Tapping threads in neck for bolt on

[Barry Daniels]

Rodger, If you are seeing top cracks along the edge of the fretboard, this is often caused by over-heating in a car. The glue joint of the upper transverse brace slips and all the string pressure is concentrated at that point causing the fractures. This type of damage is not necessarily repaired with a neck set.

[Bryan Bear]

Hmmm, I posted a reply yesterday and I don't see it now. I probably forgot to hit submit. . . Oh well, it didn't say anything that wasn't covered in the last two posts.

[Rodger Knox]

Michael, my use of "failure" is a bit misleading. What I mean is that if a neck reset is required, something has "failed". As you said, the heel block rotates slightly. My point was that this rotation is primarily the top of the block moving toward the bridge, NOT the bottom of the block moving away from the bridge. Part of my problem is that engineers use common words like stress and strain with very specific meaning. Stress is force/area and strain is the deformation due to stress. That deformation can be elastic(temporary) or plastic(permanent). When you string it up(stress is applied), the top moves a little(elastic deformation). If the strings are loosened, the top goes back to it's original position. If the stress is high enough(if it exceeds the materials "elastic limit"), then the top does not go all the way back to it's original position(plastic deformation). Unfortunately, wood is a less than ideal material, and even stress below the elastic limit over an extended period of time causes plastic deformation(usually called cold creep in luthier lingo). Plastic deformation is a type of failure, which is what I meant.
This will happen reguardless of how securely the neck is attached to the block.
Barry, I meant that cracks indicate there is a significant load on the neckblock, but it's a compressive load.

Since we're talking about the strength required for threaded fasteners, I've got a question for the experienced repair people out there. How often do you see the bottom of the heel pulling away from the body? If I'm right about the tensile force being neglidgible, it would never happen.

[Barry Daniels]

I have definitely seen heels pulling away from bodies.

[Bryan Bear]

Barry and anyone else answering about heals pulling away, do you remember what type of heal joints were used in these instances? When you have seen this, what was the failure/fix?

[Barry Daniels]

I've seen dovetails pull away, and mystery joints on asian imports.

[Clay Schaeffer]

One of the main reasons for using a dovetail joint rather than a straight tenon is to resist the forces pulling the heel away from the body. A sloppily made dovetail will fail and allow the heel to pull out some. The fix is to shim and refit the dovetail joint.

[Bryan Bear]

I was going tomask how a dovetail joint can pull away without wood failure in the cheeks or tail. If I'm being honest, I have never made or fit a dovetail neck joint. I suppose -- even if I intend to always use bolt on buttjoints -- I should force myself to do a couple as part of my education in this hobby.

[Greg Robinson]

I've seen heels pull away on dovetail joints when the wood has become crushed and the bottom of the joint has started to pull out. Not very often though. The fix is to shim and re-fit.
More commonly I've seen the neck block rotate and come loose from the back and popped bindings. This is usually on mystery joints. The fix for this is to clean joints and reglue, sometimes requiring removal of the back plate.

[Mark Swanson]

I've seen dovetails give way and pull out, and almost every time it's because the joint is poorly made. Most of the factories, especially the cheap guitar ones, don't take the care to fit the dovetail joint as well as an individual luthier does.

[Michael Lewis]

Rodger, thanks for the nudge toward proper usage of terminology. I keep forgetting that in the scientific world words have very specific meanings, my apology but I'm just a civilian.<G> This reminds me of the time I first signed up to advertise with the new magazine, Acoustic Guitar. The company had been putting out Strings, a violin centric magazine, and the violin world is rather traditional in the terms they use. When I told the person on the phone I was a mandolin and guitar builder I was corrected that I was a "maker" not a builder. A builder makes very large things like houses and ships. I remember that and do prefer and use the term "maker".

The populace of the world could use a good education, myself included. Then we would all know that heel is a noun and heal is a verb. People seldom have theories, they usually have vague ideas. As I understand, a theory is a well thought out concept supported with lots of math and or other substantiations. We do tend to misuse words, and the situation seems to be getting worse rather than better. We should all try to express ourselves clearly and keep in mud that words do have specific meanings. Some have several.<G>

[Chuck Tweedy]

Did you say "keep in mud" intentionally Michael??

[Barry Daniels]

That does have a certain poetic elegance, doesn't it.

[Bryan Bear]

Thanks Michael, I didn't know anyone made a distinction between builder and maker. I don't know if I agree with the rationale though. I think I like it, but I think it would be needlessly pretentious when referring to my level of workmanship; I'd better stick with "hack!" I routinely mix up heel/heal, even though I know better. . .

[Rodger Knox]

I've been thinking about this a bit, I've realized my mistakes and errors. My first impules was to discount the heel pulling away as poor materials and/or construction, but if there's no tensile force, that shouldn't matter. By the way, a mistake is a blunder and can be corrected, but error is the difference between a measured or calculated value and the actual value, and can only be minimized.
While I was thinking about cheap guitars, I realized the action got higher as the neck rotated, which increased the bending moment. My error was assuming 3/8", I should have used at least 1/2" to be conservative. My mistake was in superpositioning the stresses, there's actually about 60 lbs or so of tensile force in the heel. I have to admit to a senior moment.
Many trades have their own jargon, it's just more standardized with engineers. It's helpful in grasping concepts for some words to have very specific meaning.

[Clay Schaeffer]

Hi Bryan,
Take to your heels and your heels will take to you (or some such). <g>

[Bryan Bear]

Hi Bryan,
Take to your heels and your heels will take to you (or some such). <g>

Clay, go-ahead and mark yourself down for 5 points! Save up enough points and you get to pick from the prize drawer. <G>

[Michael Lewis]

All in great fun, gang. Make those little gray cells work, keeps 'em happy

[Rodger Knox]

I guess we've decided a couple of things.
Wood is an imperfect material that deforms under stress over a period of time.
There is a lot of stress on the heel of a guitar, or,

Time wounds all heels.

[Ron Belanger]

Prize Drawer!!! There's a prize drawer?