Sunday, May 31, 2009

Jig for Cutting The Dovetail Joint

Saturday, May 31, 2009.

In researching how to cut myself a dovetail style neck joint, Mr. Siminoff shows a jig for holding the neck at the proper angle when using the bandsaw. Unfortunately I find that he does not give any indication just how to build this jig, only a couple of photos of his in use. So, the last couple of days have been focused on making this jig and I think I’ve got it. Here is what it looks like and how I made it.

This jig has three important surfaces - the base, the flat that the peghead is clamped to, and the ledge where the base of the joint rests. The base and the ledge are pretty easy to figure since they are parallel to one another, but he flat for the peghead is a bit more tricky. Since the goal here is to hold the back of the neck joint perpendicular to the surface of the bandsaw table, this flat needs to be cut at an angle that combines the 6 degrees plus the angle of the peghead with respect to the fretboard. This means I had to find a way to layout, cut and then verify that the fretboard plane was, in the end, held at 6 degrees off of vertical.

The first thing I did was to I created a jig block by cutting up a 1x6 of hardwood and gluing it back together to create a 6x6 block approximately 12 inches long.

Next, I did some trigonometry to determine the rise and run of a 6 degree angle. This, I calculated, corresponded closely to 1 inch of rise for a run of 10-7/16 inches. Knowing this, I had a way to verify that I really had a 6 degree rise.

Then, rather than try to calculate what the angle of the flat needed to be, I chose to approximate this angle by holding neck and peghead in the approximate location and sketching a line. Once I was pretty comfortable that my sketch was close enough, I cut out the rough shape on my bandsaw and rough-sanded the peghead flat on my belt-sander.

Now that I had something to work with, I needed to be able to clamp the neck and peghead to the jig. For this, I chose to drill four 7/8” diameter holes in the sides of the jig. These holes are big enough to allow me to insert the fixed end of my small C-clamps and clamp without getting in the way.

With my peghead now clamped in place and the heal of the neck joint just clear of the ledge, I was ready to verify my angle. This was done using a flat scale I have and a framing square.

By laying the scale on the fretboard and holding the framing square against it, I am able to make a height measurement at both the “zero” position (in this case, at the edge of the peghead veneer) and then at the 10-7/16” distance. Since my goal is to determine an angle, not an actual distance, I don’t care what the actual measurements are, only that the difference is close to 1 inch. As you can see from the two photos above, that is what I was able to get - just under 3" in the first photo and just under 4" in the second one (the first time I measured I was a shallow, so I took the jig back to my belt-sander until it was good).

Now that my jig is ready, all I have to do is layout my dovetail, create a spacer block to mount to the head block, and work up the nerve to make the dovetail cuts in both the head block and the neck.

Meanwhile, here are some more pictures of the neck clamped in the jig.

And finally, here you can see that the neck joint and jig will just be able to fit into my bandsaw when it is in its most open setting. Thank goodness. I was afraid for a while I was going to have to shell out some big bucks for a larger bandsaw (or cut the joint completely by hand - I really did not want to have to do that!)


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