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| Bending Form Dilemma http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=29811 |
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| Author: | woodbutcher50 [ Sun Nov 07, 2010 11:09 am ] |
| Post subject: | Bending Form Dilemma |
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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| Author: | runamuck [ Sun Nov 07, 2010 11:52 am ] |
| Post subject: | Re: Bending Form Dilemma |
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. |
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| Author: | woodbutcher50 [ Sun Nov 07, 2010 12:17 pm ] |
| Post subject: | Re: Bending Form Dilemma |
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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| Author: | TonyKarol [ Sun Nov 07, 2010 12:22 pm ] |
| Post subject: | Re: Bending Form Dilemma |
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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| Author: | woodbutcher50 [ Sun Nov 07, 2010 2:51 pm ] |
| Post subject: | Re: Bending Form Dilemma |
Thanks for the responses. I had really felt the same way, but couldn't seem to persuade him otherwise. ![[headinwall]](./images/smilies/headbangwalluf8.gif "Mad") ![[uncle]](./images/smilies/surrenderflagqv6.gif "Uncle") 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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| Author: | John Killin [ Sun Nov 07, 2010 8:58 pm ] |
| Post subject: | Re: Bending Form Dilemma |
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 |
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| Author: | the Padma [ Sun Nov 07, 2010 9:52 pm ] |
| Post subject: | Re: Bending Form Dilemma |
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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| Author: | bluescreek [ Mon Nov 08, 2010 7:02 am ] |
| Post subject: | Re: Bending Form Dilemma |
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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| Author: | woodbutcher50 [ Mon Nov 08, 2010 9:17 am ] |
| Post subject: | Re: Bending Form Dilemma |
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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