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Bandsaw Tension Gage Sketchup Model

7K views 10 replies 5 participants last post by  ragstian 
#1 ·
I've been messin' around with a model of a bandsaw blade tension gage based on the one on Woodgears.ca. While I was at it I decided to try my hand at Scenes for the first time.

[/url] tension gage screen shot by pat w1, on Flickr[/IMG]

For those that don't already know the scene tabs are across the top of the edit window.

The gage should produce a .001" reading on the dial indicator for each 2000 psi of tension on the 4" captured section of the blade.
 

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#2 ·
Add the dimension scenes for the clips and the hinge strap.
 

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#3 ·

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#5 · (Edited)
That's why I gave the woodgears.ca link in the first post. He has an excellent write up.

In short it's a gage for tensioning a band saw blade. It's based on the elasticity of the steel and the use of Young's Modulus to predict the amount of stretch associated with a given tension. The usual implementation is to attach a gage to one point on the blade and a measurement point at a known distance to the previous point. As the tension is applied the blade will stretch a fixed distance for the length of the blade between the two points. You then use Young's Modulus to convert the measured stretch to tension in PSI. This device amplifies the responce by placing a hinge 1" away from the blade and taking the measurement 3.625" on the other side, increasing the response by a factor of 3.625

... So ...

for each .001" on the dial there will be a stretch of .00027582".

- divide that by the 4" of blade captured between the clips and you get:
.00027582/4 = .00006896551724

- multiply that by Young's modulus for steel - 29,000,000 and you get
.00006896551724*29,000,000 = 2000 ... 2,000 psi for every .001" division on the gage.

If not otherwise specified, a rule of thumb spec for band saw blade tension is 15,000 psi so you would tension the blade till the gage showed an offset of between .007" and .008". Low tension blades like Timberwolf call for a lower spec of between 6000 and 8000 psi (I think) so a gage reading of between .003" and .004" would be needed.

Hope that helps.
 
#7 ·
Yes. The force required to achieve the tension is dependent on the cross sectional area but the elasticity and consequently the amount of stretch remains constant. This is why the tension gage in the back of some saws is so unreliable. They respond to the force applied to the tensioning spring and are calibrated to blades of a particular thickness (one source I saw on the net seemed to think it was .019"). Lets say the saw was calibrated with say ... a 3/8" - .019" blade. If the thickness of the blade installed is greater, then it will require more force than is reflected on the integrated gage scale. A 1/2" - .022 blade when at proper tension may display the correct tension for a 3/4" blade or greater.

My Craftsman 10" saw has no such scale and I've never been very confident in the validity of either the 'push the blade with your finger with "reasonable force" for a 1/4" deflection' or the 'pluck for a ringing tone' methods of setting tension.
 
#8 ·
Oh, forgot to add. I just got home with this:



1" full travel, .001" resolution, with plenty of open space between the ticks. It's not he smoothest action I've ever seen but I am pleasantly surprised for $15 from HF.
 
#10 ·
Shouldn't be too hard to make from scratch in SU. The only question is how the clamps at the top and bottom work. I'd guess wing nuts would be fine. The woodgears.ca link is pretty helpful in sorting out most questions. I've had this one on my list of shop projects for a while. Might not get to the top as the "tap for deflection" method seems to work ok for me. If I was going to do a bunch of resawing or more extensive BS work, it would probably zoom to the top.

And, the total geek in me says it would be cool to use a cheap chinese caliper with a connection to something like an Arduino for a direct readout of the actual tension. Then you wouldn't have to set it exactly at 4" - the Arduino's programming would compensate for the actual starting point. But that would cut into my time for making things in wood...
 
#11 ·
Hi.

Thanks, will give sketchup a try.
Your Arduino idea is a good one.
I have made a python program which calculates the tone pitch you need to get the correct tension, I have a microphone connected to my PC and can measure the frequency when plucking the blade, I then tighten until I get the right frequency. The downside of this method is that you have to get the dimensions of the blade measured right to calculate the "string mass".
The tension meter circumvents this altogether.
 
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