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I've just bent my first set of sides using a home made bending machine following the John Mayes dvd's. Everything has gone well with the bending except that the sides have sprung in all sorts of directions and won't fit the outside mould. I've made the bending machne mould about 3/16" inside the shape of my outside mould.

Does it need to be shaped to compensate for the springback, and if so on which parts of the mould?

Thanks for all the suggestions for my last problem (humidity) I think I've got that sorted.

The bending form should be thickness of the side+ thickness of a slat if you put your heating blanket on the out side + maybe a little at the end of each bout for over bend/ add blanket thickness to this if you put your blanket on the inside.

Now be aware. Spring back is normal on most hard woods. Usually there is not enough so that you have problems fitting it into the assembly mold and pushing it flush using inside cauls. There is a good bit of leaway due to the flexible nature of wood.

If you leave your bent sides clamped up inside the bending machine for 24 hours after you bend you will eliminate most of the spring back. the need to have the bend set by cooling while under the load of the bending machine. It takes a while for the wood to set. they need to equalize to room temp for this set to take place. Even then thee is a bit of spring back. If you take them out of the bender while hot or warm right after bending get them in the assembly mold right away or they will spring back a bunch.

The bending form should account for the thickness of the side+ thickness of a slat if you put your heating blanket on the out side + maybe a little at the end of each bout for over bend/ add blanket thickness to this if you put your blanket on the inside.

Now be aware. Spring back is normal on most hard woods. Usually there is not enough so that you have problems fitting it into the assembly mold and pushing it flush using inside cauls. There is a good bit of leaway due to the flexible nature of wood.

If you leave your bent sides clamped up inside the bending machine for 24 hours after you bend you will eliminate most of the spring back. the need to have the bend set by cooling while under the load of the bending machine. It takes a while for the wood to set. they need to equalize to room temp for this set to take place. Even then thee is a bit of spring back. If you take them out of the bender while hot or warm right after bending get them in the assembly mold right away or they will spring back a bunch.

A little springback is to be expected. You can minimize it by letting the sides cool in the bender and heating a second time. Be sure to allow them to cool before removing and clamp into your outside mold immediately. Also, the hotter you can go (without scorching, of course), the less springback.

Compensating the bending form works too. I've done it by trial and error. Not too hard to figure out.

Another option is to touch up by hand after the side comes out of the bender.

Good luck.

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And in addition to the very good advice provided by my bros above consider this:

First, what thickness did you bend at?  Many of us bend any where from .075 - .090.  If you bent to thick this will contribute to spring back as well as increase the risk of cracking.

Also how much heat did you use and for how long.  IMHO many issues with spring back can be eliminated by bending hotter.  Are you using a blanket or bulbs or both?

And lastly, concentrating the heat and steam from lightly misted wood is helpful too.  So what does your stack of wood, slats, blanket if you use one look like?

Typically when I have more spring back then usual I did not heat long enough or hot enough.

I also agree with cooling AND completely cooling, twice - it is very helpful.

When I make my bending form, I compensate the waist by 1/8" and both ends by 1/2", this is after I remove the thickness of the material, slat and blanket (I bend with the blanket on the inside). When my sides are finished, they fit to my body mold perfectly with less than 1/16" of spring back. Depending on the wood, it can even be slightly over bent like 1/8".

You don't want to have a lot of spring back to your sides otherwise there will be stress in the sides that don't need to be there which may lead to all sorts of future issues (though it may not be confirmed by the induced stress of forcing the sides into shape). By compensating the bending form you can eliminate spring back almost entirely, there is no exact number though and what I've done may be considered excessive to others.

If you've followed John's DVD you should have hit all the pertinent issues that the others have mentioned, so maybe it's just the type of wood, or the misalignment of the stars or a black cat running through the shop or.......

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OK... I know I do thing a bit weird, but here it is....

I bend by hand over a pipe. I bend the sides until they line up with my pattern. Then I put the sides in my Fox bender, heat them up and let them cool. The form is the exact shape of the guitar, minus the thickness of the slats, etc. The result is a side that fits the mold pretty much exactly, no springback.

For me, bending by hand reduces breakage (I can feel the wood as it bends), and eliminates springback.

Putting the side in the Fox bender after hand bending helps smooth out the curves, reduces any rippling that may have occurred, and insures symmetry.

Mark

That's weird!

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Ok, so everyone has their own recipe to bake a cake....

My bending molds are .100" smaller than the form. No over compensation.

I cycle the bender 3 times. I go as hot as I can without scorching and I let it return all the way to room temperature before reheating. I rarely get any spring back on rosewood, and very little on mahogany.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Mark,

I just finished my first with a luthier who has been in this forever and we bent it same way, set up the initial bends on a heating iron close to the mold, and then over to the fox bender for final shaping.

Since this was my first and it is the only way I know how to bend, am I weird too?

I was hoping to turn out to be a cool luthier guy. (Though weird would be OK if I could end up with something half as good as yours, I'ld even settle for an 1/8 as good)

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Kurt Thomas

"There's a first time for everything even if you do it by mistake."

I bend my waist bend over a pipe, then toss it on the bender for the easy parts....

Springback, not much, I compensate my forms at head 3/8" and tail 1/2".

I yank 'em and clamp in the mold and let 'em cool.

It all works, heck I saw a guy bending over the tailpipe of his truck the other day!      That was right before he keeled over from CO poisoning.... (not recommended)

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http://www.dickeyguitars.com

I spray my sides with SuperSoft II the day before, then let them dry out flat overnight. The next day,I spritz the wood with plain water and then I line up the waist in the bender with the blanket on top of the wood and a slat under the wood and over the blanket. I tighten down the waist just enough to keep it from moving on me, then turn on the heat. I heat at medium high until I can feel the heat on the underside of the wood, then clamp down the waist......s....l....o......w.....l....y. When that is all the way down, I bend the lower bout and then bend the upper bout. I leave it on medium high heat for 15 minutes, then let it cool for about 2 hours. Then I reheat it again for 15 minutes and then let it cool to room temperature (over 2 hours).

So far I have had really good luck with no breakage and very little if any ripples or springback.

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Reguards,

Ken H

Lance and I know a guy who is successfully bending his sides at .150 thick.......  No prebending, no SuperSoft.

Thanks for the advice.

I used John Mayes sandwich method- blanket,slat, wood, slat and blanket. I left the first side in the bender for about 2 hrs, then put it straight into the outside mold. The 2nd side stayed in the bender  overnight, then got put into the mold. I left them there for two days, but when I took them out they sprang out about an inch at each end and the middle bout had also moved.

My sides were rosewood at 80/1000 , and I took them to about 320 degrees then down to 290 cycling four times.

Charles.

That is a bit unusual to spring back that much.

I would suggest that you get your sides joined with in a 24 hours at least of taking them out of the bender. All wood will want to return to their normal state if left unsupported. Of course they will not ever completely return to flat once bent

I use my blanket on top one slat above and one below. I have accounted for the thickness of my slat .09 for the wood and .125 over bend at the ends of the upper and lower bouts.

I bend and cool for 10 min. keep clamped in the bender over night. and place in my assembly mold supported with internal cauls at the waist and center of lower bout and some shapes like the SJ and MJ also use an upper bout caul. Once the second side has been bent, cooked and set over night clamped up in the bender; I then put it in the assembly mold mark up the trim cuts, take the sides out and trim them and install my blocks. i never allow my sides to be unsupported after the bends cor more than just a moment to trim them.

to me It sounded like maybe you took the temp at the blanket and not the wood there by not actually having the wood to full temp at the bend and therefore the bend did not set as well. but i don't know as I was not there. rose wood is usually a pretty cooperative wood to work with and takes the bend quite well.

That is a bit unusual to spring back that much.

I would suggest that you get your sides joined with in a 24 hours at least of taking them out of the bender. All wood will want to return to their normal state if left unsupported. Of course they will not ever completely return to flat once bent

I use my blanket on top one slat above and one below. I have accounted for the thickness of my slat .09 for the wood and .125 over bend at the ends of the upper and lower bouts.

I bend and COOK for 10 min. keep clamped in the bender over night. and place in my assembly mold supported with internal cauls at the waist and center of lower bout and some shapes like the SJ and MJ also use an upper bout caul. Once the second side has been bent, cooked and set over night clamped up in the bender; I then put it in the assembly mold mark up the trim cuts, take the sides out and trim them and install my blocks. i never allow my sides to be unsupported after the bends cor more than just a moment to trim them.

to me It sounded like maybe you took the temp at the blanket and not the wood there by not actually having the wood to full temp at the bend and therefore the bend did not set as well. but i don't know as I was not there. rose wood is usually a pretty cooperative wood to work with and takes the bend quite well.

[QUOTE=Bruce Dickey]I bend my waist bend over a pipe, then toss it on the bender for the easy parts....

Springback, not much, I compensate my forms at head 3/8" and tail 1/2".

I yank 'em and clamp in the mold and let 'em cool.

It all works, heck I saw a guy bending over the tailpipe of his truck the other day!      That was right before he keeled over from CO poisoning.... (not recommended) [/QUOTE]

Bruce,

Thank you for your concern. You'll be pleased to know that I'm back on my feet. Doc says I should be bending sides again soon. (Me thinks he likes repeat customers!)

J.R.