Today I am addressing guitar necks that use dovetail joints.
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| Sliding a Martin D-18, dovetailed neck out of the neck block after steaming. |
What Holds the Neck On?
While glue obviously keeps the neck from coming off, dovetailed joints are designed to slide together, wedge and "lock". When a dovetailed joint is not shaped or fit properly the glue is being asked to do far more work than intended. On many occasions I have seen a loose neck joint blamed on glue failure when in fact, a poorly fitting dovetail was the underlying cause.
I have had quite a few people bring me acoustic guitars that have loose necks. Understandably they are hoping for an easy and cheap fix ...like simply gluing the neck back on. Unfortunately that is rarely the case when doing the job professionally. Dovetails must be shimmed and fit to insure the joint is locking. Simply adding fresh glue to a loose or sloppy dovetail joint would not be a professional repair. (In more transparent language ...it isn't anything I would offer to do.)
Glue Nightmares
Between the chisels and the reamer lay a bottle of Tylenol. I'm going to need it.
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| Ack Ack! A Martin neck block filled with epoxy. Yellow lines = this is where the dovetail locks and glue is applied. Red line = this open area has been filled with epoxy making removal a nightmare. |
Above you see a neck pocket that has been completely filled with adhesive, and not just any adhesive ...it was epoxy. After steaming the neck the traditional way I quickly realized it was having no effect. The neck joint only takes a couple of minutes to loosen with steam, mysteriously, this one was still securely set and showed no sign of yielding.
Necks should be glued using water soluble adhesive only. This would include Titebond regular(woodworker's glue) and Hide glue
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Because there was evidence that this fingerboard extension had previously been removed I had my suspicious and removed it to have a looksie.
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| Another fingerboard which had been sawn thru at the 14th fret. |
Exactly! And that's not the way we do it today. However, once upon a time a far more conspicuous and somewhat destructive method of removing guitar necks was sometimes used. Instead of simply drilling holes down thru the fret slot into the joint, the fingerboard was sawn in half at the 14th fret. This allowed the fingerboard extension to be removed and set aside, exposing the dovetail joint.
While this made the joint physically (and visually) accessible it meant the fingerboard would have to be repaired. And when they were bound it was far more obvious as seen in the picture to the right.
Although this is obviously not recommended it is still a part of guitar repair "history" and something I have seen several times.
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| Oh Goodie, another one! |
Just to reiterate ...a locking dovetail requires very little glue. When I see something like this I assume the individual was trying to use adhesive to keep everything together instead of properly fitting the joint.
Fitting The Dovetail
The arrows point to mahogany shims that I have glued into the neck block where it will contact the neck's dovetail. This (and the fingerboard extension) are the only area's that will receive glue.
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| New mahogany shims have been glued into the neck pocket and sanded to provide a locking fit for the dovetail. |
Shims are required when resetting acoustic guitar necks so the joint can be sanded to fit properly after removal. When a neck is removed from an instrument all old glue is sanded away and the contact surfaces are smoothed, this process removes wood making shims necessary. Even on new construction, shims are often present.
Once the shims are glued in place I begin the sometimes arduous task of sanding and test fitting. I still prefer good old pencil lead as it allows me to see where my joint is rubbing, sand high spots and insure good coupling. Instead of merely placing shims at the tip of the dovetail I create a locking joint which mates from top to bottom.
For more details, you can read my acoustic guitar Neck Resetting article here.
