Rib Bending/ Avoiding spring-back

[Dr Rod]

Hi friends


I just bent a black poplar rib, let it dry overnight clamped on the mould (about 18 hours in a very dry northern city).

Well, unfortunately I got quite a bit of spring-back, I was wondering what you think I might have done wrong.


Thanks


Rod

[Don Harris]

Was the rib holding the shape before you clamped it? I think I've read that 10% or so of spring back is normal so you may have to overbend a bit and then clamp. I did some touch up before glueing to get everything to fit without tension.

[Don Harris]

No problems with the black poplar creasing in the tight bends?

[Dr Rod]

thanks Don

I actually bent it with the clamps onto the mold, while heating and brushing with water (my wife helped). I didn't have a bending iron and wanted to experiment/learn.

If over-bending is the solution, guess I could not do it unless I had a bending iron.

and yes, the black poplar was not such an easy wood to learn with, but I certainly enjoyed the experience. There was creasing in the tight bends and there was a knot that really didn't want to cooperate.

[Matthew Tucker]

how did you heat the ribs?

[Dr Rod]

please don't laugh, I used a clothes iron.

[jimmyduded]

my guess would be not enough heat to bend, an iron is also very hard to manuver around a mold so im guessing there were a few spots that did not bend too well thus resulting in a spring back

[Dr Rod]

Originally Posted by jimmyduded

my guess would be not enough heat to bend

It bent really well at around mid-length, then towards both extremes I could not get a lasting bend.

[Darren Molnar]

Clothes iron, aye, there's the rub ( maybe ). It would be tuff to get a good even heat in the bending area, around the tightest radius corners. Because of the flatness of the iron.
when I'm bending, I like to visualize the heat as if I'm wearing heat seeking gogles. The point of contact of the iron would be the red-est, and losses color rapidly the further from the point of contact. With the clothes iron, at the tightest radii, you're only touching the edge of the iron with the wood. Super-heating that sharp area, only warming the area right next to it.Picture the infra red image.
Ideally, the area of the radius you would like to bend, should be the same temperature all over.
wet wood cools very fast, and I think the temp range for bending is narrow. Which might mean that using the iron in an "ironing" motion, trying to heat the area evenly by moving the heat source, might not work well.
So the iron was effective enough to warm the wood in those areas, allowing enough deformation to get the clamps on, but not heated to the point of allowing a set of the fibers into a permanent bend.

[Darren Molnar]

I may have completely miss-interpreted Dr. rods bending method. I was thinking that you were bending the ribs into the final mold. meaning iron on the outside of the ribs, iron on the inside of the bending radius. That's what my reply was reffering to.

Did I miss the bus?

[1st Bass]

You could try a heat gun, but be careful--you can easily scorch your ribs.

I made my bending iron of a piece of 8" heavy-wall aluminum tube. I had a friend mash it slightly with a press, to achieve an egg-shape. The aluminum did not appreciate the attention, and cracked along the pointy end of the egg-shape, so I welded up the crack, and also welded a flat plate protruding from the bottom end to be gripped in the vise, and another flat plate completely closing the top end.

Then I drilled several holes in the top plate to allow the exhaust of a propane torch to escape, and I was ready to go...sort of. It turned out I should have drilled more holes in the top. The heat I would like to have would take two torches, but when I put the second torch in place both torches were instantly extinguished.

So I am considering cutting a 2" hole in the top and welding in a section of 2" aluminum pipe, to simultaneously allow a full flow of exhaust and provide a tighter radius for when I want to make a violin-cornered bass instead of the gamba-style.

As it is, I use one torch, and it takes about 20 minutes to get up to operating temperature, but I don't scorch the wood either, so I guess that is OK.

[Dr Rod]

Originally Posted by Darren Molnar

I may have completely miss-interpreted Dr. rods bending method. I was thinking that you were bending the ribs into the final mold. meaning iron on the outside of the ribs

Thanks Darren, you understood correctly this much (above quote)


but I don't know exactly what you meant by "iron on the inside of the bending radius"

[Dr Rod]

Originally Posted by 1st Bass

You could try a heat gun, but be careful--you can easily scorch your ribs.

Thanks for sharing your ideas 1st Bass.

do heat guns blow hot air? or do they radiate heat?

hot air could burn your hands, I'm guessing. But I like the simplicity of the idea.

[ctregan]

Check out this link: http://www.youtube.com/v/ebu_TbRIKaI

[ctregan]

http://www.youtube.com/swf/l.swf?vid...xnLwdJzQjcQyDP
This shows some rib bending also.

[Dr Rod]

those linings bent pretty quickly, and I didn't notice any steam.

[Matthew Tucker]

It's the heat more than the steam.

Wood structure: a large number of short pieces of small pipes glued together, all in a parallel direction. The short pieces of pipe are called wood fibers.
The fiber pipes are "glued" to each other by a substance called lignin. Lignin softens at the temperature of about 170 degrees centigrade. Moisture lowers the softening temperature below water boiling point.

The steam helps get the heat through into the wood, allows bending at a lower temperature and protects a bit from scorching, but it is not essential.

[Darren Molnar]

What Mathew said. When i'm bending violin/viola stuff, linings included, I'm not using water at all. The wood is thin enough to heat through efficiently.

I think it's not good to think of the water as "softening" the wood, making it easier to bend.( I think a lot think that way) It doesn't, it's really just a heat carrier. Less is more, esp. with highly flamed woods.