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brace deflection
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=24550		 Page 1 of 1
Author:  arie [ Wed Nov 11, 2009 4:09 pm ]
Post subject:  brace deflection
So given a standard 6 string flat top dred, sitka spruce top and spruce bracing, how much deflection should the top braces have before they are shaved? Dana Bourgeois mentions testing deflection and then matching the braces with like values using a drill press vise as a weight. How much movement is "ok"? How much weight would be used?

thanks
Author:  Tom West [ Wed Nov 11, 2009 5:32 pm ]
Post subject:  Re: brace deflection
ARIE : I measure my X brace stock after sizing to a certain cross section size. They are placed on a jig with two beams 18 inches apart and I hang a pipe wrench in the center and measure the deflection with a dial indicator.Have no idea of the weight and only measure for matching purposes and to check overall stiffness. Or the lack of same.Not sure you will get a direct answer to your question.Hopefully others will give you more info. Good luck.
Tom
Author:  Steve Kinnaird [ Wed Nov 11, 2009 6:17 pm ]
Post subject:  Re: brace deflection
Tom, I could hardly believe my eyes...your unit of weight is the same as mine. "One Pipe Wrench" of weight, applied to the center of the brace, and each brace is measured for deflection.

Attachment:
Brace test Sm.jpg


Look closely, and you can see that wrench dangling at the end of a wire hook that fits over the brace.
Also like you, I don't know what the weight is, but it has become my standard of measurement, and each brace is subjected to the same test. Records are kept, and values recorded.

How 'bout that?

Steve
Author:  arie [ Wed Nov 11, 2009 6:53 pm ]
Post subject:  Re: brace deflection
guys: how much deflection are you measuring?
Author:  Steve Kinnaird [ Wed Nov 11, 2009 10:19 pm ]
Post subject:  Re: brace deflection
arie, I actually test twice. I test the brace at 3/4" height, and then at 11/16" height. So I'm getting two sets of numbers.
At 3/4" height, that brace is considered quite stiff if it deflects only .017". Pretty loose up in the 25+ thou range.
The 17's and 18's become "X" braces.
Also, you can test with this simple getup which way is actually stiffer. As in, there's an up and a down for this little piece of wood.
Make sense? Test your brace, then flip it over and test again.
One side will often offer more resistance. Glue that brace then in the direction of the string's pull.

Steve
Author:  Tom West [ Thu Nov 12, 2009 9:40 am ]
Post subject:  Re: brace deflection
Steve : Set up is just about exactly the same,jig at the edge of the bench.Must admit that I have not thought of turning the brace over and measuring twice. Thanks for the tip.Arie,my deflections are a bit more than Steve's,but I measure at 5/8" depth.Rather than worry about actual deflection values folks get with unknown weights,think it's more important to measure and get the stiffest.My braces are about .280 wide and the deflections have varied from high teens to low 30's.Think this is only one part of a much bigger puzzle and if you are just starting would not get excited about it until you have a number of guitars under your belt.Please excuse me if I'm being presumptive.
Tom
Author:  Dave Rickard [ Thu Nov 12, 2009 10:08 am ]
Post subject:  Re: brace deflection
Steve Kinnaird wrote:
arie, I actually test twice. I test the brace at 3/4" height, and then at 11/16" height. So I'm getting two sets of numbers.
At 3/4" height, that brace is considered quite stiff if it deflects only .017". Pretty loose up in the 25+ thou range.
The 17's and 18's become "X" braces.
Also, you can test with this simple getup which way is actually stiffer. As in, there's an up and a down for this little piece of wood.
Make sense? Test your brace, then flip it over and test again.
One side will often offer more resistance. Glue that brace then in the direction of the string's pull.
Steve

Steve, Do you put the stiffer side on the bridge side of the top or the neck side?
Author:  Pete Licis [ Thu Nov 12, 2009 11:34 am ]
Post subject:  Re: brace deflection
Steve, Do you put the stiffer side on the bridge side of the top or the neck side?

I think he's referring to the top and bottom, not the ends ....
Author:  arie [ Thu Nov 12, 2009 3:07 pm ]
Post subject:  Re: brace deflection
Thanks for all the help guys. I'm new to building and was wondering what numbers to look for in brace deflection. Thanks again for the replies [:Y:]
Author:  Steve Kinnaird [ Thu Nov 12, 2009 11:50 pm ]
Post subject:  Re: brace deflection
Dave Rickard wrote:
Steve, Do you put the stiffer side on the bridge side of the top or the neck side?


Dave, I think I understand your question, but let's see for sure:
It's not that one end of the brace is stiffer than the other.
Rather, I meant to say that one face of the brace was yielding a lower number. Any brace, in an unprofiled configuration will have two potential gluing surfaces, an "up" and a "down" as it were. It struck me one day that the brace might perform better if glued on one edge or the other. With some braces it doesn't matter.
But flipping the brace over in that deflection jig and testing again can give a stiffer reading at times. Critical? Hardly.
Useful? Mayhaps. I conduct this extra test now as a matter of course...just one more little bit of info that you can take into a system where you're moving as close as you can to the edge, and hopefully not falling off.
So--the stiffest piece I've got usually gets glued under the treble side of the bridge, while the slightly looser one goes on the bass side. Tone bars and finger braces are made from the less stiff brace stock.

Did that make sense? Is any of that clear?

Steve
Author:  Dave Rickard [ Fri Nov 13, 2009 10:01 am ]
Post subject:  Re: brace deflection
Got it.
The older I get the dumber I get.
THANKS
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