Slight Cupping In Maple Sides

[Steve Gonwa]

I successfully bent a pair of curly maple sides in a fox-style bender using a heating blanket. After being in the form for a period of several weeks and thoroughly dried, I noticed that the sides have cupped slightly across their width, mostly in the lower bout. Is there a recommended way to re-flatten the sides in these areas ?

I do have a bending iron but am not sure of the proper method to correct the cupping issue.

I have also seen it suggested to use a bending blanket in the form, one side at a time, to reheat the wood and use metal slats to help re-flatten the side. If this is an option, suggestions on a correct procedure would be helpful.

I would say the greatest amount of cupping is about 1/16 inch across the width, maybe 3/32. Thanks for any advice.

[Chuck Tweedy]

Puckering out, or sucking in?

[Steve Gonwa]

The center of the side is flat against the form, but the upper inch or so at the edges are cupping towards the inside. Sucking in I guess !

[Chuck Tweedy]

Picture would be the best way to see, but that does not sound too bad to me.
Once it is boxed up and bound, you may not be able to tell at all.

There is almost no way to fix this. You can only do everything in your power to keep it from happening in the first place.
Are the sides well quartered? Flatsawn sides can get really squirrly.

[Michael Lewis]

Oh yes you can. You can correct any section or all of a side with your blanket and slats of metal in your body form (not the Fox bender). Put one slat in the form, then the side to be corrected, a layer of paper, the blanket, and finally the other slat. Oh yeah, spritz the side before you put it in the form. Use clamps and sturdy sticks(one for each clamp, laid across the slat) to clamp the whole thing against the inside of the form. Once heated bring the clamps to tension starting at one end of the 'correction' and continuing to the other end. Or you can start in the middle and work out in both directions, but don't clamp both ends and then try to clamp the middle. Use as many clamps and sticks as you need to make the side conform to the shape you want.

The reason for using two metal slats is to force the wood to conform to the shape they hold, which is pretty flat across the side, meaning no ripples or cupping. You have to clamp it firmly or you may retain your ripples.

[Chuck Tweedy]

I've re-bent sides in the bender, and they come out of the bender looking great.
But once re-equalizing they went back to cupped/rippled.
I get a better bend (less springback), but it does (did) not correct cupping or ripple along the grain.

I'm pretty sure my wood was bad.

[Doug Shaker]

I laminate my sides with hardwood on the outside and softwood on the inside, gluing them and putting them in a mold after they have been bent. This is way more trouble, but so far (and it ain't far - I'm a beginner) I haven't ever gotten wobbly sides.

It may be that side splints with a little more height than usual (for extra stiffness) could straighten the sides sufficiently. One might apply such splints with hide glue now and see how they work. If they don't work, one could trash the sides or go to lamination. If they do work, one could easily remove them and reapply them as needed later.

Slight cupping outwards doesn't look so bad. Cupping inwards does.

[Michael Lewis]

Chuck, if you force the sides into conformation when hot and keep them there they will stay there when cooled. I have not been able to do this in a Fox bender because I can't get enough pressure against the sides with a drawstring or springs. It takes clamps and some pressure to force the ripples out of the sides, and that is why it is best to do it in the body form where you can get a good amount of clamping pressure. When heated and forced with C clamps against the metal on the inside of the body form and allowed to cool, the surface of the wood when removed from the form will be very smooth and straight across the side.

I generally use a Fox side bender most of the time, and sometimes the sides turn out fine, but when they don't I depend on the method I just explained to fix any unevenness. It is also a good way to repair any areas that start separating at the curl. As long as enough fibers are still holding the pieces together so all the fibers align well, you can make a nearly invisible repair and save an otherwise wasted side. Heat and form it first and let it cool enough to handle, add hot hide glue to the affected area and put it back in the form and clamp it over night. When dry and sanded the repair is usually invisible.

[Randy Roberts]

After being in the form for a period of several weeks and thoroughly dried

The first thing I'd ask is how wet were the sides when bent. I would suggest that the wetter the wood the more likely to get problems, especially with figured wood. I'm convinced that at the thicknesses used for sides, most woods will bend just fine totally dry. Of course there will be exceptions (Spanish cedar for one.)

[Steve Gonwa]

Thanks for the responses from everyone. I suspect had I continued the construction process right away insteads of leaving the sides in the mold for a few weeks after bending, the cupping may have been avoided. But from the responses I think I can remedy the problem. Thanks.