measuring BS tension with a micrometer?

[Toller]

I thought this was a brilliant idea, but it failed completely. Why?

I opened a digital micrometer to 4" and clamped the two ends to a band saw blade. I then tensioned the blade. I figured the micrometer jaws would be pulled apart a few thousanths and I could determine the tension.

Nothing happened. I flipped the tension lever back and fourth, but it stood relentlessly at 4". The ends were clamped securely and the micrometer took very little force to move, so I don't think they were just slipping.

Shouldn't this work, or at least do something?

[Chris Friesen]

Yes, and in fact FWW had an article on a tensiometer based on similar technique (except they used feeler gauges).

Are you sure you were tensioning the blade properly?

Chris

[Toller]

I hope so. I have the indicator set to 3/4". With the lever in one position the blade is floppy; in the other it is taut. That's right isn't it? Hey, I just bought the saw last week; I don't discount the possibility that I don't know what I am doing.

[kkfitzge]

I've seen this method written up before. In principle, it should work, but you're looking for only a few thousandths of an inch of stretch. Might be close to the limit of resolution of your measuring device.

[Frank Boettcher]

I can assure you it will work, at least with a dial indicator and probe.

and the winner is?

[Bruce Barnett]

So you are measuring stretch? What if you apply a deflection?

[John Martin]

If you clamp a micrometer spindle it won't turn. Why would you expect the reading to change?

John Martin

[todd]

I assumed Toller meant to say he was using calipers, rather than a micrometer.

todd

[todd]

I'd be very dubious of any results you would get if you could even get this to work. The amount of error in your measurement is bound to be a large percentage of the measurement, which means your calculated readings would be all over the map if you factored in measurement error.

The really cool way to do this would be with a strain gage. Glue it on, load it up to a predetermined change in resistance, and voila.

todd

[Larry Kraus]

I'm no machinist, but I think you are using either the wrong tool or the wrong name for your tool. A micrometer looks like a C-clamp, with a rigid steel frame. If this is what you are using, you are applying tension to your micrometer frame, which I would hope is much thicker and more stretch resistant than your bandsaw blade. If you manage to somehow clamp the blade tight enough to the frame to get a reading, you will have bent the frame and forced your anvils out of parallel, making the micrometer useless, though I doubt many band saws will generate that much tension. It might be possible to clamp the jaws of a digital caliper to a bandsaw blade, but you are probably better off using a dial indicator, as the professional tension gauges do. I've seen plans for homemade tension gauges in magazines (FWW?) using cheap dial indicators. Maybe someone here can give you a link.

[Toller]

That is correct, a caliper rather than a screw micrometer.

[JeffB]

The article indicates that over a 5" length (open the calipers to 5") the blade should stretch .001" per 6000 psi tension. Therefore a properly tensioned blade should read about .003".

[Toller]

My caliper gives repeatable measurements to .001", so it should be accurate enough to do the job. Maybe it has more resistance than I think.

[CW]

No.

[Enoch Root]

Not to hijack this thread or anything, but I'd like very much to try this with a dial indicator. You don't have a reference for the stretch properties of the various metals for bandsaw blades, do you?

I'm using timberwolf silicon steel blades, but I didn't read anything on this in the Suffolk brochure...

er

[mac davis]

ok.. I guess it's better to ask a dumb question that make a dumb mistake...

Before measuring stretch and deflection, should the blade be at some preset tightness or something? Opps.. sorry, tightness wasn't listed, maybe preset tension?

Maybe I'm just not seeing something here, but if I take my cheap chi-wan-ese saw and loosen the tensioning knob 5 turns would I get the same results from this test as if I'd tightened it 5 turns instead?

Mac

[mac davis]

yes and no, but not right away...

Sorry.. loan business flashback.. *g*

I find that I just don't CARE how correct my blade tension is... sinful by rec standards, I admit, but the truth..

I set it to the gauge and "pluck" it like a guitar string... I've never found out if it's the right pitch, but it comforts me, somehow..

If it cuts straight and sounds ok.. doesn't slide on the tire or bog the motor down, I figure it's right enough..

I loosen it a couple of turns at night and try to tighten it about the same in the morning..

I just can't get myself to care enough about a $12 blade to spend a lot of time getting it ready, and then checking it again the next day..

This sort of leads (ok rambles) to another question... on the machines with levers, does it leave the tension EXACTLY where it was when you loosen it, wait over night and retighten?

Mac

[bridgerfafc]

problem with measuring deflection is variation from blade with and tooth pattern. if you measure stretch the only variation you need to account for is steel type, and that difference is tiny.

[Bruce]

On Mon, 3 Apr 2006 11:29:16 -0600, Toller wrote (in article ):

For my MM 16" saw I used a dial indicator with mounts about 12" apart. The calculations are straight forward (I posted the process w/pictures on the yahoo MM group several years ago). I don't remember exactly, but it was somewhere about 0.003" to tension a 1" blade and my indicator was graduated such that a full revolution is 0.004". You really need a sensitive instrument to tension a blade especially if your mount points are only 6" apart.

-Bruce

[Swingman]

Recently 'inherited' an Iturra "Blade Gage" which works on the same principle.

Made me immediately upgrade the tension spring in my US Delta 14" bandsaw.