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Re-reading my original post, there is a glaring error. I said "... putting the heat on the tension side allows the requisite dimensional change to be mostly relaxation and stretching of the tension side."

The deformation in bending is primarily and obviously compression and not stretching, regardless of heat distribution. However, there is a distinction between the stretching quantities of very little, and none. At certain bending extremes, having just a little more room to move may well be the difference between success and failure, and this little bit of extra movement can only be afforded by supplementary heat to the tension side. Mind you, one could also make a further and fair argument that that by adding heat to the tension side, one is actually putting more heat toward the compression zone closer to the neutral axis, thus improving plasticity through and through.

I'd like to say I knew this perfectly well, but miswrote in haste. One can perform work successfully and almost automatically based on ideas absorbed long ago. It had been a while since I thought about bending in specifics.

Having said that, some quick reading reconfirmed that wood is indeed stronger in tension, as I indicated, but one does have to distinguish between axial, radial, and tangential axes. Of course all bending done by luthiers is axial, where wood is stronger in tension.

Here is a quote from the book The Testing of Engineering Materials: Davis, Trauxel, Hauck - McGraw Hill 1982, that indicates basically what I said earlier

Therefore, the first visible signs of failure may be in the tensile face even though the wood is stronger in tension than in compression.

Reference to those texts, please. The failure strength of the wood beam under a load from cell rupture is not the same thing as it's failure when heat bending. And it's not a reason to apply heat to the side less likely to fail. On the contrary, I'd suggest that the side more likely to fail under a cold load is the one that is more in need of heat to facilitate the cells slipping instead of rupturing.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

How do you know it is different? Or rather, indicate precisely what the difference is. and I would be similarly grateful for references of any forthcoming answer. Heat bending is indeed bending under load, with heat. And a long plate is nothing more than a lateral collection of connected bars or beams.

Regardless, if I am going to cite something, I need to know exactly what point or points of mine you want me to elaborate or validate by citing. Please be more specific.

Note: you edited more quickly than I could respond to : -) I will need some time to digest your points. But I will quickly say that I cited a source in reference to how failure can be misleading in appearance, just above your post.

Even if you were correct (which I am still ruminating on) there is a big difference between more heat and inordinately more heat. My contention is that conventional bending allows significantly less heat to conduct to the tension side compared to the compression side. In addition, what heat does reach it, is easily conducted away by the backing strip, which being metal, conducts well. I believe in many bending instances, the temperature differential is too great.

Edit: O.K. Howard, I concur. The side more likely to fail under cold load (the compression side) does indeed need more heat.

However, this is hardly proof that the tension side does not benefit from a certain quantity of heat during bending, which may be less than the compression side, but more than that sometimes provided by traditional bending irons and processes.

I enjoy an oblique and tasteful insult as much as anyone, but they cannot be so oblique as to be indecipherable.

I've bent Bloodwood on a pipe, and I'd say that all of the breakage issues I've had have been related to runout, due to the angle of the breaks. However, it repairs like a dream with CA glue. When I bend, I have the best result using little blocks on top of the bend to keep tight contact with the pipe. I usually slide them apart as I sense that the wood is hot enough, and put pressure. This keeps tight pressure on the bend part of the wood. I bend in small increments, take my time, and sometimes it breaks and sometimes not. I generally don't even attempt to bend it at less than 350.

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Waddy

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And what is the typical thickness of bindings, like those mentioned by the OP, and to which all my remarks were directed?

Steve, I'm sorry but I have no experience to help with your problem. If you want help, please ignore the rest of my post.



OK, I'll bite.

This is just my interpretation.

I interpret it as real world personal experience specific to the issue at hand as opposed to siting academic research (with all of it assumptions and simplifications) that merely resembles the issue.



Kevin Looker

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I'm just a guy who builds guitars in his basement.
It's better than playing golf.

That helps. Just for clarification, I have done my share of wood bending, not necessarily guitar bindings, but then I made that plain in my first post. In addition, I have experience bending wood in some unconventional ways not likely attempted by many guitar makers. And it is a mistake to assume technical books are written by people with no practical experience in the domain about which they write. Maybe the odd time, but not universally.

Well Todd, that's thinner than I imagined when I posted my response to the OP. I provided some comments I thought would be helpful to him, like ensuring that sufficient heat reaches both faces. I'd write it just slightly differently now, but what can you do.

That wood is stronger in tension, and usually fails in compression, are not remotely my ideas, but are the observations of wood engineers and material scientists. The full theory of wood bending and failure and every possible exception are obviously way beyond anyones scope to elaborate in a forum, even if one did know it all.

Thanks for all the replies guys. I love the exchange and the wit .

I made the bindings out of dimensional Bloodwood with the purflings glued on with TB prior to slicing and thicknessing to .095. The I hit the old trusty BBQ Pipe for the bend. At that thickness, the pipe internal temp needs to reach about 450-500 degrees before the wood "lets go". Otherwise I'm sitting there for an hour waiting for the slow relax. I've not had any Bloodwood bindings break on me, although, I've seen some potential run-out wannabees trying hard.



Well, at that temp, I get the expected delam; and at the waist, I get the unfortunate binding cramp/crinkle where the surface area of the bindings does not take kindly to being squeezed. Maybe, I need to lam AFTER I bend - but that seems like such a pain in the ....


Questions, for those using pipes or even benders - I assume the same applies - reducing the surface area of the laminations seems to me it would still want to resist reduction and its only option is to crinkle/crimp.

Thanks again for the help. I LOVE Bloodwood bindings. Maybe I need to find a wood that relaxes at lower temps.
Steve

I am still confused as to why one must glue up the binding and purfling straight. What exactly would prevent one from making a mold and gluing up in some rough pre-radius that is approximates the waist? I am positive that doing this would mitigate delamination. Perhaps there is some technical reason I am not aware of, but I would like to know precisely what it is. I mean at first glance, it might appear as more steps or labour, but if it prevented delamination...

Just read this on a second read:

Well, at that temp, I get the expected delam; and at the waist, I get the unfortunate binding cramp/crinkle where the surface area of the bindings does not take kindly to being squeezed. Maybe, I need to lam AFTER I bend - but that seems like such a pain in the ....

I have done exactly this, i.e., pre-bend wood laminations (although not for bindings) before gluing, precisely to avoid delamination in later more precise bending. You do not have to be perfect. Any rough bend built in will make tweaking later under heat vastly simpler. Then again, tweaking might not even be necessary in the case of bindings.

The question is, what is more of a pain in the ..., setting up a nice repeatable system (once) that delivers excellent results every time, or gambling every time? I really don't know in your case or anyone else's, but I will go out on a limb again anyway and say the latter, and by no small measure. It's a problem I find interesting.

Actually, de-laming IMO isn't such a big deal, as it all gets re-lamed as you tape the aggregate into the channels. I've quasi bent purflings with a clothes iron on a flat board, but of course they don't like going sideways which is the problem in the first place. They are many times thicker than tall in the direction of the bend.
So I guess to OP, as long as you can get the crinkle flat, either by using an iron or the vice/heat gun method, it doesn't matter if it's de-lamed a bit. It will go back to where you need it.

Now if I didn't know better, I'd say that's another subtle swipe at me. If the "luthiers" were a perfect help, maybe he wouldn't still be having problems with delamination, or "the unfortunate binding cramp/crinkle where the surface area of the bindings does not take kindly to being squeezed" as he just indicated ; -)

I was curious, so I just tried bending a .080" thick piece of Bloodwood on a maxed out Ibex with a surface temperature of 452 F. It bent very easily with a thin backing strap to a 1.375" radius (much tighter than a typical guitar waist) with no remote sign of failure. Neglecting the delamination issue -which would easily be solved with pre-bending before gluing- I am curious about your heater Steve. Maybe it has inadequate mass, and thus gives up heat too readily to complete bends. How is it heated? What is the material? I am not so sure you could trust that gauge, unless you have a wired thermocouple directly under the zone you normally bend over.

Even material .103" thick (at 438 F) to the same 1.375" radius, bent like butter over an iron that can supply heat without significant loss.

I've built 3 guitars with Bloodwood bindings at .085" on a Fox bender. I'm glad I never knew they were difficult to bend!

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JJ
Napa, CA
http://www.DonohueGuitars.com

I too have been the beneficiary of not knowing something was difficult! I love not knowing something is difficult or if it can't be done.

But to address the OP - I've used Super Soft II and I've thinned them down to .080. I like the look of them thinner, especially bloodwood.

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Filippo...I've actually never invested much time in making bending molds. If one is careful when cutting out ply for building molds, the plugs can be used to make the bending molds. That way, you'll only waste the sawdust! <G>

You're right about them taking up space though...but that can be reduced as I have done. Make the 9-10 spacer rods removable. That way the storage thickness requirements go from 6" to about 2".

_________________
JJ
Napa, CA
http://www.DonohueGuitars.com

I tend to build a bending form for any shape I plan to build. Yes, they're a storage problem, but as soon as you need it the second time, you'll be glad for it. I also use solid forms, not the pipes. I find I get better results and less of the waviness issues popping up. Of course it's been a few years...

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Actually, I do the top purflings separate in a separate channel (glued in a form so they're pre-bent; they only need to be cut) I only lam the bottom purfs. It is interesting that I didn't realized that my final thickness is MUCH thiner than .095 - so why do I start at that thickness in the first place?

On my next, I'm going to take everyone's advice. Bend just the binding (even if it isn't bloodwood), then add the bottom purflings. I think, as stated, that it will be much less stress and make it much easier.

Sometimes I just have to do things the stupid way before those with much more wisdom point out the errors of my ways.

You have all provided wonderful advice and some insight into the bending process I had not considered. A big thanks to everyone!
Steve

So I just walked away and looked at my guitar again and had an old man moment.

Building the top purflings is easy. They can be glued flat in a bending form (i.e. stacks of _ ). But gluing the purflings that go on the bottom of a binding must be glued in a form in a vertical position (i.e. ||| ) where again, you are gluing in a form that is going against vertical stresses. Maybe at the thickness required for a single purfling, that is irrelevant and I'm over thinking this. But I wanted to get some ideas on how those of you that glue and attach the bottom/side purflings to the bindings are doing it.

Can you post pictures of your form? Is it the same as the side form and at a thinner thicknesses, they veneers don't split along the vertical stresses? Maybe, I need to bend them wet to allow the moisture to keep the veneers flexible?

Thoughts?
Steve

If you are bending them separately, stack the bindings and the purflings in a taped up bundle and bend the whole thing at once. Like this.



Bent on a pipe, all at once, side, top and back purflings and bindings. Only the thin layered purfling strips(b/w) were glued together.

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Waddy

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