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Hello,

Does anyone here do this, and can anyone share their techniques and jigs?

Thanks....

Much less sophisticated then Todd. Cut stock to .625" x .280"x 20". Place in jig with supports 18" apart, zero dial indicator at top/center of brace, hang pipe wrench at center and note dial deflection. Write deflection on brace with species then match into pairs. Don't measure wood for other braces. Have not measured a lot of samples but have noted that of wood worthy of sampling there has not been a big variance. Some folks measure in two directions (both with grain vertical) and have noted slight variation. They then average the two readings . I have never done this. As an aside I don't know the weight of the pipe wrench and don't care. Just sort for degree of difference and to insure a proper degree of stiffness only based on what I have sampled.
Tom

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A person who has never made a mistake has never made anything!!!

a piece of aluminum angle plate, two dowel pins, a 2.5 lb weight, and a dial indicator. very simple.

Do you specifically want to know deflection....or Young's modulus?

If Young's modulus, you don't need a jig, just a tap test of the stick having measured the dimensions and mass. Generally quicker and takes less space than a jig/fixture.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Sctitch scritch scritch....

Good question. I don't actually know the answer. What I'm doing is trying to sort the brace stock by stiffness for different purposes, like sorting the good stuff for UTB and x's, and using weaker stuff for transverse top braces and back 3 and 4's.

I think I'm doing what's already been described, more or less. Pinning the end of the brace on a base at one end, and putting a weight on top the other end and a dial underneath. The way I have it set up though is the dial pin is at the very top of its extension, and it doesn't really feel 'right' at the beginning of the pin travel, and sometimes I get some pretty sketchy results. I'm thinking it might make more sense to get another rig that has a dial that comes from above, so that you can have the pin putting pressure on the top of the brace so it can be somewhere in the middle of its travel.

Here's a photo of mine. I get repeatable results with it. After the deflection I calculate young modulus and combine that with a density measurement to sort by stiffness/weight ratio.
I tend to sort these into three piles sorted by the stiffness/weight ratio.
What I've found over the years is that if the braces are all dimensionally the same, I can sort them readily by just holding them and tapping them with my finger and listening.

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Jim Watts
http://jameswattsguitars.com

Yet another reason to love this hang out. A picture is worth a thousand words. Thanks Jim!

Cheers,
Danny

It sounds to me like you need to be measuring both density and Young's modulus, using SI units, and then you'll have data that is comparable to published data, which is never a bad thing.

The nice thing about the tap method is that you can use any wood of any rectangular cross section and any length (within reason), so no need to reduce to a standard size. Just tap into a mic/computer to measure the frequency. Google "vibration of a rectangular bar" or "marimba bar design" which should get you to the right formula to compute Young's modulus pretty quickly.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Todd, I was thinking about what I might put together with some scrap and I see you've already done it - not too big and easily repeatable. Thanks for the photos.

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Steve Smith
"Music is what feelings sound like"

I've got an old Grizzly indicator that will be just right. Roughly how much weight are you using? Maybe 1/2 to 1 lb?

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Steve Smith
"Music is what feelings sound like"

Todd, what going on at the right hand end of the jig? Safe to assume a sliding holder to accept differing brace thickness?

Awesome jig, btw....

Being a simple soul, have to keep things simple.

Here's my rig.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Any practical reason why one would want to use one or the other of these 2 approaches? Or just a case of same thing only different.

Thanks for the photos. It's time for me to start doing both bracing and tops on all guitars. Pretty quick and simple once you're set up.

Two different ways to get the same information, no difference really accept in the formulas.
These things seam to be made out of whatever you have lying around. I had a carbon fiber beam laying around and a test indicator, so this was the easiest configuration for me at the time.
Todd made his from scraps and Trevor had a mic.
We're all getting the same information, although if you use the mic you can get damping information too.

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Jim Watts
http://jameswattsguitars.com

Todd, I would appreciate some bigger pics. I'm not quite getting how yours works but I like the look of it.

Trevor, are all the equations (including with the mic set-up) for brace testing in your book?

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We are all in the gutter but some of us are looking at guitars.

Yep. Section 4.4.4 of the Design book; equations for both the static and dynamic (using the mic) testing methods for bars. The two methods correlate very well. (Plates are in a different section). As Todd says, data entry is about the same for both, but I find the mic (dynamic method) quicker as I always have a mic set up with my laptop as I'm always tapping or tuning something; plates, guitars, braces, bridge blanks, strings and whilst I do have a static rig (3 point bending), it really only gets used in teaching demos to show how its done.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Can't wait to get the book.

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We are all in the gutter but some of us are looking at guitars.

Has any one found that the E value of braces makes a whole lot of difference assuming you are using brace stock of "good" quality? in my research I have found that altering the height of the braces slightly makes up for small variations in modulus of the brace it's self

I would say that you are correct in your assessment. But it's nice to use your stiffest (probably heaviest) brace stock for things like the UTB, and i use my most flexible stock for finger braces. But I can't support any claim that it makes much of a difference. I also use my lightest stock for classical fans, since as you pointed out, you get stiffness real quick through geometry.

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Jim Watts
http://jameswattsguitars.com

How does one express a stiffness to weight ratio? How is it arrived at?

Whoa, whoa, whoa!!! That was almost Somogyesk!

Have another go, Todd (before the bourbon, this time, or after a coffee, if the other end of the day...!)

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Some notes for the Meddlingfool (who I'm sure has a real name...)

Guitars pose something of a dilemma. They have to be both stiff enough and strong enough to stand up to static string tensions over a long period of time whilst at the same time being flexible enough and light enough to radiate sound when excited by the meager forces exerted by the vibrations of the string, the vibration forces being close to 3 orders of magnitude less than the string tension forces.

In round numbers it's best to keep the bridge rotation under the torque of the strings over the saddle to around 2 degrees (otherwise the deflections look excessive, if nothing else) which implies quite a stiff structure in that plane, yet the soundboard has to be accelerated (sound radiation is proportional to soundboard acceleration) under those meager vibrating forces, so needs to be low mass* and compliant enough to move. So it helps if the soundboard structure is built of high stiffness, low density materials (and is very carefully designed) to best meet those conflicting requirements. That's why the ratio of stiffness to density of materials is of interest, usually expressed as Young's modulus E (the stress in a material that produces a certain amount of strain, therefore a measure of stiffness) divided by density (mass per unit volume).

That explanation leaves out a lot of detail and also contains a lot of words that have specific meaning in an engineering context, different from common usage, so if it comes across as nonsensical let me know and I'll have another go!

* because low masses accelerate more under a given force.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Brace Deflection Testing....hmm, well me
stick it in a vise and reef on the end...real simple eh.

blessings

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Audiences and dispensations on Thursdays ~ by appointment only.


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Trevor,

meddlingfool= Ed Bond, builder of Tinker and Halcyon Guitars in Vancouver, BC, quite possibly the first person to have bought your Most Excellent and Useful Book.

Now you're asking, if you have the book, why are you asking these dumb questions? Well, it's cause I can only understand the parts written in English. I flunked out of math early. However, the English bits have been a real gamechanger.

For now, I'm content to sort brace stock into two piles, one for x and UTB, the rest for the rest...

Hi, Ed. I hear a lot of good things about your guitars, so I'm sure you don't need very much help!



Yes, Tom Rossing regards maths as another language, used primarily for conciseness and avoidance of ambiguity. Most of the maths is to show derivations, primarily there to indicate to people how I got to where I got and for people to challenge in the same vein if they so wish.

Pleased to hear that you're finding plenty of stuff in there that you're using.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au