Experience With And Modifications To Cumpiano’s Barrel-Bolt Neck Joint Method

by John Whiteside

© 2008

 

This neck joint is an alternative to traditional dove-tail or tapered dovetail neck joints for acoustic guitars. It is based on an online addendum by William Cumpiano to his well-known guitar building book, Guitarmaking: Tradition and Technology. In the book he gave a pinned joint method but readers had a lot of trouble with it.

These notes are based on my experience using this idea with a number of modifications, additional considerations, and other research.

The trick in this neck joint is to have fine control over the angle between the neck and the body (the setback angle). This angle needs to be accurate to within a fraction of a degree of optimum in order to achieve the best combination of low action (playability) and high saddle height (sound power) in the finished guitar. Unfortunately, it’s impossible to know what this angle should be until the guitar is completely assembled and strung up, because stringing up the guitar puts strains on it, bending and shifting various components. Furthermore, this bending and shifting takes place over a period of time.

I feel these restrictions rule out any type of joint that must be glued before the guitar is strung. Instead, the joint must be easily disassembled and allow an easy way to alter the setback angle in a predictable manner.

The barrel bolt joint matches these characteristics. You build the joint to best approximation, attach the neck, string up the guitar, and measure the action. Then you leave the guitar strung for a week or two, recheck your measurements, calculate the adjustments needed, take the neck off, make the modification, and you are left with a perfect setback angle.  In addition, this procedure can be repeated years later in case the neck warps (probably upwards) during that time.

The joint is basically a straight-sided mortise and tenon, with the mortise cut out of the headblock inside the body and the tenon cut into the heel block on the neck. The tenon and the mortise are shown in the two photographs at the right.

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