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Deflection testing revisited...
http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=11757		 Page 1 of 1
Author:  John Elshaw [ Mon Apr 23, 2007 3:38 am ]
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After building many classical guitars, I'm in the midst of building my first steel string and have found the bracing to be quite a bit different. I'm a proponent of Al C's advice of keeping good records and taking a systems approach to building. With that in mind, can some of you tell me what your target is for deflection testing on a top? I'd like to know what your target is for cross dipole (width) and long dipole (length) deflection testing of various model guitars.

I have a braced top I bought from John Mayes (along with his voicing videos) and it was very insightful into the steel string top. I would now like to start a database for my own records of top charateristics. Other than the obvious like deflection testing, weight, and thickness, are there any other stats you guys find particularly useful to keep track of?

Along with your deflection testing results, could you each include the type of sound you are shooting for with your particular measurements?

Thank you!

John
Author:  old man [ Mon Apr 23, 2007 2:12 pm ]
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Way tooooo technical for me, John, but I'll give you a bump.

Ron
Author:  John Elshaw [ Mon Apr 23, 2007 2:48 pm ]
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Thanks for the bump Ron. Deflection testing is just where you rest the sound board on two supports either on each side, or top to bottom. Then, you put a weight in the middle and measure how much the soundboard sags, or deflects downward. This helps you determine how stiff a soundboard is. Even if your topwood or braces all vary a little, deflection testing will help ensure you get your tops to a consistent stiffness.

Cheers!

John
Author:  burbank [ Mon Apr 23, 2007 2:54 pm ]
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Probably not much help here, John, but I recall seeing where some shop measured deflection on perhaps a dozen points around the lower bout of the top.

Since there doesn't seem to be any sort of standard (too bad), I'm figuring on just finding something that works for me. That will be my plan and I'm sticking to it!
Author:  Andy Zimmerman [ Mon Apr 23, 2007 3:11 pm ]
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Deflection testing!!!!!!
Well there are many ways to do it and different people believe in different
things. The key is to make a system yourself and keep track. My system
will be different from yours. Everyone uses a different method and
different weights. The key is to be consistent!!! Set up a sytem and use
it. Keep a record of everything

Type of wood
size
mass
deflecton measurement
I have never tried deflecting to a certain tone, just to a degree of stiffness.

Then when you finish the guitar put notes in about the sound. What you
like and what you don't like. Also, take pictures of your bracing and the
measurement.   You might find that 2 tops with the same deflection but
different bracing creates 2 different guitars.
Author:  Andy Zimmerman [ Mon Apr 23, 2007 3:28 pm ]
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More points. The advantage to deflection testing is you can get some hard data with regard to stiffness. Each piece of wood is unique and should be treated accordingly.
Why thickness each top to the same thickness???? My analytical brain thinks that is rediculous. I have felt many pieces of wood of the same type that were DRASTICALLY different in stiffness. If you thicknessed all of them the same and made guitars with them, they would all be different.   Also each body size changes everything. A smaller body guitar is different from a dreadnaught.

My set up is just a table with 2 bars and a dial indicator.
Simple as that!!!! I just put a dumbbell on it.

Author:  John Elshaw [ Mon Apr 23, 2007 11:49 pm ]
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Good input fellas! I have another question as I get ready to brace my first steel string. I already have deflection results for the bare top, but does anybody do deflection testing once the braces are on to see how much of the brace to shave away and try and record a stiffness value after the top is braced? I know top stiffness is important but it seems like it wouldn't matter if it was way overbraced. Is deflection testing a viable way to measure top stiffness after the top is braced? Are there any other ways to measure top stiffness after it's braced?

BTW, I was AMAZED at how stiff and light the braced top was I bought from John Mayes. Compared to my classical tops, this steel string top was in a whole different category. I know there are differences in string tension, but I had no idea how big a difference there was between the two types of tops.

John
Author:  matt jacobs [ Tue Apr 24, 2007 12:59 am ]
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[QUOTE=azimmer1] Why thickness each top to the same thickness????
[/QUOTE]

Andy, maybe a little clarification here. When you said the above quote you meant just going with a standard thickness on a finished instrument(e.g. .110"). What about when doing your defelction testing do you thickness the testing tops to a standard thickness like .140". You said the key is consistanancy. Or does it not really matter that much. Or do I not make sense at all?
Author:  Andy Zimmerman [ Tue Apr 24, 2007 1:44 am ]
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I don't know how to deflect a top when the braces are carved. I carve mine
when the top is already attached to the rims and carve to tap tones.

Matt I dont thickeness to a standard thickness, but to a stiffness
Author:  Philip Perdue [ Tue Apr 24, 2007 7:04 am ]
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Check out David Hurds's site. www.ukuleles.com He gives some info on how he does defection testing. Check out his site map for a lot more information.

Philip
Author:  PaulB [ Thu Apr 26, 2007 10:14 am ]
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[QUOTE=azimmer1]
Matt I dont thickeness to a standard thickness, but to a stiffness[/QUOTE]

Andy,

Is that only the cross-grain stiffness that you measure (as in your pic), or does longitudinal stiffness come into it too?

Seeing as most of the tension from the strings is acting against the longitudinal stiffness (or so it seems to me). Then the X brace, tone bar(s) and finger braces help to stiffen up the cross grain stiffness of the top, and by carving them, give some control over cross grain stiffness (and I guess the X braces are going to contribute to longitudinal stiffness too).

Seems to me; that might be where people are coming from when they say you can thin the tops a lot more than most of us do. Get the longitudinal stiffness right where you want it, and you have cross grain stiffness adjustment through brace carving.

I'm thicknessing an Adi top, and the longitudinal stiffness/cross stiffness ratio is way out of whack compared with the sitka - engelmann - and lutz that I've used in the past. The Adi has heaps more longitudinal stiffness than any of those, but at 0.110 the cross grain stiffness seems about the same, at least that's what my fingers tell me. I don't know whether to thin it down a little more or not - it's for a 12 fret 000.

Anyone want to comment/shoot me down/put me out of my misery/ or even educate me?

Cheers,

Paul
Author:  Alan Carruth [ Thu Apr 26, 2007 10:42 am ]
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Dave Hurd wrote a whole book, 'Left Brain Lutherie', mostly on methods of deflection testing, with some of the physical thinking behind them. It's not light reading, but very worthwhile.

One thing he did not do was give a lot of information about what the 'right' numbers would be. That's altogther proper. For one thing, he probably doesn't know what those are except in a general sense, and certainly would not have an idea of what your design should do.

What he did do was give some good guidlines on how you can figure out your own numbers. One of the best is simply to measure a lot of guitars, and not just your own. If you can find some instruments that are just plain too stiff, and some that are 'way too loose, you can at least set up some plausible outer limits. It's pretty likely that short-term deflections under given loads don't change a lot even when the top has distorted over time from string torque, so if you find an older guitar with a collapsed top, you can still get some idea of what doesn't work by measuring the deflection with the string tension on and off.

One rule of thumb from the aircraft world that might help: given the cold creep of wood, the initial deflection under the maximum load you need to take should be no more than about 1/3 of the allowable deflection. If you can figure out what an 'allowable deflection' for a top is over time, you can make a guess at how much deflection you can stand when you first string it up.

I have only recently started to measure all of the tops I get as they come in. The ranges of some values for tops of the same species are pretty amazing. Using this information, and comparing the top I have in my hand with the best guitars I've made using these measured tops has already helped me work a little 'closer to the bone', so to speak, and that's where the sweetest meat is.

Yeah, I know this might not help much on your first one, but at this point, with so little general understanding of these things, you're sort of on your own.
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