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Air-tensioned bandsaw?

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Forum topic by William Shelley posted 05-20-2014 05:23 AM 1265 views 0 times favorited 17 replies Add to Favorites Watch
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William Shelley

150 posts in 933 days


05-20-2014 05:23 AM

Topic tags/keywords: bandsaw question

Hi there,

I’m looking to get some feedback on bandsaws tensioned with an air cylinder and a regulator vs. the traditional spring and screw mechanism on 99% of the bandsaws out there. I’m in the construction phase of a shop-made 20” bandsaw project, inspired by Matthias Wandel but the design is entirely my own. When I got to the point where I needed to come up with a design for the tensioning mechanism, I initially assumed that a screw and spring was the best option, but then I got stuck because I would have to buy a really heavy duty spring that could survive a lot of tension on a 3/4” or 1” blade, but which would be too stiff if I was using a 1/4” or 3/8” blade.

I realized that an air cylinder could apply a LOT of force (a 3-1/2” bore cylinder @ 100psi is about 962lbs), but had the added bonus of providing a cushioning action at the same time to absorb vibration. This would make the mechanism simpler overall, and air cylinders are incredibly cheap on ebay. In fact, even including fittings, valves, the regulator, and other items, I’m fairly certain it would be a price-competitive alternative to the spring and screw tensioning design. I’m trying to stay on a pretty tight budget but at the same time I’d like to be creative and innovative if possible.

I also like the idea of being able to de-tension the blade with the press of a button or the flick of a switch.

If I end up going with an air cylinder, I will probably use a pressure-actuated switch as a safety interlock to prevent the motor from starting if the blade is not tensioned, as well as to shut the motor off if the air supply dips below a certain threshold. I’m using a Square D variable frequency driver to run a 1hp 3-phase motor, which I can DC-injection brake to stop all motion in a matter of seconds, in an emergency like an air line bursting.

However, I’ve read a few articles or forum threads on other sites where a bunch of users are poo-pooing the idea of using an air cylinder to apply tension, saying things like “its a solution looking for a problem”, “whats wrong with just cranking a handle?”, and “not a single person i know has ever de-tensioned their blade when done working”. These kinds of attitudes are making me doubt my idea.

-- Woodworking from an engineer's perspective


17 replies so far

View TheFridge's profile

TheFridge

5765 posts in 950 days


#1 posted 05-20-2014 05:31 AM

Never know till you try it. If there are doubts, leave room to modify it with a spring and such. Never hurts to be inventive.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View William Shelley's profile

William Shelley

150 posts in 933 days


#2 posted 05-20-2014 08:10 AM

Ordered two cylinders just now, new old stock (cheap), 1-1/2” bore, 145 PSI max… $15/ea. Even if they don’t work for this project they will be fun to play with, I am sure. At 100 PSI, each cylinder can apply 235lbs of force and at 145 PSI it goes up to 341lbs. With two of the mounted in tandem to the wheel mounting mechanism, this should easily be able to tension even a 1” blade although I imagine that my saw frame (which is made of wood) will not enjoy that kind of abuse.

-- Woodworking from an engineer's perspective

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TheFridge

5765 posts in 950 days


#3 posted 05-20-2014 12:38 PM

Give it a shot.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View mramseyISU's profile

mramseyISU

419 posts in 1009 days


#4 posted 05-20-2014 01:01 PM

The bandresaw mill my grandpa had used a hydraulic tensioner. It was basicly a bottle jack with a pressure gauge and we used that to keep tension on the blade (1.25” wide). It worked pretty well you might think about that as an option.

-- Trust me I'm an engineer.

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crank49

3981 posts in 2435 days


#5 posted 05-20-2014 02:55 PM

It all sounds perfectly do-able to me.
I have had belt grinders that were tensioned with air cylinders.
The best small regulators I ever found for these types of applications were made by Wabco (Westinghouse Air Brake Company). They make a little regulator with a 270 degree dial that goes from no pressure to full pressure in that 3/4 turn of the knob. You could make your own dial with set points for each size blade right on it.

-- Michael: Hillary has a long list of accomplishments, though most DAs would refer to them as felonies.

View RandyinFlorida's profile

RandyinFlorida

181 posts in 1532 days


#6 posted 05-20-2014 03:15 PM

What mramseISU said. Using compressed air I would think the “cushioning” you speak of would heat up the compressed air and cause an increase in pressure. I think you would be chasing your tail trying to regulate the pressure as it heated up and cooled down. Go hydraulic. If your concerned with “cushioning;” incorporate a compression spring.

-- Randy in Crestview Florida, Wood Rocks!

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TheFridge

5765 posts in 950 days


#7 posted 05-20-2014 03:27 PM

I’d like to hear and see how it works with pneumatic or hydraulic tensioning.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View BilltheDiver's profile

BilltheDiver

250 posts in 2350 days


#8 posted 05-20-2014 03:59 PM

I de-tension my blade after every use!

-- "Measure twice, cut once, count fingers"

View bigblockyeti's profile

bigblockyeti

3668 posts in 1185 days


#9 posted 05-20-2014 04:03 PM

It does sound complicated, but with proper design should work without a problem, Tracking and tension are both controlled pneumatically on most wide belt sanders, the concept is already in place. Connected to an air supply the near zero temperature fluctuation you will see with a well designed system will be controlled with the regulator.

View William Shelley's profile

William Shelley

150 posts in 933 days


#10 posted 05-20-2014 04:21 PM

Thanks for all the feedback. I had thought about hydraulic but that would involve buying additional parts that would add cost, complexity, and maintenance. I already have an air compressor (who doesn’t?), and air is a lot ‘cleaner’ than hydraulic fluid. I do agree that hydraulic would probably provide better performance and smoother adjustment though.

Here is what I ordered: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=321406912488&ssPageName=ADME:L:OC:US:3160

-- Woodworking from an engineer's perspective

View distrbd's profile

distrbd

2227 posts in 1910 days


#11 posted 05-20-2014 06:31 PM

There was a good discussion on this subject on another WW site,most thought it was a good idea but the cost of building it with a pneumatic cylinder was /is going to be way up there but imho ,if used parts can be found ,then may be is not going to be too expensive to build one.
https://www.youtube.com/watch?v=oDCTa16aHh0

-- Ken from Ontario, Canada

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crank49

3981 posts in 2435 days


#12 posted 05-20-2014 06:47 PM

Air cylinders do not heat up due to the pressure or “cushioning” they provide.
Air getting compressed heats up at the compressor, then gets cooled in the system’s tank and piping system. Once the compressed air cools off before it goes to a cylinder it is not going to heat back up unless you do something to increase the pressure to a higher pressure and hold it there. Compression and decompression cycling will cancel each other out in terms of heat gain. In fact, dropping the pressure at the regulator will cool the compressed air substantially. Air flow going from 100 PSI down to 50 PSI through a regulator will have it’s temperature reduced by 50%.

-- Michael: Hillary has a long list of accomplishments, though most DAs would refer to them as felonies.

View SCOTSMAN's profile

SCOTSMAN

5839 posts in 3049 days


#13 posted 05-20-2014 08:30 PM

This is truly a great idea.I would highly recommend it to anyone who could make buy the materials to it work, and also have or able to buy the compressor etc.It if you already have the compressor,it would not be too costly IMHO and the results are very well worth the effort involved,In fact it would as I see it be a very enjoyable project. Have a bash at it your self I am sure I will later with help from my boys. Alistair

-- excuse my typing as I have a form of parkinsons disease

View woodchuckerNJ's profile

woodchuckerNJ

1154 posts in 1098 days


#14 posted 05-20-2014 08:42 PM

It sounds like a great idea.

Just one thing to think about.
When mounting a blade, you need to just get the blade tensioned so you can spin the wheels and get the blade to center.
Then you tension.

How will you accomplish this, where you just get the light tension?

I certainly see merit in this even as a retrofit for a regular band saw.

-- Jeff NJ

View Fishinbo's profile

Fishinbo

11362 posts in 1640 days


#15 posted 05-20-2014 08:55 PM

It’s complicated. Never tried air tension on my band saw.

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