Saturday, September 5, 2009

Mortise and Tenon

The traditional way to join the neck to the body is with a dovetail joint and glue. This is a very strong joint, but complicated to execute and makes re-setting the neck a fairly big job. Every steel-string guitar at some point in its life is going to need a neck re-set. The constant pull of the strings over many years results in the neck being pulled forward, toward the bridge, which raises the height of the strings over the frets resulting in high action i.e. it makes the guitar harder to play. The only thing to do is to loosen the glue, detach the neck, re-establish the proper neck angle, and glue it all together again. Fortunately this doesn't have to happen very often. To illustrate, my Yamaha is over 30 years old and shows no signs of needing a re-set. It doesn't hurt that I use light or extra-light string which exert less pull than heavier strings.

There are a number of different options, but the one I have chosen to use is a combination of mortise and tenon and bolts. The only glue I use is on the fretboard extension, so when a re-set is necessary all that will need to be done is to heat the fretboard extension to loosen the glue, then unbolt the neck.

To cut the mortise into the body I use a router with a template bearing and a jig which holds a mortise template.

Here's the jig:

You can see the centerline on the body centered on the template opening. This is very important, as you, unfortunately, will see.

The bearing of the bit rides against the template and the bit cuts into the body and heel block.

It's kind of scary to cut into your body, but you gotta do it. I cut just a little at a time, as I don't want to overheat the router.

When it's over I inspect the opening, expecting to see a perfectly cut mortise.

Crap. I used a few cam clamps to hold the body to the jig, but I don't want to clamp it so hard that I crack the body. Unfortunately, while I was cutting the body shifted and the mortise is cut off the centerline.

I hate it when that happens. So now, instead of going on to cut the tenon, I have to plug the cut and re-do it. I forgot to take some pictures to show you, but you'll see what I mean in a minute.

I trace the off-center mortise onto a block of mahogany and use the table saw and band saw to cut a plug, then glue it in the mortise. After it dries I'll cut the mortise again.

This time I clamp some 1" dowels to the jig at both waists which prohibits any side-to-side movement. I re-draw the center line and make another cut. This time it works fine. Here you can see the mortise, and you can see a little bit of the plug. See how far off-line it is?

Fortunately this is going to be covered by the neck heel, although I'm going to have to make it a little wider than normal to cover the plug.

Now it's time to cut the tenon. I draw the lines for the cut on the neck block, and use the table saw to make the cuts. I'm cutting the width of the tenon here, and next I'll turn it on its side and cut the cheeks. I undercut the cheeks a bit to make sure there is a good seal against the body.

I use a chisel to shape the round end of the tenon, and I'm done. The tenon will receive inserts into which I'll screw some bolts from inside the body through the heel block, and that will hold it tight.

Next up is to carve the heel.

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