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I am currently building an acoustic bass using mahogany for the back and sides. I have bent the sides and had they clamped up in the hold for the last few days. My mistake was in making the hold too thin. I used 3 layers of 18mm ply as I would for a normal acoustic.

The trouble is the sides for this are still 6" deep so there was plenty of overhang either side of the hold. I have gone to start scribing the sides today and noticed they have warped on one side, the bottom side, but not the other.

The photo show the level of distortion, certainly too much to sand out. Is there any way to get this back to being warp free?

I don't know a way to get your sides perfectly flat at this point but you can improve the situation quite a bit by being generous with the number and rigidity of side braces. I would recommend installing the braces before the linings so that you can make the braces go all the way to the edges of your sides. Make them rigid enough and clamp them firmly when gluing to take out as much of the side warping as you can. After doing this, you'll still have some ripples but after some sanding you'll likely be within acceptable range. When you are level sanding your finish, keep in mind that your sides aren't perfectly flat and sand accordingly so you don't sand through.

Hope this helps.
Pat

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There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Difficult to say how bad that is because even the mearest glint of light under a straightedge always looks much more than the reality. At that depth of side you would expect a certain amount of cupping/distortion, they are DEEP!
I accept a certain amount of distortion and only scrape it out if it's over a certain limit. Very glossy, ruler flat sides aren't exactly my thing but nor do I like the roller coaster appearance. After you have finished I suspect these will be OK. If you keep the finish to a semi gloss I doubt you'll notice much. If you were getting recurves (it can happen) I would be a lot more concerned.

It does look like something you can work with.

Putting it back in a bending for and reheating of that area may be an option.

If the ply layers are glued in I would heat then and remove.

Then make a one layer ply and use that to smooth out the outer layer. When you reglue the inner ply I would place a shaped cawl where you have the bowing and clamp well.

If none of that works then a side brace clamped firmly in place should do it.

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Joe Beaver
Maker of Sawdust

Lee: Three layers of 18mm, 1.8mm or .018" ply....?? To me these dimensions don't add up so it's hard to comment on your present situation. I assume the sides of the bass are deeper then a regular guitar thus adding to the problem. I also assume you possibly bent with water and glued with water based glue. I would suggest if that is the situation then epoxy might be a better choice next time around. Also the heavy side braces are worth a try.
Tom

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

I think he means that his mold is 54mm deep.

I don't think your hold or mold had anything to do with it. Looks like normal cupping from perhaps a bit too much water. Try, the side braces, I bet you'll find they get things within tolerable range. And don't forget that the upper and lower edges will be backed up with linings...

Iron it flat.



Put it on your form, make sure you have the whole width supported. I also put a piece of paper over the area being ironed with it getting a light mist of water.

Don't know but that might actually make it worse? The iron will be drying out one side of the wood, so you have a 'dry' side and a 'moist' side. That can lead to cupping, just like it does when you wet just one side of a veneer or when you heat one side of a veneer. The normal procedure would be to treat both sides of the veneer the same.

Totally agree -- plus a fully supported mold length and width works best for preventing cupping.

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Ken Cierp

http://www.kennethmichaelguitars.com/

I've had sides as bad, probably worse, as the ones pictured (too much water and rift sawn) that were saved by Pat Hawley's suggestion. In places they were cupped about the same as the thickness of the sides, 2 mm.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

That happened to me once, & I used the side brace technique. I made the brace heavier in one spot that had the worst cupping and made a matching one on the other side of town. It did the job and looked great!

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Pat

Thanks to you all for your comments. I meant 'mold' and not hold - dang auto-correct! So meddling fool is right, a 54mm deep mold. I have only ever used bias tape with HHG for side reinforcements so it would probably never have occurred to me to use something more substantial if you guys hadn't put it out there!