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Anyone here care to share their measurements of the action at the 12th before and after initial stringing up? Wouldn’t this be an accurate measurement of neck deflection due to string tension? Thanks.

Joe

I suppose you could do it with a straight edge from the first fret to the saddle along the fretboard after string tension is applied. Honestly it's not something I ever do. I just set the action under string tension and that's that.

I do it with a long straight edge when planing the fretboard final but I never felt the need to compare the difference with tension added. If I would feel my necks are not stiff enough, or too stiff, under tension, I'd work on the amount of relief, wood density, and shape. As a rule I pick the denser cedar pieces for thin necks.

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Build log

I do as Alex does, and I haven't measured the difference either. I will say, however, that though I have not measured, I have noticed that the differences are very small, in the .2 to .4 mm range.

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Waddy

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Thanks for the replies everyone. Filippo, you may be right about having more deflection of the top than neck; still I’m interested in the overall consequence. I’m doing what Alex describes to arrive at a final dimension/taper on the fingerboard. My problem is that I have no previous experience to go on, as Alex mentions, neck wood stiffness, density, shape, previous builds, etc., so I have no idea about what factor may be needed to accommodate expected changes in action that may arise due to string tension.

Waddy, I think 0.2 to 0.4 is probably very close as a general approximation. I have a 45 year old classical that I can measure a 0.10 mm change at the 12th as string tension is taken off; but my thinking is that this measurement doesn’t fully represent the initial deformation or set that happens when the wood sees the string tension for the first time. I have a tendency to over-think things, especially when I have no direct experience to apply, and this is one of those times I guess.

Joe

I had a 10 year old flamenco in for repair earlier in the year which had at least 2.5mm rise in the bridge between strung and unstrung, and consequent high action.
I had to repair it with a wedge shaped fretboard, but I had to have faith in my calculations, because unstrung it looked fine.
I measured the soundboard at 1.8mm in the upper bout, I suspect it was a fair bit thinner in the lower bout.

Thanks Jeff, sounds like that's a pretty flimsy soundboard. I hope mine doesn’t move that much.

Filippo, I am essentially following Brune’s 1937 Hauser plans. Most body dimensions are within a mm or so. The top thickness is generally within 5% of the plans. The top and braces are Engelmann. Fan struts are laid out like the Hauser but I omitted the bridge plate (didn’t like the idea of notching into the skinny little fans after spending all that time making them from straight grain, perfectly quartered and zero runout stock). The soundboard was braced to 19ft radius but now measures 21ft (spring back or humidity fluctuation?).



The neck angle was set using a 2mm drop from 12th to nut on the solera neck extension. Back and sides are EIR, also to within about 5% of plan thickness. Neck is Spanish cedar (final thickness with fingerboard will be close to 21.5mm at 1st and 24.5 at 10th), African ebony fingerboard about 6.5mm average thickness.

Joe