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Help: Neck Backbow....


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Hi!

Yesterday I glued a perfectly straight fingerboard to a perfectly straight neck. Now that I removed the clamps and put a straightedge to the fingerboard I found out that it has a massive backbow. It is about 2-3mm higher in the middle then at the ends. As I am 100% sure that both pieces were perfectly flat before gluing I suspect that the humidity of the glue caused the neck to backbow. My question:

Will this backbow dissappear after some days of storing the neck in a warm room, or do I have to level the fingerboard again? Or should I compensate for the backbow with the trussrod?

Thanks in advance,

Marcel Knapp!

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This is very common with waterbased glues like titebond - when you apply glue to the neck it aborbs the water in the glue, and expands, forcing the neck into backbow as the gluesurface gets longer. Some builders use epoxy to attach the fretboard, thus avoiding introducing waterbased glues to the neck, and I plan to adopt this approach on my next guitar. In the past I have clamped the fretboard using a plywood and cork caul, then a deadstraight piece of mild steel square section to prevent it from moving whilst it dries - this has worked OK, but I've still had a couple of necks backbow, hence the decision to go to epoxy next time.

I very much doubt the backbow will depart of it's own accord, since the glue will have 'set' the neck in the backbowed position when it dried.

You will probably have to heat the neck, then clamp it to a flat surface with a small shim in the middle to create a mild foreward bow whilst the neck cools. I have sucessfully straightened necks by heating the fretboard with a clothing iron, until it is too hot to touch, then clamping it fretside down to a piece of mild steel square section, with a 2 mm thick cardboard shim in the centre, and a well padded clamp at each end. The heat softens the glue holding the fretboard, and allows the neck to adopt a straight position, which it should retain once the glue has cooled and rehardened. The shim forces the neck slightly past dead straight, to account for springback when it is unclamped - you may get away without it, but I find it helpful.

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I find this happens 90% of the time, I hate it. I've tried epoxy and it hasn't really done any better, my biggest problem with the epoxy is it doesn't absorb into the wood and didn't give as strong of a bond.

However, here is my method of gluing and the problem has since gone away.

First, I think, part of it has to do with clamping to tightly (why this would affect it I don't know, but it seems to) I used to use a bunch of clamps and tighten them really tight. Now, I use one of those huge rubber bands that stew mac sells, I am not even sure what they sell them for, I think it's for gluing binding, but they work great for fretboard glue ups. The neck seems to stay pretty straight when doing this. I had previously tried gluing the necks with an extreme front bow in it as well and only found this to do nothing, I still got a back bow.

As for your neck, what I have done to cure even fatal backbow in old necks, is get a good strong piece of wood or even metal square, something that won't bend easier than the neck, and over the course of about 2 weeks, I force a front bow in that is double the amount of backbow, for example, if you have 3mm under both ends, that will require 6mm of front bow measured in the middle of the neck (assuming your steel bar stays dead flat) don't worry, if you progressivly do this you won't break the neck, I had a JEM77FP neck that had a wicked back bow, almost 10mm, and I ended up having to front bow it 25mm. I did it over 3 weeks and it didn't crack at all. I also spray the back of the neck with some water once a day and leave it over a heat register, this allows the neck to slowly warm and cool. After a couple weeks I usually have a neck with a slight front bow which is desireable and no twists.

I sure wish I understood the physics of why they bow, but I don't, so all I could do was come up with a solution that worked for me :D

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I've also been considering using epoxy to glue my fretboards down, I guess that the only reason I'm halfway scared to take the plunge is because I know I won't have as much time to make sure it's lined up right.. lol Also, if I needed to take the fretboard back off for any reason it would be almost impossible. But I'm sure the neck would probably be more stable using the epoxy anyway.

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The physics is simple, you wet the front of the neck, it gets longer. The back stays dry, and stays the same length. The only way for the neck to accomdate this uneven expansion is to backbow. The neck can't return to straight once the moisture is gone, because you've glued a fretboard to it, which is very dense, and has a lot of compressive strength countering the necks desire to shrink.

The trick is to soften the glue with heat, but without reintroducing moisture. That may the glue can 'slip' allowing the neck to straighten, then cool in the correct orientation. I'm surprised you still had the problem with epoxy though, I guess I'll have to see how I get on with the next guitar...

:D

BTW frenzy.... less open time with epoxy? I wouldn't advise using a quick setting epoxy for the fretboard connection, and a regular slow set should have a much longer open time than titebond. It's also no harder to reverse epoxy joints than titebond - plenty of heat (but no need for moisture) and it will separate just as easily as titebond, especially on a fretboard. My only problem with epoxy is it's talent for getting *everywhere* without any respect for conventional physics, plus it's a bitch to clean up. Maybe it's just me, but I find epoxy creeps right up to my armpits before I've even applied it to my gluejoint... hell I barely need to look at it and it's in my hair!

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wow..

i occur this matter..

my accident is fingerboard glue.. -> clamp remove -> fretting -> neck back

after neck back,, neck backbow.. ^^

so i decide.

(router)trustroad -> free neck back -> planning( fingerboard joint for neck) -> glue -> fretting -> sandding neck back..

my teacher say, glue have moisture.. so this is natural ..

and i decide when make neck.. at least more 3 weeks needs.

how long time is need for making neck?

fingerboard glue & make neck back is enable 1 Day.. but this neck is not good.

need long stability time for fingerboard & neck..

wow this forum is really lovely forum..

thank you all...

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wow, i didn't realize this many people have had this problem... out of my 4 necks, only one had a backbow, it was less then a 1mm and it's the experimental neck i made out of basswood....

the rest are dead flat.. and i use titebond, with as many clamps as i can find as tight as i can get them in 10 minutes...

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Holy crap this is a great forum. :D

That's all I gotta say. Ok, no it's not-- B)

Would it help matters at all to clamp more of a sandwich? Ie. instead of just clamping the neck and the fingerboard, clamp something sturdy and straight on either side? Some sort of caul would be needed for the fingerboard, too, I guess.

Or is it a matter of nature overcoming brute force? Ie. would the neck be just as prone to backbow, even with extra 'reinforcement'?

Also, does it matter which way the grain is cut? Would quartersawn be less prone to backbow, or do the physics have basically the same effect no matter which direction the grain runs? Along the same lines, do necks made from laminated pieces stay truer?

Cheers,

Greg

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Also, does it matter which way the grain is cut? Would quartersawn be less prone to backbow, or do the physics have basically the same effect no matter which direction the grain runs? Along the same lines, do necks made from laminated pieces stay truer?

The neck in question is laminated from five perfectly quatersawn pieces in this order: Maple, Bubinga, Maple, Bubinga, Maple.

As it looks this did nothing in order to prevent that issue....

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You know GregP has got me to thinking and maybe someone else can confirm this. I just now realized that the only neck I've had problems with was before I was using Carbon Fiber Rods on my necks. Just curious if KrazyD's necks that seem to not get a backbow in them had carbon fiber rods in them? Anyone else can confirm this theory? Maybe that didn't have anything to do with it, but It does make sense since the rods being epoxied in wouldn't let the neck backbow as much. Was anyone else using carbon fiber rods when they experienced a backbow?

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my latest neck, 3-piece mahogany with an ebony board also developed a slight backbow after i glued it. This is the first time this happened, and the first time i used this type of glue. beforehand i had used a powdered resin glue.

john.

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Just curious if KrazyD's necks that seem to not get a backbow in them had carbon fiber rods in them? Anyone else can confirm this theory?

nope, haven't tried out the carbon yet.

I do leave quite a bit of time between stages with everything i build, partly because of that "let the wood adapt to it's new shape" stuff that jean larrivee talks about in his factory tour video, and partly cause i'm just busy.. So my wood moves back and forth from the cold garage, and warm house for weeks or months at a time before i get a chance to get back at it.

I think this was the idea hyunsu was trying to explain??

lol with my luck, my next neck will probably be shaped like a harp after glueing...

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made my first neck not so long ago, my 31 fret scalloped wonder, and i had no back bow, totally perfectly straight. this just luck, cos i really dont want to make another with any bowing.

PS i used a glue called Cascamite, ,or Extramite (the more modern name of it) binary glue, mix with water into paste yadda yadda, very very strong! also i did all profiling of the neck after the fretboard was glued on.

Mike

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Just wanted to let you know that I head great success in straightening the neck with a heating iron and water. I heated the fingerboard until it was too hot too touch, then I clamped the neck lying on the fingerboard onto a flat surface and applied some water to the back of the neck. After this most of the backbow was removed. Then I had a good idea I think which I can definately recommend:

I took one of the long Stewmac fret levelling metal bars and clamped the neck with many clamps onto the bar, so that the fingerboard is in contact with the bar. Now i heated the bar with a clothing iron and a very powerful electrical heater. This resulted in a perfectly straight neck. It only has about 0,5 mm of backbow left, but this comes from the fingerboard, as I did no final levelling on it yet.

Thanks again for your help!

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Just wanted to let you know that I head great success in straightening the neck with a heating iron and water.

(Snip!)

This resulted in a perfectly straight neck. It only has about 0,5 mm of backbow left, but this comes from the fingerboard, as I did no final levelling on it yet.

Thanks again for your help!

Great - Glad it worked for you :D .

The .05 backbow should dress out, but you can also get rid of it by using the shim I originally suggested. Given the choice I prefer to have a hair of relief rather than back bow, because that way I can always remove it by tweaking the rod a bit.

BTW, when do we get to see some pics? I'm looking forward to seeing what you've come up with!

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