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MGREEN
11-04-2006, 10:59 PM
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Hi everyone, Just finished my latest project, a band saw blade tension gauge based on a Starrett gauge. I printed a picture of one and cut it out to use as a model. Mine uses a .001" cheap dial indicator, so the scap piece of aluminum I used to mill body of the gauge needed to be modified to house the indicator. The Starrett gauge uses an indicator that reads directly in tension, and appropriatly so, list price $340.00 US dollars. A friend recently read an article, relating to a woodworking bandsaw about a home made tension gauge made from a couple sticks of wood, steel pins, some clamps, and some feeler gauges. In the article it stated that on a .025" thick blade, a .001" stretch in 5.0" length equates to 6000 psi tension. Not sure how to verify this, so I just assumed it to be correct. Using a second dial indicator, I have determined that a .001" movement on the pivoting leg on my gauge at the blade clamping thumb screw, causes a .001" deflection of the needle on the dial indicator housed in the body of the gauge, which is a one to one ratio. Next, I need to find out what the proper tension should be on say, a .025" x 3/8" wide blade, or any other blade for that matter. I'm hoping that this will turn out to be a usefull instrument for my friend. It sure was a fun project to make. Could anyone confirm any of these figures? Any comments would be welcome. Thanks Mike Green http://i34.photobucket.com/albums/d140/MIKEGREEN555/2005_0630jigthreadingNew0051.jpg

GKman
11-05-2006, 05:11 PM
Mike,
Thanks for sharing. Nice looking job. Have the same on my to-do list.

Found an old article in Fine Woodworking Feb 2001 about the subject. Regardless of the cross sectional area of the blade, a tension of 6,000 psi will result in a strain of 0.001” in 5”. The FORCE required will increase linearly with blade area.

MGREEN
11-05-2006, 07:38 PM
GKman, Thanks for the reply. That must have been the article my friend was refering to.
You may notice that the shaft of the indicator is shifted to the side of the slot milled in body of the gauge. This is due to the pivot pin position not being equidistant between the thumb screw center and center line of the slot. I should have thought about that issue before I picked a spot to locate it.http://i34.photobucket.com/albums/d140/MIKEGREEN555/2005_0628jigthreadingNew0023.jpg
http://i34.photobucket.com/albums/d140/MIKEGREEN555/2005_0630jigthreadingNew0041.jpg

Your Old Dog
11-05-2006, 07:56 PM
simply fantastic! Great job. Does your friend realize how good of a friend he has? You didn't knock that out in no hour !! Let us know how it works out.

The one thing I'm confused about is does it meaure twist or tension? I thought a tension measurement would require 3 points for the blade to come into contact with and the measurement is taken against a fixed load against the center point? Up till now my tension indicator has always been by how red my palm gets from cranking on the adjustment :D

J. R. Williams
11-05-2006, 08:08 PM
Mike
Check out "The Home Metal Shop Club" newsletter for an article on "Band Saw Tension Readout" designed and built by a Club member. He fabricated a hydraulic load cell and is able to get a very accurate load reading on his band saw.

JRW

MGREEN
11-05-2006, 09:17 PM
YOD, Thanks for the kind words. We all have at least one good friend that we are willing to help out in any way we can without hesitation don't we?
Your right, it did'nt take an hour and that's alright with me.
Besides, while working on projects like this, I get to practice and learn along the way as I'm still new at this stuff. Got a SB heavy 10 about a year ago and a small knee mill about three months ago.
The gauge measures tension. As GKman stated, a tension of 6,000 psi will result in a strain of 0.001” in 5”. For example if a blade requires a tension of 18,000psi my gauge(my friend's gauge) should read .003"

JRW I checked out the article you referred to, and found the gauge very clever. With his system you get an instrument built right into the machine, very convenient!
Thanks for the suggestion, looks like I may have another project on my hands.
Mike

Fasttrack
11-06-2006, 04:44 PM
Beautiful work! I'm new at this stuff too; i've had a three-in-one machine for just over a year but there's very little chance i could do something that nice!

I would get to the final cut in the piece and then bugger something up... :D

winchman
11-06-2006, 05:20 PM
"I thought a tension measurement would require 3 points for the blade to come into contact with and the measurement is taken against a fixed load against the center point?"

What you're describing is a tool for measuring tension ON the blade, measured in pounds. MGREEN's very nice tool measures tension IN the blade, measured in pounds per square inch.

The three-point tool is commonly used in aircraft rigging for adjusting the control cables. It looks like this:
http://www.aircraftspruce.com/catalog/topages/cablemeter.php

http://www.aircraftspruce.com/catalog/graphics/12-01050.jpg

Roger

mochinist
11-06-2006, 10:33 PM
Nice work, I never knew I needed one of those, I always just use Your old dogs method. Might be something to make when the shop is slow though.

MGREEN
11-06-2006, 10:34 PM
Fasttrack, Thanks for the compliment. The gauge is'nt very pretty, but seems to work just fine. I"ve had my share of buggers, and am sure I have'nt had my last.
The cylindrical pocket for the dial indicator was turned on the lathe in the 4 jaw chuck,most of the rest of the work was done on the mill, with the exception of the outer curved section, which was band sawn following the circle that I scribed in while still on the lathe and then smoothed on the stationary belt sander. Mike

MGREEN
11-06-2006, 10:55 PM
winchman, Thanks for clarifying the way the gauge functions for YOD.
I knew someone could put it in better words than mine.

mochinist, Thanks. There's probably lots of things we see on this forum that we did'nt know we needed til we see them. lol
This one did'nt cost no $340.00 niether!

Sparks325
04-25-2007, 08:56 AM
That is a great gage. It is just what Ive been looking for. I have a couple of questions. GKman says that The FORCE required will increase linearly with blade area. If I have a inch blade what should the indicator read for say 6000 lbs of tension? Would you take the .75 times 5 inches and get 3.75 for the area? I think that is about two times the area of the 3/8 blade. Would .001 equal 12000 lbs of tension? My other question is about the pivot arm. What is the correct distance from the blade attachment point to the pivot, and how far is it from the pivot to the indicator feeler?
Thanks again for sharing this design.
Brian.

Rusty Marlin
04-25-2007, 10:13 AM
Very nice gage. I only have one point of contention. If you are looking for values in the thousanths .001-.010" you should be using an indicator with 10% resolution ie .0001" for readablity and error prevention.

Other than that... I like it. :)

Evan
04-25-2007, 10:51 AM
If I have a inch blade what should the indicator read for say 6000 lbs of tension?

The value given above was .001" per 5 inchs for 6000 psi of stress. This isn't the same as 6000 lbs of blade tension. .001 is the amount of elongation at 6000 lbs tension per sq in unit area of material so to determine how much tension that is for your blade you need to calculate the cross sectional area of the blade and divide accordingly.

Mcgyver
04-25-2007, 11:30 AM
neat project. to be clear it is a strain guage - its' measuring the elongation of the blade caused by force, stress and strain. Hooke's law says that the amout something will stretch depends on its length so the variables are load on blade, cross sectional

here's the formula

total strain = (load*length)/(cross sectional area*modulus of elasticity)

for example, if you put 200 lbs force on the blade, and modulus of elasticity is say 30 000 000 for steel (most steels are very close to this) and blade dimension are .02 *.5 and length between guage points are 6", then:

(btw complete guesswork on load & blade dimensions)

strain = (150* 6)/((.02*.5)*30 000 000)
= .003

you'd expect the blade to be .003 inches longer over 6" with a 150 lb load. that's the theory anyway


btw that 150 lb load on blade that dimensions .01 sq inches = 15000 psi

Bill in Ky
04-25-2007, 12:00 PM
Very nice work !!

J Tiers
04-25-2007, 01:08 PM
That is a nice-looking device.... Good work!


As an alternative, if you can add it, which may not be easy on many saws, the old Atlas saws had a neat feature......

One pulley was spring-loaded, and you set it to a certain dimension, which really set the amount of compression of the spring.

If you know the spring constant, open length, and the compression, you can directly get the force the spring is applying. Then if you know the blade cross-section, you have the tension right there.

My Atlas one sets to 1/4" with a knob, which is supposed to be the correct setting per the manual. I don't know what the corresponding tension is, and I sorta don't have to.

Sparks325
04-25-2007, 03:43 PM
Mcgyver,
Thanks for the explanation. Im not the greatest when it comes to math
Ok, Let me see if I have this correct. What I need to find is the load on the blade. I can determine that by measuring how much the blade stretches. Once I know the load, I divide that by the cross sectional area of the blade and I will have the tension in psi. For a 20,000 psi tension on a blade (mfg. recommendation) that is .035 inches by .75 inches.
I took 20,000 psi and multiplied it by the cross sectional area of my blade ..0263 and got 526 pounds for the load. Then with the rest of the formula it went like this.
(526*6)/(.0263 *30,000,000)= .004 inches.
Does that sound about right?

J Tiers
04-25-2007, 05:16 PM
By the way....... I assume this may have been covered, but I didn't read every post.....

One type (3 point) reads deflection of blade under a certain side force, which has a "base load" at zero corresponding to the stiffness of the blade. That has to be zeroed out somehow.

The more tension, the less the "toggle" force can deflect the blade. The "base" is the amount it deflects at zero tension.

A direct "extensiometer" actually reads the blade's extension under the tension, which needs to be corrected for the blade area if you want pounds(force) or newtons. Essentially you are reading the "spring" extension and deriving the tension from an assumption of the spring constant of the blade. The EFFECTIVE blade cross-section will depend somewhat on the tooth count, shape, number, and gullet depth. There could be a 20% to 40% or better error in that, depending on the type of blade

Each has its advantages, and in each case you need some external information.

Mcgyver
04-25-2007, 05:49 PM
Sparks, I think you you;ve got it.

ps, also agree with Rusty, would be better to use a10th's indicator given how small a movement you are detecting

MGREEN
04-25-2007, 07:50 PM
Sparks325,
I just kinda "winged " it when I made that gauge.
Just recently, another member of the forum contacted me and
asked if I had any dimensions written down as he was interested in building
a gauge for his band saw.

I decided to hand sketch a drawing ,as built, and sent it off in an email.
The correct length between the thumb screw on the fixed leg and the
thumb screw on the pivot leg should be 5.00"

If one was to build a gauge as per the dimension on the drawing, then a correction in the reading must be used.

I would guess that 0.0009" rather than 0.001" would equate to 6000psi
over the stretch length of 4 1/4".

For the average home shopper I think that would be close enough.
Maybe someone here could re-calculate?

Here's the drawing;
http://i34.photobucket.com/albums/d140/MIKEGREEN555/tension_guage_drawing.jpg
Mike Green

Sparks325
04-26-2007, 09:44 AM
I just wanted to comment on the considerate nature of all the replies to my questions. This a great forum. Having little experience (but having a great interest to learn) it can be intimidating to enter into a discussion. You have extended a warm welcome. Again, much appreciated.

MGREEN
04-27-2007, 10:24 PM
oops I just looked at the drawing again and noticed an error on
the dimension for the OAL.
Or rather over all hieght, it's listed as .875"
Should actually read 3.875"
Sorry about that.

Mike Green

Frank Ford
04-27-2007, 11:33 PM
Mike -

THANKS a bunch for posting this item! I've been getting set (slowly) to do some sensitive ivory resawing, and have been thinking seriously about blade tension. So, this came at a great time for me. Here's my version:

http://www.frets.com/ForumPix/bandsawgauge.jpg

I made mine a bit taller because I had a chunk of aluminum that size. I've ordered a cheapie tenths indicator from Shiraz to replace the one I have set in here.

Cheers,

Frank Ford
FRETS.COM (http://www.frets.com)
Gryphon Stringed Instruments (http://www.gryphonstrings.com)
My Home Shop Pages (http://www.frets.com/homeshoptech)

MGREEN
04-28-2007, 12:32 PM
Hi Frank,
Neat! Yours looks so cool.
Of course all your stuff looks cool.

How did you calibrate your gauge?

To calibrate mine, I set it lying on it's back and held clamped by the projection sticking out the backside of the indicator in the gauge body and then set up a second indicator with its tip touching the side of the pivot leg
in line with the center of the thumb screw.

Then put a deflection on both needles, and set them at zero.
I swung the pivot leg slowly back and forth to observe wheather or not both needles were reading the same.

If the reading of the indicator in the gauge body was greater than the second indicator, then that meant that the tip was touching the pivot leg at too great a distance from the pivot pin.
And had to be adjusted by loosening the set screw holding the indicator firm in the gauge body and rotating the indicator in the pocket thereby moving the tip down closer to the pivot pin then locking the set screw.

If the reading of the indicator in the gauge was less than the second indicator, then that meant that the gauge indicator tip was positioned too
close to the pivot pin and needed to be adjusted a little further away.

This procedure seemed like a logical way to insure that the amount of movement at the thumb screw end of the pivot leg could be read accurately.

Mike Green

awake
04-28-2007, 02:39 PM
... to make the legs of the pivot in a known ratio so as to give yourself more effective resolution at the dial? For example, if the legs of the pivot were set so that .001" of movement in the blade produced .010" of movement at the gauge, you'd effectively get a 10th indicator just using a regular .001 indicator.

Or am I way off? (It's been known to happen ...!)

Frank Ford
04-28-2007, 04:42 PM
I "calibrated" the pivot on mine by cheating, I guess. I centered the indicator in place, and locked it down with the set screw in the holder.

Then, I took the pivot leg and clamped it in position, so it was sitting level with the bottom of the holder. Then I simply drilled the pivot pin hole through both the holder body and the moveable arm in the center between the point of contact of the plunger and the blade clamp screw.

davidfe
01-19-2010, 06:59 PM
Whilst reading this thread, I found your photo is missing.

Is is possible to correct the link?

Regards


Mike -

THANKS a bunch for posting this item! I've been getting set (slowly) to do some sensitive ivory resawing, and have been thinking seriously about blade tension. So, this came at a great time for me. Here's my version:

http://www.frets.com/ForumPix/bandsawgauge.jpg'

I made mine a bit taller because I had a chunk of aluminum that size. I've ordered a cheapie tenths indicator from Shiraz to replace the one I have set in here.

Cheers,

Frank Ford
FRETS.COM (http://www.frets.com)
Gryphon Stringed Instruments (http://www.gryphonstrings.com)
My Home Shop Pages (http://www.frets.com/FRETSPages/Machining/index.html)