tapping plates

[mac007]

In Design, the procedure for determining final backplate thickness mentions reducing the plate(s) to no less than 4mm before starting the analysis. It also says the plates must have a smooth surface to provide predictable results.

My EI rosewood arrived at about 5mm rough sawn and by the time I had drum sanded the sides smooth it's barely 2.6mm which is about the minimum I would like to go but this renders the Design procedure invalid.

Any others have this and how important is the thickness and surface finish?

thanks in advance

mac007

[Trevor Gore]

The reason for the 4mm thickness number is so that (across a variety of different species) you still have the ability to reduce it to your target.
Looking through my EIR data, I have target thicknesses ranging from 3.1 mm (medium sized guitar, wood with high long-grain runout) down to 1.9mm (small body), which just demonstrates wood's variability.

Tap test your back panels it at their current thickness (all the equations will still work fine, but the frequencies measured will, of course, be lower) and see what your target comes out to be. Rough sanded wood has "mashed" material on the surface, which adds mass but little stiffness, so measures up as less stiff, predicting a larger target thickness. You should then be able to make a call as to whether you have enough thickness left. If you are just marginally under thickness, you should still be good to go on a steel string guitar by leaving the braces full height until you can tap test the box without the top (see Build Section 12.4 (last paragraph)).

[mac007]

Thanks as always for the detailed reply Trevor

MK