Official Luthiers Forum!

Yes the snugging of bolts would address this hypothetical issue. In practice I have not seen this as a prevalent issue. The purpose of suitable torque on the fastener is that the threads under compression do not release. I have guitars with bolt on necks with 20+ years and have never seen this problem. Nor have customer guitars ever come in with loose necks - aside the aforementioned fenders that have been messed with and glued-in Martins with a bolt. But Martin has so many quality control issues that I generally consider them a bad actor. Either joint works just fine, sounds just fine, holds up just fine over time.

In both construction methods, the skill comes in fitting the heel to the body with no gap, and getting the neck angle right. On the gap, I periodically see luthier guitars with gaps because of a variety of reasons - side area is not perfectly flat, upper binding has been rounded instead of kept flat, and sanding of the heel during fit most often showing gap at the bottom of the heel. The second issue, fitting for neck angle AND centered string projection is something that shows up with luthier-built instruments and ... cough cough ... Martin, who delivers guitars with varying height bridges as their manner for dealing with lack of geometry control. Last Eric Clapton Martin in the shop had a bridge so tall that it was downright gaudy. That being said, I observe those two areas - fit and projection - as the two places that demonstrate skill. They are both needed in either design.

Belleville washers would be useful.

In a factory, quality problems generally are personnel problems rather that process problems, and lucky for so many employees, they cannot be changed as easily as socks...

May I provide a counter point?

• Gibson headstocks - nearly 70 years of hogging out for the truss rod without reinforcement of the headstock to compensate for the material being removed and stress riser being created in the channel.
• Martin neck angle - a failure to control body geometry, process-wise. Comparison-wise, Taylor/Bourgeois/Collings/et al has near zero problems, all bridges same height. Doubt we are going to relegate that one to hiring practices.
• Martin (sorry) binding coming off since 1960s (whether material or glue, an engineering issue)
• Martin (sorry again) gluing pickguards down to bare wood, and I won't get in to all the reasons this decision is poor, which has caused I don't know how many cracked and buckled Martin tops.
• Countless guitars with the bridge located in the wrong place, hence the saddle being in the wrong place. Just had an Ovation in with this issue.

Not trying to make a definitive list of course. Just providing example in the counterpoint, where individuals are really not the issue.

Back to threaded fasteners, torque is critical to deformation and how a fastener holds.

I was speaking of along-grain compressive strength for genuine mahogany...the paltry 150-180 pounds of load on a guitar neck is not going to cause much of a dimensional change due to compression of the neck wood...that would take a much larger load. I do worry about changes due to RH variation and the cross-grain nature of the tail-socket construction, but practically speaking, we see a lot of 60-90 year old guitars that are handling those environmental variations without issue.

On mechanical fasteners, I believe auto and aircraft engineers and mechanics have the nut of it - torque values are essential to stretching the bolt enough to ensure that things stay put under tension - whether in metal or wood structure. If builders are uncomfortable with considering metallic structure as examples, then consider massive timber constructions like railroad trestles we saw come into common use in the 19th century...these structures were bolted together, and exist to this day. Pay attention to proper torque values and you will not see issues with bolted necks.

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For the times they are a changin'

- Bob Dylan

Ms. G. ... yes!

How does one determine the appropriate torque for neck bolts?

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

That's nifty! Too bad I don't know the yield strength of brass inserts and have no idea what the applied load is. I suppose I'll have to stick with "yeah, that feels about right."

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

The issue will be with those that thread inserts directly into the end grain of the neck or fail to reinforce a tenon to prevent bearing failure of the barrel bolt. For a 1/4-20 KD furniture bolt, even the 10,000 psi number of 16 in-lb seems like overkill, but I suspect many builders apply closer to a 25,000 psi tension load in the fastener with the applied torque (3” Allen wrench and 15 lbs of force = 45 in-lb...getting close to the 75% number).

http://www.repairengineering.com/bolt-torque-chart.html

_________________
For the times they are a changin'

- Bob Dylan

Yes. So that said, I gave you a very technical answer to your question, as the calculator is somewhat exhaustive. While I own 6 calibrated torque wrenches, I don't use them on guitars. Here's my general method, which I have found works for most cases: simply tighten the fastener until I feel resistance, then add another 1/4 turn (90 degrees) which should be with some resistance.

I prefer to use steel inserts. https://www.mcfeelys.com/1-4-20-threaded-inserts-for-hardwood-clear-zinc-plated-qty-100.html. Most ACE Hardware stores I've visited carry these in the fastener bins.

Torquing the bolts properly is ease. I give it one Grrrr and it's done.

3/8" brass bar in the neck heel, 2 x 6mm bolts with Belville washers - tight as I can get them.
Going nowhere.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Sounds like a Trevor Gore style neck attachment, Colin. How do you like it overall?

It is, and overall I like it. Little heavy, think an extra oz or so. But you can seriously torque it up.
IIRC, Trevor said he has tried to shear it out and couldn't.
It was Gerard Gilet's idea originally.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Lols, this thread requesting info about dovetails is getting very educational about bolt-ons...

Hey, I got the info I needed. The rest is icing on the cake. Although thinking about mortising out the tenon to put in a brass insert I’d have to drill and tap makes me laugh at the self-flagellation remark. Sounds bomb-proof though.


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Good to know you have no problem selling guitars with bolt on necks. I certainty prefer them if not for future resets but even for the initial set. I'm definitely going to try it on my next guitar.

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Hutch

Get the heck off the couch and go build a guitar!!!!

Andy said

"While I own 6 calibrated torque wrenches, I don't use them on guitars. Here's my general method, which I have found works for most cases: simply tighten the fastener until I feel resistance, then add another 1/4 turn (90 degrees) which should be with some resistance."


Andy - how does the thread-per-inch count of the bolt come into play with that rule?

Ed

I think reference table fastener torque specifications become irrelevant when we're discussing putting a metal fastener into another metal fastener separated by wood. Any of these fasteners is vastly under-stressed when all we do is snug up the fastener firmly. In a similar vein, When mounting wooden propellers on aircraft engines the torque specifications are assigned by the propeller manufacturer, not the bolt torque charts, and are a small fraction of the loads imposed while installing metal propellers (same bolts).

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Peter Havriluk

Are we talking about a rotating propeller creating thrust and other loads (e.g., precessional, etc.), or a static propeller on a hangered aircraft which sees compressive load? I assume the later, given my lack of familiarity with any guitar neck installations which undergo appreciable rotation in operation relative to the guitar body.

_________________
For the times they are a changin'

- Bob Dylan

Presumably the same propeller exists in both states, a bit like Schrodinger's cat.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

So if we posit a closed hanger and an operator willing to crank the engine in a closed, confined space...

_________________
For the times they are a changin'

- Bob Dylan

How so? It's a general rule of thumb. If I was to reduce it to every aspect of a fastener (thread pitch, which you point out, is one aspect) that goes into its ultimate torque setting, we'd be back to a deterministic use of a torque wrench and either a specification or mathematical determination (i.e. the calculator I passed along).

BTW I don't suggest anyone use this rule of thumb when tightening fasteners on aircraft, spaceships, or critical ground-speed vehicles. But for bolting a neck onto a guitar, it works and no one dies.

I was thinking more along the lines that the same bolts and torque exist fastening a propeller whether the aircraft is stationary, engine off parked in a hangar, or rotating when flying.
But it's not the first time I've missed the point as it goes flying over my head....

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I believe those prop bolts get retorqued every 50 hours or so, so not sure that's all that applicable a model.

That said, I do like the cat-in-box meme, but my own two felines would likely not appreciate that sort of treatment.

_________________
For the times they are a changin'

- Bob Dylan