Sunday, May 24, 2009

Routing for the Truss Rod and Shaping the Headstock

OK, I'm catching up on some posts and just realized that I got out of sequence on one item. Before gluing the peghead veneers I have to rout the channel that will hold the truss rod. The truss rod is a metal bar that is inserted in the neck. Actually, it's two bars, one of which has threads and a nut, the other is fixed. When one bar is shortened, the fixed one can only bow because it can't shorten or lengthen. When strings are attached to the neck and brought up to pitch, 170 lbs. of pressure causes the neck to bow forward like a banana. A certain amount of bowing is needed, otherwise the strings would simply lie flat on the fret board. But too much bowing would make the guitar difficult to impossible to play. The adjustable truss rod give the neck some additional strength, but allows us to counteract the pulling force of the strings and reduce the bow to just the amount that we need.

I measure the width of the truss rod, divide that in half, and mark that distance on either side of the center line. I then set up my router table to rout the channel. Since the bit isn't as wide as I need the channel to be, I set it up so that it cuts to one of the outside lines, make the cut, then reset it to rout the other half of the channel.

Cutting the full depth would overload the router motor and could possible also result in blowing out a large chunck of wood, so I set the depth to just take off a smidge.

Each time I measure the depth with the actual truss rod, cutting to both line, checking the depth, resetting and doing it all over again. I want to leave as much wood as possible while making sure that the truss road is close to the surface.

Here's the truss rod in the channel.

Maybe a smidge deep, but it's ok. I'll put some silicon caulk in the bottom to keep the rod from rattling, and that will take up some space. (A loose channel will cause a rod to rattle when it's natural resonance frequency is played. It will actually vibrate in sympathy to the frequency, so I want a nice tight fit.)

OK, now is actually when I glued the veneers onto the peghead. The next morning I unclamped everything. Looks good. Next I cut the end of the veneers to a 90 degree angle to the neck (not the peghead) using a 15-degree block as a saw guide. This is an important surface since the back of the nut will bear against it.

Next I screw a template of the headstock shape onto the back of the peghead, and rough cut it at the band saw.

Then I set up a router bit with a bearing to ride agains the template, and with the veneers face down, shape the headstock.

And there it is. I'll put some decorative binding on the headstock, but that's for later.

This is all I want to do on the neck for now. I want to wait to cut the tenon until the body is done because I need to make sure there is a correct neck angle at the body joint--the neck tilts back from the body 1 to 1 1/2 degrees, but I won't know how much until I can attach the neck to the body and measure the gap at the bridge. You'll understand when it rolls around. I also need to wait to shape the neck until at least the fretboard is glued on, and there's much to do before that happens.

So it's on to the soundboard.

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