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Old 04-04-2022, 09:37 AM
H165 H165 is offline
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Default Neck/body joint final face pressure - dovetail and bolt-on

Questions concerning my current guitar build. Any help, comments, information will be greatly appreciated. If there is an existing thread on this subject please point me to it.

The areas I am interested in are the "faces" of the guitar body and the neck when these two components are separately made and then later joined.

It appears to me that a "good, tight uniform fit" is required at this joint. The problem is, I do not know exactly what a "good, tight uniform fit" means. I stipulate the basic physics: the forces on the faces is equal to the forces applied by the dovetail wedge action or the fastener faces. My basic question:

What is the optimum final face-to-face pressure of an acoustic guitar neck-to-body joint?

The related mechanical questions for the two most common joints:

1. What is the optimum final face-to-face pressure of a glued-and cured dovetail joint?

2. What is the actual final face-to-face pressure of a glued-and cured dovetail joint from any specific manufacturer or builder?

3. What is the optimum final face-to-face pressure of a bolted/screwed joint?

4. What is the actual measured final face-to-face pressure of a bolted/screwed joint from any specific manufacturer or builder?

5. What is the measured joint clamping pressure required to achieve the optimum final face-to-face pressure of a glued-and cured dovetail joint?

6. What is the measured torque in inch-pounds per fastener required to achieve the optimum final face-to-face pressure of a bolted/screwed joint?
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  #2  
Old 04-04-2022, 10:23 AM
Skarsaune Skarsaune is online now
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I'm going to guess you're an engineer, or scientist. Prone to perhaps overthinking.

Not to worry, so am I.

I have not ever seen any " actual measured final face-to-face pressure" of any guitar feature published.

I believe you are suffering from analysis paralysis.

As far as fit of the faces - a technique many use is to relieve the face of the neck that contacts the body so that only the outer 1/16" to 1/8" is in contact with the body. These smaller edges are then "flossed" with sandpaper so the fit is as near perfect as possible. No visible / measurable gap.

For the bolted joint - tight enough to not move under string tension and normal use. How tight is that? Most of the fasteners on bolt on necks that I have seen are 1/4" or smaller - so you are not going to apply a ton of torque. Also, you will get deformation of the wood surface if you get too enthusiastic with the wrench. Hence most folks employ a washer to spread the load.
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Old 04-04-2022, 11:01 AM
Rudy4 Rudy4 is online now
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Quote:
Originally Posted by H165 View Post
Questions concerning my current guitar build. Any help, comments, information will be greatly appreciated. If there is an existing thread on this subject please point me to it.

The areas I am interested in are the "faces" of the guitar body and the neck when these two components are separately made and then later joined.

It appears to me that a "good, tight uniform fit" is required at this joint. The problem is, I do not know exactly what a "good, tight uniform fit" means. I stipulate the basic physics: the forces on the faces is equal to the forces applied by the dovetail wedge action or the fastener faces. My basic question:

What is the optimum final face-to-face pressure of an acoustic guitar neck-to-body joint?

The related mechanical questions for the two most common joints:

1. What is the optimum final face-to-face pressure of a glued-and cured dovetail joint?

2. What is the actual final face-to-face pressure of a glued-and cured dovetail joint from any specific manufacturer or builder?

3. What is the optimum final face-to-face pressure of a bolted/screwed joint?

4. What is the actual measured final face-to-face pressure of a bolted/screwed joint from any specific manufacturer or builder?

5. What is the measured joint clamping pressure required to achieve the optimum final face-to-face pressure of a glued-and cured dovetail joint?

6. What is the measured torque in inch-pounds per fastener required to achieve the optimum final face-to-face pressure of a bolted/screwed joint?
First, there's no way to measure any of those forces accurately.

The traditional dovetail joint isn't about creating force as much as simply providing a well-fitting joint that provides a small amount of clamping force once the glued faces of the dovetail are drawn together. It's actually easy to cut the joint so close that it locks the neck to the body BEFORE it gets all the way to it's seated position. There are a ton things to go south when a traditional dovetail is used, and even more down the road when repair eventually becomes necessary. It's the reason why so many builders have opted for the easier to manage bolt on M/T joint.

Years ago Bob Taylor reasoned out the NT neck joint, which provides a solid joint that's easy to manufacture, fit, and later adjust and work on. Taylor could just as easily produce a "perfect" dovetail using their CNC technology, but doesn't. That should clue us in to the sham that dovetails are.

The icing on the cake for me was seeing examples of hand-fit dovetails from the main advocate of dovetail necks when they are removed for resets. Many of those dovetails are far less "perfect" than they would like you to believe. It's no wonder the heel / body fit shifts over time.

As Skar relates above, a undercut heel and hand lapping the heel edges allows the casual builder to easily produce a good fitting joint that locks together using simple bolt pressure that doesn't count on a specific amount of force. Since we're locking hardwood surfaces together there's no need to go all Mongo on the bolt tightness. If there's a worry about loosening of the joint you can always go with stacked Bellville washers to provide a calibrated and consistent amount of force.

Last edited by Rudy4; 04-04-2022 at 11:10 AM.
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Old 04-04-2022, 11:12 AM
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Bruce Sexauer Bruce Sexauer is offline
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I can only address the dovetail joint that I use for all of my work. The goal is to join the neck and the body so that they function as one object. The mechanical aspect of the joint merely requires that they fit as well as possible, and does not require any actual force that could/should be measured as pressure of one part against the other. The purpose of the glue is merely to keep the neck and body in the required proximity to maintain the relationship.

Bolt on systems operate in a number of different ways, and cannot therefore be treated as one thing. Some are fairly simple involving brute force and requiring relatively massive wooden components, while others are amazingly complex (to me) and would require more thought that I am willing to give them for clarity about their function.

I love the simplicity and elegance of the well fit blind dovetail mortise to attach a guitar’s neck to the body, and have never seen any reason to abandon it.
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Old 04-04-2022, 11:33 AM
Alan Carruth Alan Carruth is offline
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Guitars have been made with 'three point' contact between the neck and the body. One maker uses two small setscrews in the fingerboard end, which allow for some fine adjustment of the string length and left-right alignment. There is a bolt through, or into, the heel, to keep the neck from folding up, and it's all held together with string tension. I've used something like this on 'test mules', and see no difference in the sound when a filler block is used so that the heel screw can be fully tight, as opposed to allowing the neck to 'float' backwards.

In one recent experiment I did I found that a bolt-on neck is a bit stiffer at low frequencies when the fingerboard extension is glued down. This primarily affects the low-pitched fundamental corpus ('neck') mode, which only influences the timbre if it happens to match the 'main air' resonant pitch. There was no appreciable change in the high frequencies that I could see in a spectrum plot.
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Old 04-04-2022, 03:45 PM
H165 H165 is offline
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Thank you for your replies. I appreciate this discussion.

I am using a variation of Laskin's joint; a fitted traditional "Martin-style" dovetail, with no glue. I am using two bolts mated to two threaded inserts in the neck. I have a pretty good understanding of the geometry and forces on the joint under load.

My (admittedly pedantic) interest leans towards the mechanical coupling pressure of the joint and its eventual effect on the sound produced, although I am peripherally interested in the point of component failure (I used to own and operate a building materials testing laboratory).

I was applying torque to the bolt heads, and thinking this operation was similar to the installation of my carbon bicycle handlebars. With the bike bars, a precise torque setting is provided to enable assembly of a stable, consistent joint, and to avoid destruction of the bars.

I was thinking a set of torque recommendations must exist at Taylor or Ryan, or in some other bolt-on makers' QC specifications; all of them using engineering to remove variables from their assembled products.

There are obviously differing opinions of the value of this information, and that's one of the fun parts of being on this particular forum. It's not a player's forum, or a music forum, it's all about the machine/tool/toy itself
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