the tension.
Contributor
Van
You're catching me a little off guard here???
I do not know of the response by Louis Iturra that you refer too???? I don't believe I've ever talked too, or met him. If you have a link, that would be great.
My role was to simply test the after market band saw tension gauges I was supplied, and correlate a comparison to the installed OEM bandsaw tension gauge on the back of the Delta 14" bandsaws and clones.
Marks position was that people were applying too much tension to the band saw blades using after market tension gauges, resulting in broken upper wheel yokes, by trying to achieve the "15,000" psi ideal value, and that the OEM tension indicator was sufficient. At least as far as the 14" Delta style clones are concerned.
Of the 4 mechanical gauges I tested one was broken and I had to repair it prior to testing. This gauge had been in use, as is, by a forum member here.
The Lennox I received was poorly machined and had some serious "stiction" issues that manifested themselves as hysteresis . Once I polished the poorly machined surfaces of the Lennox it worked fine.
The Starrett was the only after market gauge supplied that I did not have to repair prior to use. The two from Iterra had a wear dimple on the force arm created by the steel ball bearing it exerted force on. The curved surface of the bearing had, in fact, become the math of the tension indicator. Whether or not this is how they are made, or whether they were misused,I have no idea.
Each dial indicator was calibrated against known traceable standards, and then reset into the respective gauge. The tension gauges were then affixed to a length of 1/2" band saw blade and place in a test fixture I made. The actual force applied was measured by a traceable strain gauge, and the gauges tested in the increase and decrease force direction to quantify actual calibrated indicator readings and mechanical hysteresis.
Once this was completed. I then performed a cross cal check against the installed OEM Gauge stamped on the tension block on the back of my Jet 14" bandsaw with 6" riser. I tested blades from 1/4"-3/4", and then ran through the cal's again. Believe it or not, the OEM tension indicator is pretty darn close.
I did find that all of the mechanical gauges benefitted from being pre-loaded to ensure they weren't on the stops of the internal dial indicator mechanism when in use.
At the end of it all I found that the actual component that was probably having the most dramatic effect on over tensioning, was the OEM spring. I tested the spring in my bandsaw and found that it was only capable of maintaining it's properties to about 8600psi. I added a carter Cobra Coil and the value went to about 13,600psi.
The point being that in order to achieve the "15,000psi ideal value on my 14" Jet would mean that the spring was completely compressed and the entire tension system was now "solid. All of the force beyond what the spring was capable of maintaining was being transferred to the Yoke assembly, resulting in broken yokes. It would be very easy to lose sight of the spring/yoke, when focusing on the tension gauge. and not realize that everything had gone solid.
From my testing I determined that my Jet 14" bandsaw could only tension up to a 1/2" blade. A 3/4" blade, on my Jet, cannot be tensioned without completely compressing the spring, and transferring the additional forces to the upper yoke assembly.
I also have an 18" Rikon and used it as a test platform also. The results were similar enough.
I think the band saw has always been my favorite wood working machine simply because of the versatility it brings to the table. I'd give up my table saws before I'd give up my band saws.
Out of the box, the band saws I've been exposed to have pretty "rough". But some time spent going through them is well invested. Both of mine were pretty much ripped down to the bearings within days of buying them.
"The first thing you need to know, will likely be the last thing you learn." (Unknown)
So, what tension range are we looking for for each blade size? I don't recall my blades coming with any specific numbers for tension.
Lee Schierer
USNA '71
Go Navy!
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Lee
At the time I did all this, the "magic number" being bandied about was 15,000 psi. The wider the blade, the more force required to achieve this value. As blade cross section increases, so too would the force required.
Personally, I've never been a big believer in the 15,000 psi number. I don't use it, or tension gauges.
"The first thing you need to know, will likely be the last thing you learn." (Unknown)
Member
John ... this was so incredibly helpful. Thank you!
"the mechanic that would perfect his work must first sharpen his tools.” Confucius
Member
If you want to play with gadgets, by all means measure the tension. If you want to cut wood, the the flutter method. I've used the flutter method for 15 years.
I've never hand a blade break or fly off the wheel.
Try it, you might like it.
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