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Industrial bandsaw blade tensioning (pneumatics!)

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Forum topic by William Shelley posted 01-18-2017 04:37 AM 900 views 0 times favorited 8 replies Add to Favorites Watch
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William Shelley

479 posts in 1309 days


01-18-2017 04:37 AM

Hi all,

I’m finally getting back to my shopmade high-tech 19” bandsaw project. Specifically, I’m finalizing the pneumatic tensioning system and redesigning it a little.

My original design used two 40mm bore cylinders to apply upward force to the upper wheel carriage assembly which would tension the blade. At 120 PSI, a 40mm (1-1/2”) cylinder applies about 232 pounds of force. So my theoretical maximum force was about 464 lbs. Based on information I’ve read online, for example Matthias Wandel's excellent writeup about blade tensioning, This would be enough for most blades. My new design would use a single cylinder instead.

I plan on running this saw at exceptionally high speeds, like 6000-8000 SFM which is almost twice the blade speed of most 14” bandsaws. My bearings are fantastically over-spec’d so i’m not worried about trashing them with too much force.

What I’m trying to get a handle on is whether 230lbs would be “enough” or if I need to step up my single cylinder to be able to use 1” and 1-1/4” wide blades.

-- Woodworking from an engineer's perspective


8 replies so far

View AHuxley's profile

AHuxley

652 posts in 3161 days


#1 posted 01-18-2017 05:40 AM

Not even close.

Lets take a 1” carbon blade which needs a minimum of 15,000 psi of strain for optimum cutting:

The distance from the gullet (say a 3tpi blade) will be about 7/8” and the average thickness will be about .035 so the cross section is .0306” multiplied by 15,000 give you ~460 pounds, double that since blade load is half of the spring/wheel load so you need 920 pounds of spring pressure.

A bi-metal or carbide blade will need twice the strain (30K psi) so twice the spring pressure.

Going down to a thin spring steel resaw blade like the Kerfmaster/Blade runner/Woodslicer in 3/4” x .022” to get 15K psi you only need 206 pounds of band load (412 pounds of spring load).

Are you planning to use a load cell to monitor pressure or simply monitoring pressure in the cylinder?

I don’t know what type of frame you are using but to give an example a 1,000 pound Minimax MM24 is just about to cry no mas when you fully tension a 1 1/4” .035” carbide blade like a Trimaster.

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William Shelley

479 posts in 1309 days


#2 posted 01-18-2017 06:22 AM

Thanks for the info!

I plan on using the cylinder pressure as a measure of force. I think even a super cheap and basic analog pressure gauge is going to be accurate to within 1-2 PSI at least.

The frame, I think, should handle the stress. There’s some photos available on the blog post I did several years ago:

http://lumberjocks.com/cathode/blog/43306

Based on your information, I’ll probably jump up to a 3-1/4” piston that is rated for 250 PSI, which will give me about 1800-1900lbs of usable force.

-- Woodworking from an engineer's perspective

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bigblockyeti

4698 posts in 1560 days


#3 posted 01-18-2017 02:38 PM

Another thing to consider is what blades you’ll be using, pick the largest and reference the manufacturer’s recommendations for tension while giving yourself a little extra on the top end.

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AHuxley

652 posts in 3161 days


#4 posted 01-18-2017 04:33 PM

I actually found your blog posts after I posted last night. Not sure how you plan to test the frame be careful at those pressures.

If you are planning to use the same motor listed and at the tooling speeds you mentioned here (6000-8000 sfpm) it seems overkill to use a blade that wide. With that low speed motor you are going to be running the wheel speed at or over the motor speed and with only 1hp I don’t think your feed rate will be near high enough to take advantage of the high beam strength of the wider/thicker blades. My point being without more like 5 HP (at least 3) the wider blades are of no real use and run a much higher risk of turning the frame into a pile of splinters.

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William Shelley

479 posts in 1309 days


#5 posted 01-18-2017 04:43 PM

5HP to 7.5HP was something I had in mind from the beginning. I went with a 1HP motor for the initial build because I already have a pile (literally, a pile) of 3-phase VFDs rated for 1HP that are designed for single-phase input. I wanted to test out things like varying the motor speed and soft-start/ramp-up, as well as electronic braking. I plan on buying a larger motor once the saw is complete enough to at least make cuts.

How would you suggest I test the frame itself? I have a dial indicator, I suppose I could put one of my pneumatic cylinders as far away from the spine as possible and just apply force between the bottom mounting area and the top mounting area. Measuring the deflection should give me an idea of how strong the frame is, I guess?

-- Woodworking from an engineer's perspective

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AHuxley

652 posts in 3161 days


#6 posted 01-18-2017 05:09 PM


How would you suggest I test the frame itself? I have a dial indicator, I suppose I could put one of my pneumatic cylinders as far away from the spine as possible and just apply force between the bottom mounting area and the top mounting area. Measuring the deflection should give me an idea of how strong the frame is, I guess?

- William Shelley

I honestly have no idea how to test a wood frame. While you can measure the deflection with a push/pull gauge to get an idea how it stacks up against other saws I don’t know how to determine where the point of failure will be without testing to destruction, something you would not want to do. My point was be careful when loading the frame with that much stress as I imagine the failure mode would be catastrophic.

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AHuxley

652 posts in 3161 days


#7 posted 01-18-2017 06:55 PM

One thing I forgot to add is will wheel speeds in the 1200+ rpm range you will need to use glue on tires, the centrifugal force will likely sling tires that aren’t glued on. 1200-1300 rpm is about as fast as you see on vertical bandsaw wheels.

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William Shelley

479 posts in 1309 days


#8 posted 01-18-2017 09:28 PM

The wheels I’ve built are constructed from hard maple. Do you think I even need tires? I was considering just using a thinned epoxy treatment on the wheel edges and calling it good.

If I busted my frame, I could build another one without too much issue (and probably do a much better job!).

I plan on using a limit switch wired to control e-braking on the VFD, and placed such that if the air cylinder extends to it’s full amount, it would trip the switch. The only way the cylinder would fully extend is if the blade (or frame) broke and removed all the tension from the cylinder.

-- Woodworking from an engineer's perspective

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