PDA

View Full Version : Band Saw Tension



GKman
02-19-2008, 04:35 AM
A no brainer for all you home shop rocket scientists. I followed CCWKen's lead on artificial ivory to Grizzley's guitar section and happened to see a audible frequency counter (chromatic tuner 27.5 to 4,186 Hz) for $16.95.
Isn't there a simple formula that relates length, modulus of elasticity, weight per unit length and harmonic frequency? ie 20 inches center to center band saw wheels 0.5" X 0.035" steel blade tensioned to 20,000 psi would "twang" at XXXX Hz when plucked with one of those fourth steady rest fingers. :cool:

Your Old Dog
02-19-2008, 05:50 AM
Nah, the rubber tire would mess it up on the vertical band saw. You'd probably never make it low E before it snapped and you'd be seeing birds as well as hearing things on the horizontal:D Let us know what you find out !! You need one that measures in "thunks" and not the higher pitched "tinks" !

rantbot
02-19-2008, 06:34 AM
Well, it's too late to invent the musical saw, as that's old hat -

http://mysite.verizon.net/vzeehv3p/dennisteeth1d.jpg

But the musical power saw might be a field ripe for development.

GKman
02-19-2008, 08:34 AM
[QUOTE=You need one that measures in "thunks" and not the higher pitched "tinks" ![/QUOTE]

What, like the bass riff's in ZZ Tops "Sleeping Bag"?

Jeez! Don't you love this board? You ask for a rocket scientist and get two clowns.

sandman2234
02-19-2008, 02:06 PM
Can we go for 3 clowns?
David from jax

Forrest Addy
02-19-2008, 03:09 PM
HM. Raised an interesting thought in my pointy head. Steel saw bands are elastic and have a modulus of ellasticity of 30,000,000 lb/in^3. So if you were to C clamp two short lenghts of bar stock to the band about 10" apart you could determine their center distance with a vernier caliper with the band relaxed and agian tensioned. You know the band's safe unit tension (30,000 PSI is a good number). 30,000/30,000,000 equals 0.001 per inch elongation for a safe working tension. If you have a 10" gage length and you stretch the band 0.010" as determined by the calipers you have sentioned the band by the numbers to its correct loading. You could build a clamp-on gizmo with a little dial indicator to do this with much less hassle.

Naturally you'll have to adjust the numbers to suit your circumstances but the point it all you need is the elongation and the material modulus. The band material's section doesn't have to be considered.

The plucked musical note is a good indicator of band tension. You could use the elongation trick to initially set the tension and the musical note thereafter. I don't think the tooth pitch will make that much difference unless you go to very fine or very coarse pitches for some reason.

You might gain some notariety for having a finger-plucked bandsaw.

JCHannum
02-19-2008, 03:26 PM
HM.
You could build a clamp-on gizmo with a little dial indicator to do this with much less hassle.

Naturally you'll have to adjust the numbers to suit your circumstances but the point it all you need is the elongation and the material modulus. The band material's section doesn't have to be considered.


Do you mean like this one made & sold by our own Cybor462? I believe it is made of aircraft billet aluminum.

Joel
02-19-2008, 03:31 PM
Or this one:
http://www.toolcenter.com/62126.html

rantbot
02-19-2008, 03:44 PM
The equations of motion for a tight string (from which natural frequencies fall out as part of the solution - actually a set of eigenvalues) are dependent, naturally, on the details of the model. One of the major assumptions of the model of a vibrating string is that a string is limp - it has no bending stiffness. If it has non-negligible bending stiffness, then it's modeled as a rod (which, mathematically, is a beam).

The behavior of a beam is a bit more complex. A major complication is how to model the end conditions. Is one or the other end clamped? (that is, derivative of displacement is zero) Or is one or the other end "simply supported"? (that is, displacement is zero) In practice, a beam has an annoying tendency to be neither. For this reason, resonance experiments - say, of a plate excited with narrow-band (sinusoidal) or wide-band (white noise, most commonly) signals via a loudspeaker voice coil glued somewhere to the plate - don't match theory at all. The best approximation I have personally seen was a plate, bolted down solidly all around its edges to a frame of hefty beams ("hefty" means much beefier than the plate), with a groove milled all around the plate so that it vibrated at basically a "hinge" of thin material around its edge. Then the visible resonance patterns (measured very simply, by photographing the distribution of salt grains scattered over the surface - they migrated from the high-amplitude regions of vibration to the vibratory nodes) fit the theory very well. Adding tension to the plate in one or both axes would not have been difficult. But it wouldn't have had much to do with a bandsaw blade.

So what's a string, and what's a rod? The harmonic overtones of, say, the strings of a piano are not quite the even multiples we might expect (2nd harmonic is a frequency 2x the fundamental, 3rd harmonic is a frequency 3x the fundamental, etc), and the difference is clearly audible to the ear. The reason is that piano strings are not well approximated as "limp" strings. If you want to be accurate enough to know what's going on in piano strings, you have to model the damn music wire as a beam. And then those annoying end conditions will plague you.

In short, it's not as simple as it seems.

Your Old Dog
02-19-2008, 04:45 PM
What, like the bass riff's in ZZ Tops "Sleeping Bag"?

Jeez! Don't you love this board? You ask for a rocket scientist and get two clowns.

Sorry, thought you were kidding. I never thought for a moment that anyone would think you could get a bandsaw blade to make enough sound energy when "plucked" to have it be measured by an audio guitar tuning device unless one was willing to add a sounding board to their bandsaw much like a guitar, violin, bass fiddle, cello, mandolin, ukulele or any other stringed instruments. Let me know how it works for you.

Forrest Addy
02-19-2008, 06:05 PM
Old dog. They work on contact too. You can hold them agaist the vibrating body and pick up enough to analyse. A buddy of mine tunes his unplugged solid body gits that way.

GKman
02-20-2008, 11:03 AM
The equations of motion for a tight string (from which natural frequencies fall out as part of the solution - actually a set of eigenvalues) are dependent, naturally, on the details of the model. One of the major assumptions of the model of a vibrating string is that a string is limp - it has no bending stiffness. If it has non-negligible bending stiffness, then it's modeled as a rod (which, mathematically, is a beam).

The behavior of a beam is a bit more complex. A major complication is how to model the end conditions. Is one or the other end clamped? (that is, derivative of displacement is zero) Or is one or the other end "simply supported"? (that is, displacement is zero) In practice, a beam has an annoying tendency to be neither. For this reason, resonance experiments - say, of a plate excited with narrow-band (sinusoidal) or wide-band (white noise, most commonly) signals via a loudspeaker voice coil glued somewhere to the plate - don't match theory at all. The best approximation I have personally seen was a plate, bolted down solidly all around its edges to a frame of hefty beams ("hefty" means much beefier than the plate), with a groove milled all around the plate so that it vibrated at basically a "hinge" of thin material around its edge. Then the visible resonance patterns (measured very simply, by photographing the distribution of salt grains scattered over the surface - they migrated from the high-amplitude regions of vibration to the vibratory nodes) fit the theory very well. Adding tension to the plate in one or both axes would not have been difficult. But it wouldn't have had much to do with a bandsaw blade.

So what's a string, and what's a rod? The harmonic overtones of, say, the strings of a piano are not quite the even multiples we might expect (2nd harmonic is a frequency 2x the fundamental, 3rd harmonic is a frequency 3x the fundamental, etc), and the difference is clearly audible to the ear. The reason is that piano strings are not well approximated as "limp" strings. If you want to be accurate enough to know what's going on in piano strings, you have to model the damn music wire as a beam. And then those annoying end conditions will plague you.

In short, it's not as simple as it seems.

Houston, the rocket scientist has entered the building.

What a treatise! Thank you.

I replaced the blade with the piano wire like you suggested but the first time I laid into a billet of Extra Virgin 6061 aviation racing aluminum, I was somewhat disappointed with the result. I'll keep trying.

Back to the tension. Seems like I remember hearing that increasing the tension to a certain point actually increases life. Bending around the wheels causes the inside of the wheel to cycle from tension to compression resulting in fatigue. Whereas a blade with adequate tension cycles from tension to higher tension which doesn't cause fatigue. Does that sound right?

J Tiers
02-20-2008, 09:26 PM
The Atlas band saws have a neat way of assuring the tension......

There is a spring setting the tension. What you do is adjust the tension screw until the space between two pieces is 1/4 inch. That sets the correct tension according to the spring rate. Done.

wierdscience
02-20-2008, 11:18 PM
J makes an excellent point,all of the better horizonal saws have either a spring or hydraulic cylinder which makes the job idiot proof.Your not loading the blade,just the tensioning medium which applies a preset amount of tension.A solid adjuster is nearly worthless since the instant the blade heats slightly it expands considerably and the tension goes away.This isn't so visable on a metal working saw,but on a woodworking saw or even better bandsawmill or resaw it's pretty dramatic.

Your Old Dog
02-21-2008, 06:43 AM
thanks Forrest, learn something everyday! Wonder how that technique would work to set the tension on guywires by holding the tuner on the cable? :D It wouldn't matter much as long as all read the same on a calm day. I'm thinking on getting back into ham radio now that I've been forced into retirement. Need something to do that ain't too physical. I can't stand long enough in the shop to get anything done.

mwechtal
02-21-2008, 12:11 PM
thanks Forrest, learn something everyday! Wonder how that technique would work to set the tension on guywires by holding the tuner on the cable? :D It wouldn't matter much as long as all read the same on a calm day. I'm thinking on getting back into ham radio now that I've been forced into retirement. Need something to do that ain't too physical. I can't stand long enough in the shop to get anything done.
Sailors use a gauge for setting tension on the standing rigging (basically guy wires). It has 3 pins, 2 fixed and one between them that is spring loaded. You thread the cable between them. The spring loaded one functions as a spring scale, and you set the tension to something like 75-80% of rated breaking strength for the cable. I don't see why it wouldn't work for tower guys.

Mike

HTRN
02-21-2008, 06:20 PM
The Atlas band saws have a neat way of assuring the tension......

There is a spring setting the tension. What you do is adjust the tension screw until the space between two pieces is 1/4 inch. That sets the correct tension according to the spring rate. Done.

Something similar exists on older big marvel bandsaws - They have spring and wedge on the tensioner. once you reach the required tension, any further tightening of the tensioner rides over the wedge as the tension in the blade is higher than the force exerted by the spring.