Friday, May 7, 2010

Bridge Gluing

There are some steps that just scare the heck out of me, and this is one of them. The bridge needs to be glued with the front face square to the centerline, and positioned correctly so that the saddle is precisely where it needs to be. Screw this up and the guitar just will not play in tune.

Fortunately there are some tools that help with the precision, and then there is a little bit of wiggle room that will help compensate--literally--for minor imprecision. We'll get to that part after the strings are on and I'm doing the final setup.

The tool is a clear plexiglass centerline finder, which as two dowels which grip the sides of the neck in two places. This naturally puts the centerline of the plexiglass jig on the centerline of the neck.

It's used first of all when attaching the neck to make sure that its centerline lines up with that of the body.

But it also has an attachment whose edge is square to the centerline, and when placed in the proper position the bridge can butt against this edge and ensure that the bridge is square to the centerline.

First I have to sand the bottom of the bridge to match the 30' radius of the top. I have a small sanding dish with a 30' radius dome that I lay some 80 grit sandpaper on and sand the radius on the bottom of the bridge.

This doesn't take much time at all, because a 30' radius on this small a section is almost flat.

Then, just in case to top hasn't quit maintained that 30' radius I place the sandpaper on the guitar top at the bridge location and do a final bit of sanding. Now I know that there is a good mating surface between the bridge and the top.

Now it's time to do a dry run with the clamps to make sure I've got the sequence down and can identify any problem areas before its time to glue the bridge and the pressure is on because the clock is ticking.

First I cut a small, thin waste piece of mahogany to use as a clamping caul and use double sided tape to attach it to the bridge plate under the top so that the clamps don't damage the rosewood bridge plate. Then I cut some smaller pieces of mahogany along with some cork to protect the top of the bridge from the clamps. I'm going to use three deep throated bridge clamps and one cam clamp:

Once I've got that done, I take the mouse trap apart, and use tape to mark the side and front edges of the bridge position.

OK, now's the moment of truth. Slather a generous amount of glue on the bottom of the bridge, position it according to the tape marks, and use the centerline finder to make sure its square to the centerline. Then I let it sit for a few minutes so the the glue has a chance to get a little less slippery and will grip the top. Otherwise, the rotational forces brought on by the screws in the clamps (and the cam in the other clamp) will cause the bridge to slide.

After a few minutes I remove the centerline finder and put one of the clamps in position, slowly tightening it to make sure the bridge doesn't skate on me. Then the cam clamp, followed by the two clamps on the wings. I can see just a smidge of skating, so I loosen the clamps enough to allow me to slightly reposition the bridge, then I slowly clamp them down tight. I use a razor blade and chisel followed by a wet paper towel to clean up the glue squeeze-out.

And there it is!

Look at the bear claw--isn't that gorgeous?

This is an important glue joint, so I'm going to let it dry overnight. After that I'll mark and rout the saddle slot, make the saddle and the nut, and soon there will be strings!

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