I have the Starrett tension gauge and it checks out closely to the "long" gauge described by John TenEyck. I only used it when setting up a new size of blade on my 18" saw to see where the built in tension needle points, ignoring the factory numbers. This lets me repeat the tension quickly. With several wood and metal-cutting saws here I consider the cost of the Starrett worth it.
I mentioned this before, but years ago I had to replace parts I ruined on my 14" made-in-the-USA Delta before I knew any better. I tried to run it with a 3/4" blade as advertised in the specs. I now consider that capacity marketing hype or an outright lie. Yes, I did get it tensioned enough to resaw 12" and process green log sections into turning blanks, even with the under-powered motor. However, it destroyed the tensioning bracket by slowly bending it until I could no longer tension anything properly.
I replaced the bracket with a stronger one from Iturra and installed their stronger spring. After that I limited the blade size to 1/2". By using the Starrett gauge, I found the 1/2" blade needed to be set to the 3/4" mark on the worthless tension indicator on the back of the saw. All was well after that.
Here is John T's photo and text from a while ago:
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Posted by John TenEyck, SMC 1/1/17
https://sawmillcreek.org/showthread....33#post2640833
(Measuring tension with digital calipers)
....it's cheap and pretty easy to measure blade tension. Flutter, schmutter - measure the tension and then you'll know. All you need is a set of 6" vernier calipers and two little c-clamps. A calculator and a little math get you an answer in a few minutes. Once you measure blade tension you'll learn where to set the tension scale for any particular blade you use and the max. blade width your saw can run (and likely find it's not as wide as the manufacturer claimed).
In a related issue, if you are using a 14" cast iron Delta or clone, those saws cannot put much tension on any blade wider than about 1/4". They will still cut OK but not anywhere as good as a larger saw would with the same blade. For example, my 14" Delta can comfortably manage only about 12K psi on a 1/2" blade. It cuts OK as long as you keep the feed rate slow enough not to belly the blade. Take that same blade and put it on my larger saw that can easily put 25K psi on it and it will cut straighter, several times faster, and last a lot longer...
bandsaw_tensioning_TenEyck.jpg
...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.
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