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I have one of LMI's new design bending machines and I have had cupping on every set that I have bent with it. All have been different rosewoods, but they were straight and vertical grained. The cupping is between the waist and lower bout. I have experienced cupping when bending on a pipe too, but it is worse with the machine.
I have been following LMI's bending instructions. Is this just something that has to be dealt with by scraping or are there magic thickness/moisture/temperature/speed/bake time formulas for each wood species?

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Good judgement comes from experience. Experience comes from poor judgement.

I've had the same problem. I suspect that the slats are too thin to properly support the side in the space between the bending form supports. A set of stainless slats just arrived from John Hall. They are over twice as thick. I'm planning to do a couple sets tomorrow and will report back with the results.

Cheers, M

I don't know how much water you use for bending, but I've gotten much better results with a minimum of water.

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

No experience with the LMI bender. I do like to use a solid form

Even with the solid form I cover it with a piece of fairly heavy (maybe 18 gauge) stainless steel. That piece stays on the form. I counter sink a few screws to keep it in place. I do make a standard sandwich using spring steel slats etc. I place that on the ss covered form and bend. That sort of scheme may help you avoid flat spots.

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Joe Beaver
Maker of Sawdust

Casey,
like a couple of others who have responded, I wonder about the thickness of your slats. How thick are they?

A couple of things to consider.

First is the water issue and I agree that very little water goes a very long way when bending sides. Too much and it can encourage cupping.

I use a Fox style bender and one set of sides that I was bending some years ago, African Mahogany just wanted to cup along the figure no matter what I did. I contacted Uncle Bob (RC Tonewoods) and he turned me on to Matt Mustapick who had had success bending the same wood figure and all. Matt is a great guy and told me that he had modified his bender specifically to help avoid cupping.

His fix was a third spring-loaded hold-down call used about half way down the lower bout. I promptly retrofitted my bender with some springs and another caul from our friend John Hall and my problems were solved. Since then I have always used a third caul regardless of if I thought it was necessary and the cupping never happened again.

Too much water encourages cupping too in some situations and with some woods.

Super Soft II which is super easy to use and does not require any rocket science.... also would help make the wood relent to your wishes cupping wise as well.

Some folks like to bake sides long enough to make lasagna but I have had excellent success bending fast, hot, and with minimal water and maximum support from decent, thick slats. My slats came from John too and work very well. No set temps here I simply wait to hear some sizzling and perhaps see some steam and go at it. My bends are usually done in 7 minutes tops but I do leave it in the bender an hour with the blanket off until it's completely cool. Then I heat again for five minutes and leave it in the bender overnight.

There is one more potential issue that had reared it's ugly head before on Lutherie forums and it has to do with bending rigs with solid forms. It seems to be the case that more heat and applying heat longer may be necessary with solid forms because the form can act as a heat sink at times. No direct experience with this here I use open forms but I recall the posts and I also recall that it came up every so often.

Related is the issue of case hardening the wood making it even more brittle when leaving the heat on too long. John Hall is my expert on this subject and maybe he will weigh in.

I definitely see cupping when I use water. For my home-made fox-style bender (w/ slats from John Hall) I bend it dry. So far, no cupping.

I did get cupping with water on a pipe. Man, what a hassle.

I was thinking the same. Also, depending on the body shape (have all your problems been with the same form/shape?), you may need to put more pressure at this area you are getting the cupping. I don't know what your bender looks like, but mine is basically an open form and easy to throw an additional clamp across.

Exactly

Something I find that will help me flatten a side, my least favourite task, is a substantial side brace. There is a noticeable difference before and after glueing them in.

On the ends of my top slat there are attachments that take bungee straps that pull straight down and they hold both bouts tight. Minimal water, as this seems to be the main culprit. SS helps. Some wood is crappy and is hard to do without some form of problem. African Mahogany falls in this category sometimes. Grain that alternates,especially if it is not narrow and evenly spaced I've found to be the hardest. Two heating cycles, then leave in tension till cool.
Tom

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

Ribbon figured Mahogany African or other Ribbon figure material can be a real bear to bend because of the structure of the cross linked grain pattern. Most 1/4 sawn materials tend to lay out flat, not the case with some ribbon figured billets. And as indicated above the wrong moisture application can make the striped grain -- well, go wild! I believe this is due to the uneven absorption rate of the different parts of the ribbon pattern. So while there are woods that bend like butter when moistened I have had the best luck with the Ribbon Hogs bent dry or at the other end of the spectrum, boiled then clamped securely to a solid form in the Sloane or David Russel Young fashion.

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Ken Cierp

http://www.kennethmichaelguitars.com/

I used my new stainless slats on a mahogany side and am pleased to say that the problem I was having with cupping appears to be solved. My bender, like the Lmii bender, uses 3 longitudinal supports for the bending form. I believe my sides were collapsing into the gaps between the supports due to inadequate slat stiffness, leaving a wavy side north and south of the waist. The Lmii slats are only .008". The new tempered stainless slats are .020". I've always been able to sand the areas flat, but that took extra foolin around.

Happy camper! M

Ends of my top slat look like this and keep the slat tight against the side from the tension of the bungee cord.
Tom

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

I use a stainless sheet over the wood and clamp it with a little pull or pressure from the de staco pull clamp. It works fine and I get a nice flat bend in the side with no cupping.

michael keller
www.kellerguitars.com