How to avoid damaging back when bending with dry heat?

[Stickhead]

Sometimes when I bend bows on the caul using a heat gun, the surface of the back crushes and gets wrinkled at the sharper bends near the tips.  I'm not talking severe recurves, just flipped tips.  I haven't had this problem with Osage, but I have had it with elm and yew.  Does this mean I'm forcing it too much?  Not applying heat long enough?  Too much or too little heat on the belly?  Something else?

Thanks for any advice.

...Tom

[DC]

I'm guessing because I don't have those woods but maybe too much bend for dry heat. Apparently Osage bends easily so you might be just at that point where the sage will bend and the elm won't.

[PatM]

Steam them and eliminate the problem.

[PaSteve]

 Yup. What Pat M said. Or boil them.

[lonbow]

You can fix the bend with dry heat after steaming!

[Santanasaur]

I’ve had something like this happen when bending hickory into tight curves. The backside wrinkled in an even wiggly pattern just like it was curly figured wood. I haven’t had this happen with dry heat, just steaming. I also haven’t noticed an issue with this wood. After locking the bend with dry heat it seems to hold up just fine.

[simk]

aluminium tape sorry Tom, wrong answer  did only read the title of your posting 

But: I've had this same wrinkling problem when bending sharp recurves on yew using steam - I really steamed intense and long with the steamer. I said to myself: Too sharp bend of too thick wood. So I don't think it's a matter of steam or dry heat...rather a problem of some specific wood.
cheers

[Stickhead]

Well, I’ve probably been trying to bend wood that’s too thick.  I’ll try steam and thinner wood.
Thanks, all.

[hoosierf]

I generally boil tips then set up a jig and use a torch to set the bend and force out moisture.  The only wrinkles using this process was on an HHB bow but it turned out fine with no long term issues.

[Stickhead]

Just out of curiosity, I did an experiment to compare with my last dry heat wrinkle problem.  All things being equal (same elm, same bow design, dry heat, same caul, same amount of bend), I just thinned a new one out more before trying to bend it.  The last one that had the wrinkle problem was rather thick, about 3/4".  This one was about 5/8" thick, and it bent with no wrinkles or kinks.

So, I conclude from from y'all that steam/boiling is safer than dry heat to bend these woods, and I also conclude from my own experiment that thinner wood is also safer when using dry heat.

[DC]

According to everything that I've read all the bend comes from crushing the wood on the inside of the curve so it stands to reason that the thicker the wood, the more it has to crush. all that wood has to go somewhere so it crumples up. It also makes me think that if it's the inside of the curve that does all the moving that maybe we should be heating the inside of the bend. That would be the back of the bow so I think that's pushing it a bit It may be an idea to heat more than just the belly side. Maybe a little heat on the back. Maybe that's why steam works better, it heats the whole thing. Just thinkin' out loud

[Allyn T]

According to everything that I've read all the bend comes from crushing the wood on the inside of the curve so it stands to reason that the thicker the wood, the more it has to crush. all that wood has to go somewhere so it crumples up. It also makes me think that if it's the inside of the curve that does all the moving that maybe we should be heating the inside of the bend. That would be the back of the bow so I think that's pushing it a bit It may be an idea to heat more than just the belly side. Maybe a little heat on the back. Maybe that's why steam works better, it heats the whole thing. Just thinkin' out loud

That sounds logical to me

[PatM]

It is.   But heat allows more of a squeezing of the cells rather than  true crushing.

  DC you can see how your thought works with bending bamboo backing to fit a curve.   That's why I suggested heating the inside of the curve.

[gutpile]

when steaming or boiling remember do not correct minor imperfections with dry heat until the wood has stabilized.. you are just asking for it to check badly... gut

[DC]

It is.   But heat allows more of a squeezing of the cells rather than  true crushing.

  DC you can see how your thought works with bending bamboo backing to fit a curve.   That's why I suggested heating the inside of the curve.

That has been a real help to bending the boo tips. The boo doesn't scorch or cup at the edges nearly as easily as when you heat the other side. Describing this gets complicated when the outside of the boo is the inside of the curve Anyway thanks for that.