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Visualize Differential Expansion

Discussion in 'Luthier's Corner' started by Bruce Johnson, Dec 18, 2017.

  1. Bruce Johnson

    Bruce Johnson Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Check this out. This is a textbook example of Differential Expansion; what happens when two different materials glued together expand at different rates.

    This is an unfinished walnut veneer pickguard for one of my AMB-2 Scroll Basses. It's an optional feature that I offer on my all-walnut model AMB-2. This is a spare pickguard that I made up back in the spring, and has been sitting in a cabinet. It's made from a sheet of 1/16" Garolite plastic laminate, with 1/64" plain (non-paper backed) walnut veneer epoxied on to it using West Systems epoxy.

    We've had really dry weather here in the past few weeks, with the outside humidity sometimes down below 5%. Right now, it's "up to" 13% here in my shop. I happened to notice this in the cabinet and thought it was amusing.

    This is a pure example of differential expansion at work. The walnut has expanded just a little bit more than the Garolite, but over that distance, the difference in expansion rates adds up, and results in the uniform curve.

    Notice that all the expansion is along the grain of the walnut. There's almost none across the grain. Also, it's curious that it bowed convex. That means that the walnut has expanded more than the Garolite. I would have thought that the walnut would be shrinking in the dry weather. Huh?

    Regardless, the epoxy bond must be good, because there's no bubbles or buckling of the veneer. And it's not really a problem. With one finger, I can push it down flat. This pickguard is still good and useable. I'll be watching it to see if it flattens out as the humidity gets back in the normal range (35-50% around here).

    This does make you think about the perils of joining dissimilar materials. You know, like soft necks and hard fingerboards.....
    b3e, nouroog, reverendrally and 2 others like this.
  2. Scoops

    Scoops Why do we use base 10 when we only have 8 fingers Supporting Member Commercial User

    Oct 22, 2013
    Sugar Creek, Wisc
    I am me
    Ok, so your laminates are thin with respect to the laminating we do with say necks, bodies, and wings

    My question is, would the differential expansion be reduced by the thickness of the material used.
    Said another way, would we see the same bowing effect with the materials you are using if we were to use 1/2 inch material? 1 inch material?

    My guess is yes, but then how much?

    I can't imagine it would be as dramatic
  3. chinjazz

    chinjazz Supporting Member

    Sep 11, 2002
    Atlantic Beach, FL

    Very interesting topic...

    When I needed to harvest off pieces of top material for my peg head (from scraps of a rough body cut), some pieces came off looking like this - after letting them sit for a few days. Some pieces didn't flex due to grain orientation, size of the piece... There's most likely a math formula to figure this out, but I'd rather not try finding that this early in the morning...


    Here we have bottom to top Swamp Ash, Wenge and Maple. This piece has been in this shape for a while. I kept it out of curiosity. For necks, mass and density I would think plays into how much they move. With all the other factors added to that, moisture of the wood(s), glue strength and rate it can flex, truss rods and stiffening bars. Thanks for posting this topic :)
    b3e and T_Bone_TL like this.
  4. T_Bone_TL


    Jan 10, 2013
    NW Mass/SW VT
    I'd infer (based on what I know, or think I know, of wood movement) that the plastic may be shrinking, at a rate similar to the across the grain shrinkage of the walnut, while the long-grain of the walnut is barely moving - rather than long-grain expansion of the walnut. Never-the-less, an excellent post. Some plastics do absorb a surprising amount of water (which is a royal pain in a high-vacuum system...)

    A good scrap project is the differential shrinkage demonstrator - a long strip that's laminated to a "strip" built up of cross-grain material to match its size. With a bit of mounting and calibration, you can read the humidity by where it points. A short strip will show it, but the longer, the more dramatic the movement.
    Last edited: Dec 19, 2017
    chinjazz likes this.

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