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 I am getting ready to build a simple bending form and need some help. I have a mold that I can trace to get the basic shape of the form. What I don't know is how to modify the shape to compensate for spring back. In the waist or in the ends that connect to the head and tail block that would make sense to me. Also how much 1/8" more?

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Mistakes are mandatory-learning from them is optional. http://www.opensourceluthiersupply.com

How are you planning on bending your sides? It makes a difference. .

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Aoibeann

Bill, I pull my tail about 3/8", the neck about 1/8th", none at the waist. works for me. Others seem to go exact with no compensation and their sides work too.

If you get in the ball park with your bending, a good form and adjustable cauls will do the job.

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http://www.dickeyguitars.com

 I plan on using a blanket to heat the sides. Are the neck and tail the areas where spring back occurs?

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Mistakes are mandatory-learning from them is optional. http://www.opensourceluthiersupply.com

Most sides with a bunch of springback will look like a spradle-legged M.

The largest areas spring back the most in my opinion.

The waist seems to get all the heat it needs so it's less likely to have springback.

Some folk cook their sides twice before removing from the bender. I do mine in one shot therefore I may have some springback which the outside forms usually take care of nicely.

Another thing you'll experience is different species of sides bend and spring back differently. It's like riding a bicycle, once you get the hang of it, it's no biggie.

I started adding overbend compensation to my latter molds. Earlier molds are exact, still I'm able to continue using them. Good luck. Go for it and tell us what you find.

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http://www.dickeyguitars.com

I had to take out about 1/2 " to the front and back. Best thing to do is bend a set and see where it springs back to. Padouk and MAhogany did the same amount, no matter what I did, (380 degrees, hot iron on top ,steam cloths, 15 mintues 3 times etc etc 

I also did the waist 1/8 which was a mistake. My plan now is to build the basic Male molds, but leave the dowels unscrewed in (not glued) so they can be repositioned. Same with the the metal top, screw it in, not tacked.

Also, if one set springs back more for different woods, use less heat, this is adjustable, but it is a pain if you dont make the mold 're-doable' to some degree. Or you may hit it right off the bat. I sure didnt.

 

I compensate the waist about and eighth and the ends about half and inch.

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John Mayes
http://www.mayesluthier.com

I did the same as John and may have gotten that from him in the past.

One thing you want to remember when making your bending form, your making an inside form so you want to also remove the thickness of the slat, heating blanket (if it's on the bottom) and the wood if your using the outside mold to trace the shape. Than add your compensation to that.

I've got to make a new mold and bending form in the near future, I think I'll make sure and take some good pics of the process and post a new tutorial.

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I have never compensated my bending forms. But I do re-heat the sides and take them out hot, wear thick gloves, and put them into the outside mould to cool and set.

Colin

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good point Rod. Compensate for the blanket/slat and then on top of that I
overbend.

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John Mayes
http://www.mayesluthier.com

We cook, let cool, cook, let cool and leave them overnight or as long as we can.

I'm considering double heat blanket benders.

I also have some beastly benders that came out of the old Harmony factory in Chicago.   They are cast iron and aluminum and were gas fired. Nice antiques at this point. I'll probably donate them to the Museum of Making Music.

I account for slat thickness+ .09 for wood thickness but that is it. I bend , cook for 10 min after bend at 220-230 and stay clamped in the machine for 24 hours or at least over night.