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I'm wondering what guidelines are useful in controlling the density of spruce used for bracing. I buy my brace stock from Aircraft Spruce, and have found densities ranging from 24-34 pounds per cubic foot (pcf). In other units, that's 6.3 - 9.0 g/cubic inch, or 0.39 - 0.55 g/cc. I understand "nominal" spruce density is 28-29 pcf. I've been sorting boards and only using ones between 27-30 pcf, to try to reduce the unknowns, but not sure if I need to be that conservative. I know I could also measure Young's Modulus but would rather not get into the additional work for all the sizes involved (this is for a woodshop class, building lots of simple guitars). Any thoughts? Does anyone know what Martin, Taylor, etc. do?
Thanks, David

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David Malicky

Martin just uses it. They check for cracks and visual defects. Other than that it is just made into a guitar. I use a deflection scale for my bracing. I am not so worried about the density. I look more for the stiffness

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I make sure there is as little runout as possible.Then do deflection testing with dial indicator and spaced center points using a steel weight.Don't know the weight and don't have a specific deflection. Have done enough braces before to know what I'm looking for in terms of range.I find that I'm not very accurate in separating braces just by hand bending and feel.I look for the stiffest braces based on the deflection and record these readings directly on the braces.When doing this it important to have all your braces to the same cross section.

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

Remember that density and stiffness have an almost exactly a linear relationship in Spruce...

Your more dense pieces are also more stiff pieces per unit volume... so you can just shave them down a bit more.

I would carefully look over the very dense pieces -- check for very deep brown/red, heavy sap enriched areas that look kinda like Heart Pine (Full of dried sap).... These also smell like Pine Sol / Pine Power when split... and they burn like Napalm! Don't use these for bracing... They work great for lighting the BBQ grill or the fireplace, though...

Good luck

John

Thanks for the feedback! That's very helpful to know about Martin -- sounds like I'm on the conservative side, then. Didn't know about the sap-filled spruce... I'll lookout for that. I'm trying to keep dimensions standardized (beginning woodshop students), so I'll probably still sort by density to keep stiffness in the ballpark (yeah, that was my goal in tracking density). But based on Martin, it sounds like I can open up the range, maybe 25-32 pcf now. That'll save some $, too.

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David Malicky