Sides cupping when bending. what to do?

[Taylor Ham]

Bending on an iron, not a blanket/form setup.

cupped approximately .03" with sides 4" deep, toward the center of each arc.

I have tried easing the curves slightly and then re-bending, to no success.

As bad as .05 in one spot at the bottom of the lower bout, strangely not on the tightest curve.

I am tempted to leave the portions at .03" to be supported by the side braces and just focus on the most deformed portion, to meet my deadline.

[John Arnold]

How thick are the sides? If they are thick enough, concave sides can be scraped or sanded to flatten them, since the thinned edges are supported by the kerfing. Substantial side braces (up to 3/16" thick by 5/8" wide from a stiff hardwood) will help, but avoid the easy installation where they butt against the kerfing. That will invite cracking at the butt joint. Either cut the braces to fit over the kerfing (works best with triangular kerfing), or notch out the kerfing to accept the braces.

[Taylor Ham]

I left the sides relatively thick at .08"+. Probably a mistake. I increased the temp of the iron, eased, and rebent. problem solved.

[Shuksan]

Originally Posted by Taylor Ham

I left the sides relatively thick at .08"+. Probably a mistake. I increased the temp of the iron, eased, and rebent. problem solved.

Thanks for posting your solution to the problem. I had a set of sides that cupped more than could be dealt with by sanding which I fixed with hefty side braces as John mentioned above. If I run into that problem again I'll try what you did first to see if that fixes it.

0.080" is what I routinely thickness my sides to and lots of other builders who have given thickness numbers for their sides in various forum posts aim for around that thickness too. I go thinner only in tight bends like a Venetian cutaway. So I doubt the thickness was the problem. What kind of wood are your sides and are they flat sawn or quarter sawn? Do they have a lot of figure?

[Alan Carruth]

More heat, less water.....

[Taylor Ham]

Thanks, Alan. It turns out that was important to learn.

I am using mildly flamed maple, which i sanded down most at the tightest curves. I didnt have a power sander of any sort, which made the process quite laborious. the tangent portions I left between .09 and even maybe .10, next time i will try using a scraper and thicknessing better. I guess there's no free lunch as it were.

I started on a venetian cutaway which was very tough. it and the waist involve radiuses tighter than 1.5", since I am building a ~00 size shape.

I ended up turning the iron as hot as was recommended for long term use without damaging the heating element, and misting the sides sparingly.