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A few bending questions

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A few bending questions

Postby michael o'malley » Mon Jun 01, 2015 5:47 pm

So I'm working on my first acoustic guitar, a cut rate version of Nick Forster's "long neck" archtop. I've git a neck made and am working on the body, which will be 2 3/4 deep with a spruce top and cherry back and sides.

I've never bent wood before. I made a bending iron out of a piece of pipe and a charcoal starter, with a 600 watt dimmer as a temp. control. I started out with some figured maple sides, and was able to do the non-cutaway side easily, but the cutaway was a disaster. The maple kept scorching and cracking, and finally I gave up and milled some sides out of some very old cherry I was given by my brother in law.

Some questions/observations: the maple bends holds the bend better, when it doesn't crack. The old cherry bends really easily, but doesn't want to hold the bend very well. But I also decreased the heat, because I was getting too much scorching

Am I using too low a heat? I don't want to burn the wood, and the cherry is bending very easily. But it's not wanting to hold the bend. Clamping it in the mold works, but I'm thinking the sides should hold their final shape without the mold--is that right? I don't have a thermometer for my bending iron, and just get it hot enough to sizzle when wet wood is placed agains tit, but not hot enough to easily scorch


This bending business if kind of fun when it isn't extremely frustrating!
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Re: A few bending questions

Postby michael o'malley » Mon Jun 01, 2015 8:27 pm

Also the cherry wood I'm using looks to be quite old and it's not very dense. It likely was stored Ina barn somewhere for years. I'm wondering if that could have an effect on the bending properties
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Re: A few bending questions

Postby Rodger Knox » Tue Jun 02, 2015 12:41 pm

Two important questions, how thick are you trying to bend and how much moisture are you using?
Most woods bend OK at 0.08", and thinner bends easier. I've gone as thin as 0.06" for bloodwood and mesquite.
Some woods bend easier with almost no water, others need to be wetter.
The sides should be close to the correct shape, but they don't need to be perfect. They need to fit in the mold without forcing anything into place.
A man hears what he wants to hear, and disreguards the rest. Paul Simon
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Re: A few bending questions

Postby Dan Smith » Tue Jun 02, 2015 5:25 pm

I'm just learning how to bend.
I placed a thin wet towel over the hot pipe before bending and it eliminated the scorch marks.
Dan
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Re: A few bending questions

Postby Bob Hammond » Wed Jun 03, 2015 9:32 am

I suspect that you're not heating the wood all the way through, and it maybe be helpful to confine the freshly bent side in a form/mold until it cools completely. The wet towel as Dan suggested might be helpful too, for both bending and prevention of scorching.

There is some thought that kiln-dried stock is harder to bend that air-dried.
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Re: A few bending questions

Postby Michael Lewis » Fri Jun 05, 2015 1:06 am

For difficult bends I use a folded over paper towel about 6 or 8 layers thick and full of water. Lay it on the pipe and bend away. The pipe needs to be pretty hot and you have to replenish the water in the towel frequently. This heats the wood thoroughly and also prevents most of the scorching.

The bends don't have to be perfect but should be smooth, and as long as you can get the sides into the proper position in the form you can glue either the top or back plate on to lock it in place. Eventually any stress in the wood will normalize and relax. So go ahead and make your guitar.
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Re: A few bending questions

Postby Alan Carruth » Fri Jun 05, 2015 1:35 pm

A similar question just came up on a list of experienced violin makers, and there was quite a variety of answers. It prompted me to look up a short item by Oliver Rodgers in the Catgut 'Newsletter' #36, Nov. '81. In it he wrote:

"One can think of the structure of wood as consisting of a lose structure of cellulose fibers tightly bound together by a surrounding matrix of lignin. (He does not mention that this matrix also contains a 'filler' of hemicellulose about equal in quantity to the lignin).

...(Lignin) has a softening temperature of about 190F. This varies slightly in different wood species. Above this temperature it creeps slowly when deformed. Lignin begins to discolor noticeably when exposed to temperatures above 300F"

From what I have observed over the years, it seems as though the lignin softening temperature may depend somewhat on the presence of some bound water (which would be held in the hemicellulose). Heating wood for a long period seems to raise the temperature needed to get the lignin to creep, presumably because the water has been driven off. At some point the required temperature rises to the level that causes discoloration, and you're in trouble. Replacing the water helps, of course, but water also seems to weaken the bond between fibers, particularly on the outside of the bend. A back strap helps with this, of course, and it always seems best to put the water on the inside of the bend. This also helps carry heat through the wood.

My students and I have had the best luck with bending a variety of woods with the iron at 300F, and wetting the inside surface. It seems best to heat the wood up as quickly as possible, to avoid drying it out, bend it to shape, and hold it there while it cools. With practice you can chase the hot spot up the rib, moving more slowly where you need a sharper bend. and holding the free end in the correct bend.

Try to get a smooth bend, even if it's not the correct shape. It's usually possible to correct a smooth bend, particularly if it's slightly over bent, but it's much harder to deal with a side that has corners and flat spots. In that case it's best to wait for the wood to fully cool, and go back and bend the flats to match the over bend of the corners, THEN, correct the whole thing by straightening it out. If you straighten out the corner and then bend the flat the corner will just bend again.

Finally, for really reluctant wood, I've taken to using a product called 'Super Soft 2', available from Veneer Suppliers on line. It contains diethylene glycol monethyl ether, and the label says you're to use it in a well ventilated space. I find that on ribs it seems to work best to wet them down with the SS2 and wrap them in plastic for a couple of hours before bending. The wood will stay a bit 'rubbery' for a day or so after bending: it probably takes a while for all the stuff to dry out.
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Re: A few bending questions

Postby Dan Smith » Fri Jun 05, 2015 10:05 pm

I successfully made and bent Mesquite binding after many failures.
What worked for me was a wet piece of cloth placed over the pipe, a thin tin backer strip, and only lightly wetting the wood.
I have read that bending figured wood can be challenging.
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Re: A few bending questions

Postby Mike Conner » Tue Jun 16, 2015 6:06 am

I also use a homemade pipe with the charcoal starter and light dimmer. I insert a long temperature gage normally used for deep fryers (like a turkey fryer) inside the pipe, above the heating element and touching the inside wall of the pipe right about where I'm bending.

I have bent red maple (both plain and curly), walnut and cherry for sides and for bindings:
- sides about 0.075" thick, bindings about 0.065 to 0.070. Any thicker and they break out.
- temperature about 325 deg F
- wipe the wood with a wet washcloth to dampen a few minutes before bending. No soaking - you want some moisture available but curly figure can separate if soaked.
- heat the wood from both sides while "loosening up" the fibers. Keep damp with washcloth. You'll feel it when it relaxes.
- tighter bends like cutaways take sneaking up on, especially walnut. It's hard to make in a single pass.
- Clamp in mold overnight. Small scrap wood cauls can avoid cupping or ripples in the sides while drying.
- Distilled water helped with curly maple - it wanted to discolor using tap water.
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