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I am helping a friend with his first build.

When building his forms, he made the decision to inset the 1/2" rods about 1/2" in from the edge of the form. His rationale was that the .010" tempered steel would be stiff enough, since it was bent and couldn't form a compound curve.

So when the sides came out the bender, they were cupped. Concave at the ends and in the flatter area between the waist and the lower bout. About a 3/64" in the width of the side.

Also, a convex bump between the waist and the upper bout.

I wasn't with him during the bending process, so cannot verify if his technique was a factor.

Was interested to know if anyone could confirm if the mold construction was the main problem.

Now that they are what they are, I am looking for the best solution to cure it, if possible.

I have offered to attempt flattening out the cupping on a pipe bender, which I I have used enough to be reasonably proficient, but am not sure if it will be capable of correcting the issue.

Any advice would be appreciated.

Thanks,

Mark

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A world without music would Bb.

I think your friends' rationale is incorrect.

I'd suggest fixing the form, making the rods flush, and then re-bending the sides.

I have a question though: did he get the wood very wet before bending? That can cause cupping too.

Yes, Jim. He did mention that he had soaked it pretty good. Certainly more than I would consider necessary.

BTW, his bender is a light bulb bender, built as a copy of the one I use.

So your suggestion is to put them back in the corrected bender?

Thanks,

Mark

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A world without music would Bb.

I use 10 thou slats .. and there is no way I would ever trust them to stay flat across the shape without support when bending - he needs to fix the form. The rods are there for a purpose. even with them you may get some cupping - when making new molds I always go solid now .. it simply works better.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Thanks for the responses. I had really felt the same way, but couldn't seem to persuade him otherwise.

I am hoping to get some opinions on the best method of repair. Either rebend in a corrected mold, which is what Jim suggested or use a pipe bender which was my first thought.

Thanks,

Mark

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A world without music would Bb.

On the first and only set of sides I bent I didn't support the wood in between the sides of the form. I got some serious cupping. I ended up adding some support to the form and re-bending. It worked pretty well. This was with a light bulb bender for Uke sides. keep in mind with what I am saying that I have only successfully bent one set of sides. Hopefully that changes soon.

In this thread Todd suggests a way that I think will eliminate cupping without going full out solid form. I'm in the process of working this out on my new form now.
viewtopic.php?f=10101&t=29802

Cupping is good!

Yes, is true. If it wern't for cupping you would never learn how to do it the right way....

simple eh!


blessings
Padma

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Audiences and dispensations on Thursdays ~ by appointment only.


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Don't soak the wood ,that isn't needed a light spray of water is all you need. Also is this rift or flat sawn ? That won't help matters. It won't matter if you used .020 thick if the wood is going to cup , it will if you have bad wood. It is normal to have some cupping so plan for it. A .010 cup is fine that will sand out a .030 is too much and you have to use some tricks of the trade to make them appear flat.
I use a .015 tempered stainless slat , these will hold very well on a curve. Shim stock stainless is a different alloy and there is a difference between 301 and 302 stainless when it comes to hardness , flexibility and support.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Thanks for the info, John.

The sides are a beautiful set of quartersawn EIR with an excellent match to the back, so I am really motivated to find the best way to save them.

Mark

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A world without music would Bb.