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Springback ?’s
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Author:  JJ Donohue [ Fri Oct 20, 2006 12:00 am ]
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There were some comments recently referring to springback following bending. Some folks commented that they re-bend or reheat in order to reduce the amount of springback.

I'm concerned that I may not be sensitive enough to recognize how to quantify how much is too much. Following my bends, the sides never just follow the contour of the mold in the relaxed state. I would consider that condition as having zero springback. Instead, we coax them to conform to the shape by forcing them to accept the contour with clamps and spreaders. On all of my bends, there is usually a small gap (not more than 1/16") for a short distance below the peak of the waist area. Is that springback...and is that amount too much?

So how do others quantify the amount of springback and when is it too much?

Author:  Michael Dale Payne [ Fri Oct 20, 2006 12:24 am ]
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JJ, I can only tell you that if I feel the side is in a significant bind to fit the mold that I will re-bend. If you are having to force the upper bout or waist to fit, then the stress on the side is much greater than if you are pulling the lower bout in to fit. This is because the stress of the forced bend is carried over a longer span on the lower bout therefore you can manipulate the lower bout more.

If you use a fox style bender and mold most of the spring back will be at the lower bout and upper bout. That is why those that over bend make the compensation there. The waist is trapped in a fox style bender and the bend sets better there because of this as long as you leave clamped and allow the the fibers to full cool and take the set. But the apex of the larger curves will always want to relax a bit. But this is typically manageable.MichaelP39010.393275463
Author:  tippie53 [ Fri Oct 20, 2006 12:27 am ]
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   Hi JJ
   Springback is the wood wanting to unbend itself. I have found that Mahogany is more apt than most woods for this. If you are experienceing springback you may not be using enough heat.
   THe mold is designed to take this pretty much out of the equation. A proper set in procedure should include final trimming for the set , in the mold. You want to have the sides about 1/32 to 1/16 strong. You can tell when you close the mold as the side will want to lift off the mold at the waist.
You can then use the waist spreader to force it back on. This sets the sides tight against the mold and reduces the amount of spreaders needed for the set. I use just 1 as the waist.
    If you trim the set short you can allways open the tail area and hide that with a tailpiece.
any questions please let me know or call
john hall
blues creek gutiars
Author:  Steve Kinnaird [ Fri Oct 20, 2006 4:38 am ]
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Let me play the devil's advocate here, JJ.
I once attended a lecture by Robert Ruck, a well-respected luthier in every way. During the Q&A after his lecture, someone posed the problem of stresses in one's work, and how much was too much. (I think Art Overholtzer was one of the first champions of stress-free building... and I also think that affected a lot of people's thinking.)
So anyway, the Q. was put to him about the sides conforming to the mold, and what he did w/ springback?
I loved his answer: he just forced those suckers in there. Clamped 'em in place. Of course, Ruck's "springback" is probably much less than most. But it seems that the tension that might introduce to the build was of minor consequence to him.

I found this liberating.

I also hope I have not misrepresented him!

I'm not saying be content w/ lousy bending, but don't over-worry about the situation.

SteveSteve Kinnaird39010.5770023148
Author:  Michael Dale Payne [ Fri Oct 20, 2006 4:48 am ]
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Once the blocks and linings are glued up the rim assembly as a whole becomes ridgid and the force of the sides wanting to spring out is constrained there is no issue left. The risk of exsessive spring back is in getting the sides in, flush and square with the mold.

Now a racked side assembly is another issue al togather
Author:  Marc [ Fri Oct 20, 2006 8:34 am ]
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I get about 1 inch of springback at each end using a normal form. It would be nice to have a perfect bend with zero springback. In my case, classicals on a solera, I would like to be able to glue the top to the sides using hide glue and solid linings. Having to fight the spring back cost time and makes it difficult get everything in place in under 1 minute. So for now I use tentellones or kerfed, instead of the solid.

I tried a compensated form with a little extra bend at the ends, it worked better for the curves but the upper bout ends no longer enter the heel at the correct angle. They are slightly overbent making them prone to split while forcing them into the heel.
Author:  LuthierSupplier [ Fri Oct 20, 2006 8:52 am ]
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I recently tried a new method of bending for my 2 ukuleles, and I had 0 spring back. I used a thicker mold, about 4 inch thick. I used the inside cutout of the mold to push the sides into the outside part of the mold. I tried it without soaking the wood, and I toasted the sides. I then soaked the wood(koa) for 2 hours, then sandwiched in metal, blanket, side, blanket, metal, then heated for 2 minutes. When I saw the steam and sizzling of the sides I started to push into the outside mold using the inside cutout piece. I also used a quick grip to get it clamped tight. The whole procedure was about 2 min & 30 seconds. I turned it off and waited 2 minutes, then turned it back on for another 1 1/2 min. Turned it off and let cool for 10 min, then placed in the mold and clamped. Very very little springback with this method. A local builder showed me this technique, and he has built over 300 guitars, and has never broke a side using this method. I was amazed! Here are some pics:



Tracy
Author:  Colin S [ Fri Oct 20, 2006 9:13 am ]
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If you are finding springback a problem, as John said, especially with mahogany, then you want to cook at a higher temperature, and for a couple of cycles. Also you can get the side back up to temperature, quickly take it out of the bender (wear thick gloves) and get it into the mould and tighten the cauls up. It should then cool down to the shape of the mould. I usually do this with mahogany but don't with most other woods.

A little springback doesn't matter once the end blocks and linings are in it will be rigid. Also the amount of stress induced in minimal, with the side out see what finger pressure is need to take out that small amount of springback and compare it to the 180Ib or so of string tension your going to put on there!

Colin

Author:  mikev [ Fri Oct 20, 2006 11:29 am ]
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Tracy, I was about to build a mold this weekend, and the style you are using does look rather interestin,, and heavy . Unless you hollowed it out. But anyway, you say you use two blankets??? Do you feel that is needed with this style.. It sure looks simple, and I think it could be modified for a cutaway,, hmm I'll have to look at that.. Maybe it would have to hinge.
Mike
Author:  crowduck [ Fri Oct 20, 2006 1:15 pm ]
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That thick 2 piece bending mold that Tracy shows is similar to a rig made and sold by a guy called mandomaniac.

CrowDuck
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