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Deflection According to Body Size...
http://www-.luthiersforum.com/forum/viewtopic.php?f=10101&t=44354		 Page 1 of 1
Author:  sdsollod [ Wed Oct 15, 2014 2:10 pm ]
Post subject:  Deflection According to Body Size...
I have been using the Everett/O'Brien method of thicknessing tops. That is, shake the top until you hear the "sheet metal" sound. I am thinking about trying deflection testing. Based on Chris Paulick's youtube video http://www.youtube.com/watch?v=xDDJVAaD9n8 , the (dreadnaught?) top is placed across two beams that are 18" apart and a weight of approx. 5 lbs. and 4 oz. is placed on the center of the top. The thickness is reduced until the top deflects 1/4". Now - does that depend on whether the top will go on a dread, jumbo, 000, or parlor? I seems that additional deflection could be necessary for a smaller size top... Does anyone do deflection testing differently? Comments?

pizza food for thought Eat Drink

Thanks,
Steve
Author:  Mike OMelia [ Wed Oct 15, 2014 3:34 pm ]
Post subject:  Re: Deflection According to Body Size...
You just missed this discussion: viewtopic.php?f=10101&t=44292
Author:  Trevor Gore [ Wed Oct 15, 2014 5:51 pm ]
Post subject:  Re: Deflection According to Body Size...
The method in the book takes account of the size of guitar (length and width) that the top and back plates are to be used on. It's important. I would imagine the "shake" method works too, provided you size the plates and have the technique down.
Author:  meddlingfool [ Wed Oct 15, 2014 8:05 pm ]
Post subject:  Re: Deflection According to Body Size...
The problem with the shake method is that it's really only an indicator of cross grain stiffness, and not an indicator of long grain stiffness. As far as I can tell, there is no correlation between the two.
Author:  Trevor Gore [ Wed Oct 15, 2014 11:58 pm ]
Post subject:  Re: Deflection According to Body Size...
Sure, there isn't much correlation between long and cross grain stiffness - too many dependencies on how the wood was cut. From what I understand of the shake method, though (and I'm certainly no expert - where's Mario when you need him...), when you do the "wobble board" thing, you're getting a combination of long and cross vibration modes when you get the "sheet metal" sound and so if the piece is sized right (and you're experienced enough) you should in the ball park for thickness. If the method works at all, that, or a similar mechanism, would, I think, be the explanation of why. But I've never done any testing on that method, having backed a different horse.
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