Bandsaw Spring and other Upgrades

[John K Jordan]

For what it is worth I've had the tension spring discussion years ago with Mark and my results do not mirror his. They can be a little funky to use, but I've had as many as four lined up on my big saws, Starrett, Iturra, and Lenox and all read within 1000 @ 25000. An early Iturra had problems and I sent it back. When it returned it read like the others. I'm not sure I'd spend the money for a small frame saw as there are other methods, but when running a 20' trimaster at 25000-30000, the meter gives me quick repeatable way to set up the machine and protect the blade....

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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[John TenEyck]

Mike, it's a G0636X. I posted a review about it when I first got it a couple of years ago, I think, and another Creeker posted his impressions just a week ago. We both have measured tension values over 40K psi on a 1" x 0.035" blade, with lots of space left between the spring coils. 40K psi on a 1" x 0.035" blade would be 24K psi on a 1-3/8" x 0.042" blade. Grizzly rates this saw for a 1-3/8" blade and the data I've measured suggests it could handle one. I have no plans to use anything wider than 1" running at about 25K psi, which is a very manageable load on both the frame and spring.


John

[John Stankus]

Well, I might have stumbled upon another issue that is contributing to my problems. (will have to verify when I can get back out to the shop and move all the puzzle pieces(tools on wheels) to get to the bandsaw. After mucking with the saw some more today, I got is cutting fairly ok. But noticed that I heard a rubbing sound, which I could not locate other than somewhere in the upper wheel region. After I closed up to come in for dinner, I found a thread from 2009 about someone with a 10 year old Jet where the upper sliding hinge assembly was bent and made a rubbing sound. Apparently the original casting design had issues (for reference mine was purchased in 2001 so contemporaneous with the one with a problem) So I will check next time I can get out to the shop and see if it is bent. Now to find a beefier replacement, once I verify that is a problem.

I have been thinking on the tension gauge thing. So in the range of tensions we should be running these blades the stretch is only going to be about 0.002 over 5 inches (or two ticks on the caliper). Did I do my math right? I tried going the other direction (clamped on at five inches) and then detensioned, but didn't see any movement. Has anybody looked into a side deflection measurement of tension like they use for bicycle spokes, structural cables or sailboat rigging?

John

[John TenEyck]

Yep, 0.002" with a 5" gage length will give you 12K psi. If you use a set of Vernier calipers that can read to 0.0005" or, better still, a dial gage good to 0.0001" you will get accurate results.

John

[John K Jordan]

...Apparently the original casting design had issues (for reference mine was purchased in 2001 so contemporaneous with the one with a problem)

..I tried going the other direction (clamped on at five inches) and then detensioned, but didn't see any movement.

I wonder if that bent part you mentioned is the same as what I called a tension bracket in post #15. I have no idea what the actual name is, but the one on my Delta bent from two much tension. The upper wheel on mine was rubbing on something too.

Iturra said this part was weak on both the Delta and the Jet clones. I installed their much stronger replacement bracket and spring and all was and remains well. Someone might call the problem "casting issues" but to me it just looked like low quality material. In post #2 I gave the contact information I have for Iturra.

The tension check measurement can be made in either direction, according to logic and the instructions that came with the Starrett tension gauge. (Their flyer says the same thing.)

But I always attach the gauge before tensioning (but with the blade taut, not floppy.) That way I can keep cranking to set the tension while watching the gauge.

JKJ

[John Stankus]

Took another look at it today.
The axle for the upper wheel is a few degrees (3 or so) out of 90 with the hinge piece. (granted I don't know if this is supposed to perpendicular or not). The key issue I see here is that the hinge can interfere with the upper wheel guard sheet metal making it more difficult to adjust.

The spring definitely is an issue. That is the location of the spring without a blade on the saw...halfway between the 1/4 and 1/2 marks with absolutely no tension. Spring looks to be only 2.5 inches.


Louis Iturra indicated that my saw should have the improved hinge on it (being a 2001 vintage white saw) and that really just needs a better spring. I'll contact him again with the new observations and at least order a new spring.


John

I'm kind of jealous of my brothers Minimax MM20. I might have to see about picking up teaching some summer classes and looking at a S400P (MM16 new name) or a used one.