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| Deflection Measurements http://www-.luthiersforum.com/forum/viewtopic.php?f=10102&t=7943 |
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| Author: | Steve Saville [ Thu Aug 10, 2006 12:01 pm ] |
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As a beginner, I'm thinking that the stiffness should be measured and given a numeric value rather than just go by feel. I have not done this with my first 4 guitars, but I'm thinking that I should. Shouldn't we be sanding to a deflection rather than to a thickness? I have not figured out how to do this yet, but it seems that if you support the perimeter and place a weight in the middle, you could then measure the amount it moves (deflection). Maybe it would be better to hold one end, place a load at some distance (like 12") and measure the deflection at 12". I'm just thinking out loud. Would anyone care to share how they do it and what kind of measurement your looking for? Do you do a calculation for stiffness based on the cross sectional area? (I should probably buy a book and read it. ) |
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| Author: | Brock Poling [ Thu Aug 10, 2006 12:45 pm ] |
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I think you are right. This is the way to go. My deflection jig is a table with 2 dowels mounted at 18" on center. A dial indicator jig can be referenced on the top and zeroed. A given weight is applied and the top is sanded until a given deflection is acheived. My deflection was acheived by using a "template" top that we made in ES's class. Considering what you use for a weight your target deflection will differ from other builders, but if you always use the same weight as a reference point you will have a consistant longitudinal stiffness on the plate. |
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| Author: | peterm [ Thu Aug 10, 2006 1:34 pm ] |
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Interasting Brock....any pics of that jig? |
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| Author: | Jim Watts [ Thu Aug 10, 2006 3:05 pm ] |
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I haven't taken taken Ervins class although I think it would be great. But I do test most of my wood and do top deflection on a ficture very similar to what Brock describes. I test both direction though and just rotate the dowells and indicator for the second direction. Here's a picture of my rig. There's a 1/4 inch steel rod that actually comes in contact with the top. It doesn't show up very well but there's a base plate with slots cut into it for the dowells to sit in.  |
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| Author: | burbank [ Thu Aug 10, 2006 3:07 pm ] |
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Brock, It sounds as if you're seeking a specific amount of deflection across a span of fixed distance with a given weight. Do you compensate for different body sizes? Thanks |
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| Author: | Steve Saville [ Thu Aug 10, 2006 4:08 pm ] |
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This is done for the top when sanding to thickness. I'm guessing that this is repeated on all the braces and then again while carving the braces once bonded to the top, at various locations until the desired deflection is obtained across the top. |
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| Author: | Brock Poling [ Thu Aug 10, 2006 4:10 pm ] |
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No. The goal is to pretty much make them the same deflection regardless of the size of the guitar. The longitudinal stiffness is a constant. I also take a density measurement of the plate. |
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| Author: | Mark Tripp [ Fri Aug 11, 2006 2:08 am ] |
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I've just started studying "Left Brain Luthierie", which discusses these subjects in great detail. I'm interested in this because I want to create "quantifiable consistency". I know that Collings does this - all the bracing on Collings guitars are identical for a given model. They tweak the top stiffness/thickness instead. And Collings are some of the most consistently good sounding production guitars out there. So I'm going to start rigorous deflection measurements starting on my next one. -Mark |
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| Author: | Dave Rector [ Fri Aug 11, 2006 3:27 am ] |
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[QUOTE=Brock Poling] I also take a density measurement of the plate.[/QUOTE] Brock, how does one go about measuring the density? |
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| Author: | Brock Poling [ Fri Aug 11, 2006 4:08 am ] |
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I calculate the volume of the material height x width x thickness and divide that by the mass. So you end up with g/in^3 I have a triple beam balance to measure the mass. |
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| Author: | Dave Rector [ Fri Aug 11, 2006 6:25 am ] |
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Thanks Brock! |
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| Author: | Jim Kirby [ Fri Aug 11, 2006 6:52 am ] |
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[QUOTE=Brock Poling] I think you are right. This is the way to go. My deflection jig is a table with 2 dowels mounted at 18" on center. A dial indicator jig can be referenced on the top and zeroed. A given weight is applied and the top is sanded until a given deflection is acheived. My deflection was acheived by using a "template" top that we made in ES's class. Considering what you use for a weight your target deflection will differ from other builders, but if you always use the same weight as a reference point you will have a consistant longitudinal stiffness on the plate. [/QUOTE] Do you cut the plates being tested to a uniform width before testing? Since you are essentially point loading a wide plate (??), I'd think that variations in the plate width would have a significant impact on the center deflection. |
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| Author: | Brock Poling [ Fri Aug 11, 2006 8:05 am ] |
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my hunk of metal is about 15 - 16" long. I set it very close to the dial indicator. |
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