Tapping threads in neck for bolt on

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Bryan Bear
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Tapping threads in neck for bolt on

Post by Bryan Bear »

I suppose this is the best place to put this discussion. . .

There are a lot of people doing bolt on necks these days. It seems like there are several methods people have chosen for various reasons. I like to use hangar bolts (thanks for the tip Mario) with a but joint. I like the hangar bolts because I feel like it makes the insert unnecessary which gives me less hardware in the heal and potentially a thinner heal. I'm not 100% happy with this though because I try not to remove the wood screw threads once they are installed and sometimes it would be nice to do that during construction. Also, it would be nice to have brass allen headed bolts show on the inside of the heal block. Keeping in mind that I use a hardwood dowel in the neck heal to keep from bolting into endgrain, could I not simply tap machine threads into the dowel? I recently saw a video of a workbench being constructed with holes tapped into maple and it seemed plenty strong. I suppose eventually this may get stripped out but so could a hangar bolt or threaded insert. . . What woods would be hard enough to hold up in this application (bonus points if they are readily available in dowel form).
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Mike Sandor
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Re: Tapping threads in neck for bolt on

Post by Mike Sandor »

I have substituted the wood dowel with a aluminum rod with taped holes. Also added a third anchor point as indicated in the photo similar to Taylor bolt on's. I may eliminate the mortise and tenon and go with a butt joint. That would surely simplify things.
Mike
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Karl Hoyt
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Re: Tapping threads in neck for bolt on

Post by Karl Hoyt »

I teach guitar building to high school kids and we use a butt joint, with a single #10 sheet metal screw or 1/4" stainless lag bolt. In all candor, the screw is used to align the neck and hold it while the yellow glue sets. Way back 10-11 years ago we had a guitar that had a lot of problems, so we decided to try out the glue joint by removing the screw and smashing the guitar....(and this was not an accurately made instrument)... the neck held just fine.

I use 1/4x20 screws on my electric uprights, tapped into the maple neck wood. The biggest proviso is to know when to stop torquing the bolts AND I coat the insides of the threads with CA glue , then re-tap the threads. I think that if you have about 3/4"-1" of tapped thread surface in the neck it will hold plenty tight, especially since you add that dowel internally... it holds great. i use those allen head furniture screws: the ones that are black anodized with a flat 5/8" head: they give a really finished look to the inside of the box. For a little more insurance, you could go with a 5/16"x18 bolt.

I'd say 'go for it'. Maybe you try it on an instrument that you won't jump off a cliff over if the joint fails, but I doubt that you'll have an issue.......

Karl
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Barry Daniels
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Re: Tapping threads in neck for bolt on

Post by Barry Daniels »

Machine threads are a lot thinner than deeper wood screw threads, which is what you find on the wood side of the hangar bolt. The depth of the thread is what determines it's pull-out resistance. I would guess that machine threads in wood would have maybe 30% of the strength of wood screw threads. So this would not be a good idea.
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Erik Hokanson
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Re: Tapping threads in neck for bolt on

Post by Erik Hokanson »

This is my method. Simple. So far so good. Less than two bucks worth of stuff from local hardware store. And you can groove on the wingnut. I of course pre drill and then ease it in like I'm cutting threads and get it through the dowel that runs through the heel, then wick in CA.
Any problems with this? Aside form the tamper temptation.
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Chuck Tweedy
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Re: Tapping threads in neck for bolt on

Post by Chuck Tweedy »

This reminds me of liquid hide glue.
It will work, but if something goes wrong - it will fail catastrophically.
Then you will have to replace with a barrel bolt or a threaded insert. AND replace a broken FB and any top damage.
So, why not start there??
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Tim Allen
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Re: Tapping threads in neck for bolt on

Post by Tim Allen »

I have also used hanger bolts into a maple dowel with a butt joint, per Mario's recommendation. I start the thread with a hanger bold with its point on it, remove it, then put some epoxy into the hole, then screw in a hanger bolt with its point cut off to maximize the amount of thread in the hole. The epoxy strengthens the threads, some people think, and it doesn't seem like it could hurt. (The bolt can be removed if necessary in spite of the epoxy.)

Looking at data on the withdrawal resistance of screws into wood makes me comfortable that the joint is more than strong enough, for my own purposes, but I am not recommending this to other people. (You can hunt around on the Internet for withdrawal strength data. BTW, it verifies that cutting off the point is helpful.) I understand many people add a lot of metal in the wood in the form of inserts, etc., which increases the holding strength but reduces the amount of wood. It seems to me that the joint is as likely or more likely to fail because of the wood shrinking around all that metal. Then you get a cracked neck joint. A hassle.

If the hanger bolt suddenly pops out of the wood, causing cracking of the top and the fretboard, that would indeed be a catastrophic failure. The possibility seems remote to me but then I have often discovered I'm not omniscient. A more gradual or partial failure could be repaired with inserts, epoxy, etc.

I use small brass washers and brass wing nuts. The wing nuts were temporary at first, making it easier to put the neck on and off while setting up my first guitar. I was surprised to find they held tight enough for permanent use.
Erik Hokanson
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Re: Tapping threads in neck for bolt on

Post by Erik Hokanson »

Yes! I love the wing nut! And Brass at that. I feel vindicated.
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Bryan Bear
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Re: Tapping threads in neck for bolt on

Post by Bryan Bear »

I too cut off the point of the hangar bolt, it's nice to see another opinion that agrees this is a good idea. I am confident that the hangar bolts are more than up to the task. I suspect that well cut machine threads would also be adequate, though -- as pointed out -- the risk is not outweighed by the benefit. I don't think that (if it were to fail) it would be a failure of the catastrophic nature. It seems like you would begin to see separation/loosening before the neck snapped off altogether. This afternoon I was thinking about ways to test its failure point by stress testing mockups when it occurred to me that even if the tests showed it was more than strong enough, I would have to build 2,000 guitars before I would earn back the time invested in testing (given the very minor nature of my annoyance with the hangar bolts).
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Gerry Gruber
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Re: Tapping threads in neck for bolt on

Post by Gerry Gruber »

I use Mario's system as well. I get my hanger bolts from Home Depot, and the portion that screws into the heel has wood screw threads, not machine threads. (At the other end, there are machine threads.) I also insert the maple dowel, keeping the grain lines of the dowel perpendicular to the screw/bolt. I don't add any glue or inserts, and typically remove the bolts at least once (and sometimes more often) during construction. I've completed over 15 guitars using this method and have not had a problem yet. And my shorter bolt has only 5/8" screwed into the heel (the longer has just over 1" inserted.

To make things look pretty inside, I counter sink the bolt heads, and cover them up with a second label with my logo.
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Alan Owler
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Re: Tapping threads in neck for bolt on

Post by Alan Owler »

I had to do a little digging to find what I was looking for. Back around 1990 Lee Valley Tools commissioned some research into machine threads tapped into wood, and found them vastly stronger than comparable sized wood screw threads. The data from one of the tables (the only one I have) is as follows;

for #6 wood screws and #6-32 machine screws

for a 1/4 inch deep hole the wood screw failed (pulled out) at 35 pounds and the machine screw at 195 pounds

for a 1/2 inch deep hole the wood screw failed (pulled out) at 270 pounds and the machine screw at 521 pounds

for a 3/4 inch deep hole the wood screw failed (pulled out) at 404 pounds and the machine screw broke at 666 pounds

for a 1 inch deep hole the wood screw failed (pulled out) at 464 pounds and the machine screw broke at 666 pounds

this was in oak drilled and tapped in edge (not end) grain. Ring porous hardwoods did not tap well in end grain but woods like maple, birch, cherry, and walnut did tap well in end grain, especially if a little wax was used as a lubricant in the drilled hole for tapping.

Apparently the whole write-up can be found in "WoodCuts issue #6" a magazine I seem to recall.

Al Owler
Rodger Knox
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Re: Tapping threads in neck for bolt on

Post by Rodger Knox »

The neck joint connection doesn't need to be that strong, it really does not have any stress on it. The string pull is putting the joint in compression, and the eccentricity of the loading doesn't look to be severe enough to have any tension on the connection. It's been 40 years since I took statics, so if I have a mistake in the following diagram please let me know.
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Karl Hoyt
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Re: Tapping threads in neck for bolt on

Post by Karl Hoyt »

Interesting data from Rodger and Alan. When you stop to consider that you're probably going to use a 1/4x20 or 5/16 x 18 bolt, the pressure needed to shear the wood holding the bolts would be pretty massive. Even a direct blow to the back of the neck in a fall probably would not do anything to the neck bolts.

Being a 'try it and see if it works' guy , it's nice to get some vindication from the Engineer types in the group to corroborate my hunches.

Thanks and enjoy your weekend , all.

Karl
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Bryan Bear
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Re: Tapping threads in neck for bolt on

Post by Bryan Bear »

Interesting guys, thanks. My gut told me that machine threads (in wood) would be weaker than wood threads but would still (probably) be up to the task, since there isn't a whole lot of stress on the bolts. It is very interesting to see that machine threads are stronger. I'm really tempted to try this. This fall I want to start on a flattop mandolin that will have a few minor experiments in it. I was planning to use one hangar bolt. I may try it on this, unless this thread end up talking me out of it (it has already changed my mind twice). Discuss. . .
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Barry Daniels
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Re: Tapping threads in neck for bolt on

Post by Barry Daniels »

Rodger, I don't quite agree with your conclusion that there is no tensile force on the neck joint. If you take the nut off the lower bolt on a neck under string tension, I think you will see that there is definitely some tensile force.

The compressive force is mostly concentrated at the upper edge right under the fretboard, which is effectively a pivot point. The tensile force can be calculated by using the distance of the lower bolt from the pivot point in a simple teeter-totter analysis.

Assuming the lower bolt is 3" (.25 ft) from the pivot point, and dividing this into the 6.25 ft-lb force you get a tensile force of 25 pounds. That sounds about right to me.
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Rodger Knox
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Re: Tapping threads in neck for bolt on

Post by Rodger Knox »

I'm pretty sure Barry is correct about there being some tensile force on the joint. Otherwise you could tune it up to pitch, take the bolts out, and nothing would move. It's been too long since I've done any superposition of stresses, and I've probably made some simplifing assumptions that are not completely valid. My simplified analysis neglected the fretboard, which would resist the pivioting and reduce the tensile force. I also neglected the neck angle, which would have a small effect that could go either way. Anyway, the tensile force is "small", so the type of connection does not require much tensile strength.
The fact that many guitars need a neck reset after a period of years indicates that the forces on the neck joint do tend to rotate the joint. I believe that this rotation is primarily the top of the joint creeping toward the bridge due to compression, with the pivot point being closer to the back than the top.
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Bryan Bear
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Re: Tapping threads in neck for bolt on

Post by Bryan Bear »

Isn't most of the need for neck reset due to changes in the soundboard resulting in the bridge/saddle being higher relative to the original string path? Is there really anything gong wrong with the neck joint itself? If the neck is indeed shifting towards the bridge, unless the heal is actually pulling away from the sides, it isn't an issue with the neck joint failing (rather changes in the geometry of the box). Keep in mind, I am about as far from an engineer as you can get, so I could be way off base.
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Rodger Knox
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Re: Tapping threads in neck for bolt on

Post by Rodger Knox »

Good point Bryan. I'm sure that's one reason. But I've seen quite a few guitars with cracks in the top on either side of the fretboard, leading me to believe the neck is moving toward the bridge. I suspect there is more than one "mode of failure" that would require a neck reset.
I only brought this point up because there is significant stress on the neck joint, but that stress is primarily compression, with a small amount of bending moment. The fact that there is little tensile force to be resisted by the connection does not mean the joint is not heavily loaded.
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Craig Bumgarner
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Re: Tapping threads in neck for bolt on

Post by Craig Bumgarner »

I've been thinking about this. Personally, I'd be more concerned with stripping the threads out of the wood during assembly or perhaps repeated assemblies, than failure after assembled without damaging the wood fibers at the treads.

If neck has metal inserts of some sort like the Cumpiano method, then this seems to have been proven to work well. Where I get queasy is with the idea of tapping into wood and often not very hard wood ta boot. It may be an irrational fear indeed if the loads are as low as Barry suggests (sounds right to me). Irrational fears being what they are though, here are a couple ideas:

1) Use a three piece neck with a center line of a harder wood like ebony or maple, tap into the wood and secure w/ machine screw. or.......

2) harden up the wood in the immediate area of the threads with epoxy. This is a boatyard trick used where bolting high load items to wood is not possible. At the boatyard however, we have little concern about removablity, so we just fill the hole with epoxy, let it soak for five minutes and run a fastener in wet. That screw ain't never comin' out. Well, a soldering iron on the head of the screw for a minute or two will get it out, but we don't want to be doing that inside a guitar.

On a guitar neck, I'd probably like to think I could get it apart later, so maybe wax the screw first, still run it in wet. I'd fill the neck holes with epoxy for about 5 minutes, then drain them so as to not get epoxy everywhere during the assembly. I'd want to test removability on scrap first.

Or maybe drill the untapped hole, fill with epoxy, let it cure, redrill the hole and then tap it. Run the screw into these hardened threads dry. No figures to back me up at all, but my intuition says this would at least double the holding power of the fastener. Personally, I'd use a standard thread machine screw.

Either way, some testing is probably in order, but pretty easy to do.

cb
Michael Lewis
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Re: Tapping threads in neck for bolt on

Post by Michael Lewis »

A good method of strengthening threads in wood is to apply thin CA and let it cure BEFORE installing the bolt/screw. This is tried and true in several high end repair shops.

Neck resets are very seldom required due to failure of the joint. It is invariably the upper bout of the body that warps slightly and allows the neck block to rotate slightly in relation to the rest of the body. This seldom causes any other joint failures in the immediate area.
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