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This curly redwood adequate for acoustic (SS or classical) top?

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This curly redwood adequate for acoustic (SS or classical) top?

Postby Peter Wilcox » Sun Oct 11, 2015 7:49 pm

I've a piece that I've resawn into book matched pieces 5/16" thick for a drop top electric. Now I'm thinking of resawing again for acoustic tops. The end grain is at the bottom of the picture - it is wide and irregular - how much of a problem might this be?
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Re: This curly redwood adequate for acoustic (SS or classical) top?

Postby Randy Roberts » Sun Oct 11, 2015 10:27 pm

Peter,
A couple thoughts to consider in my mind:

1. If you are thinking of resawing each piece to end up with 4 halves and so two tops, You are running a pretty good risk of ending up with no tops. Assuming a blade with a 0.62 kerf, you need to split it exactly in half to end up with .125 thick pieces if sawn exactly, which no one I know could do. The amount of variation from dead center as you go through the cut will reduce the usable thickness to whatever the largest divergence (from perfect) ends up being. You will loose still more thickness with your sanding.

2. You should remind yourself that each of those beautiful "waves" is essentially run-out, which will reduce the strength and stiffness compared to non-figured wood, so you will probably be wanting the top a little thicker than your usual non-figured top. That's going to reduce your margin for error with your re-saw that much more.

3. I'm only guessing from the difference in the way the two halves are reflecting the light in the photo, but I think you might have a fair degree of run-out over the length of the board overall also.

Trying for two tops would take a braver man than I.
( and I just last night resawed a 14 1/2 inch wide piece of kauri [$$$!] for 4 one-piece electric tops and backs for a friend with a variation of .015 across the length and width of it. Granted kauri is extremely soft, but I had to throw that story in because I've never resawn thicker than 8.5" before, was scared speechless, and now need to go get a much bigger hat instead of new underwear. Had to talk him out of going for 5 also.)

If you meant you were just going to thin both pieces down to result in still having two halves for one top rather than two tops, then I think I'd just thickness sand them down, taking care to take most of the wood off the back sides of whatever gives you the best book match.

Just my two cents from several times ending up with no pieces thick enough due to trying to squeeze out one extra.
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Re: This curly redwood adequate for acoustic (SS or classical) top?

Postby Chris Reed » Mon Oct 12, 2015 12:58 pm

Today I resawed a mahogany drawer side, 9mm wide, by hand to make ukulele sides. I reckon I'll end up with two 2mm plates after removing the saw marks, which is fine as I'm looking to thin them to about 1.6mm. That board is a fraction wider than your boards. But my board is very plain, and was free, so I wasn't risking much.

For a uke top I'd want your wood at over 2 mm thickness, and I guess 3 mm or more for a SS guitar. Even with some super Japanese blade I'd step away from the workbench. But if you're able to cut a 2 mm kerf (after cleanup) and perfectly centred throughout, then you might want to take the risk.
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Re: This curly redwood adequate for acoustic (SS or classical) top?

Postby Rodger Knox » Mon Oct 12, 2015 1:16 pm

I finished this one about 18 months ago, and it's had string tension for about half that time.
About a month ago, I noticed a 1" long cross grain crack about 1/2" behind the bridge and loosened the strings.
I suspect with another few weeks of string tension the crack will grow all the way across the bridge and the top will fail.
I stick with electric drop tops for that redwood.
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Re: This curly redwood adequate for acoustic (SS or classical) top?

Postby David King » Mon Oct 12, 2015 1:34 pm

Most curly redwood I've come across is totally worthless as a structural element. I wouldn't go near the stuff for an acoustic top unless I was laminating it to something else, perhaps plywood or spruce /nomex/spruce. That doesn't begin to account for the difficulties in sanding and finishing it. Get a high resolution photo of redwood printed onto your white-painted spruce top.
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Re: This curly redwood adequate for acoustic (SS or classical) top?

Postby Peter Wilcox » Mon Oct 12, 2015 2:44 pm

Thanks folks, that's food for thought. It seems it would make a structurally deficient top for an acoustic, so that's out, even if it sawed perfectly.

The board was a gift from a friend, so I don't want to ruin it. However, I have a brand new Woodslicer blade on my saw (~0.035" kerf), the wood is soft, and it made a great cut through the original board. So I could make two thickish veneer tops for electrics, especially since thickness doesn't matter as long as there's enough to sand.

But I'm going to try to convince myself leave it as is. :lol:
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Re: This curly redwood adequate for acoustic (SS or classical) top?

Postby Alan Carruth » Mon Oct 12, 2015 3:26 pm

I've never trusted curly redwood, and Roger's experience doesn't surprise me.

Years ago one of my students gave me a book by her father, Robert R. Archer, entitled 'Growth Stresses and Strains in Trees'. One thing he talked about was that trees put on new wood in tension relative to the wood underneath. This is to avoid having wood in compression on the downwind side, since green wood tends to fail first in compression. The new wood in tension around the outside of the tree builds up compression forces in the center, which is not really a problem since there is not a lot of bending load there. In very large trees the weight and built in compression stress can actually exceed the strength of the wood in compression, and the cells will fracture on a microscopic scale. Even though you can't see it, the wood is still broken. and with break across the piece if you try to bend it at all.

I've seen a number of pieces of redwood that looked, for want of a better term, as if they'd been 'squashed'.They have some degree of curl in some cases, but more often have an irregular figure of changing run out. They are frequently very weak in bending. Also, although most redwood I've seen has had very low damping, with a long ringing tap tone and a well defined pitch, the squashed stuff tends to be cardboardy. I have to wonder if it's an example of the sort of thing that Archer talked about.

Even if that's not the case with the usual curly stuff, just the fact that it's got areas of very high run out is an issue. Redwood has, in my experience, a notable tendency to split, and short grain in the soundboard seems like a bad idea in that case.
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Re: This curly redwood adequate for acoustic (SS or classical) top?

Postby Rodger Knox » Tue Oct 13, 2015 3:14 pm

Alan Carruth wrote:I've never trusted curly redwood, and Roger's experience doesn't surprise me.


Sadly, it didn't surprise me either, I had been warned about curly redwood. The piece was light and stiff, both along and across the grain, and I thought I flexed it pretty hard along the grain to see if it would fail. I thought that after a couple of months of string tension it was OK, but then this crack appeared.
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