Please click on the small images to see the large views.
A job for power tools
Martin Bridge Blanks
© Frank Ford, 6/8/00; Photos by FF, 6/7/00
I spend a fair amount of time talking up the advantage of hand work in repair, but there comes a time when power tools win the day. I make quite a few replacement bridges for all kinds of instruments, many of which are Martin guitars. I've found that it pays me to prepare some "blanks" ahead of time to streamline the bridge making process.
This is primarily a job involving my old Rockwell/Delta shaper. It's just about the weeniest little shaper around - a 1/2" spindle with a one h.p. motor. Nonetheless, the spinning shaper cutter is ready, willing, and able to chew off major body parts if I get careless, so I'm wary around this critter.
Today, I'm making up some rosewood blanks.
|First, I'll cut my blanks to length. Martin guitar bridges are 6" long, and I make my blanks closer to 6-1/2" so I can use them for oversize applications, and for making bridges for other kinds of guitars.|
|The old 6" belt sander does a fine job of flattening. I'm not particularly interested in the total thickness, as long as it's not less than about 3/8". I'll work the thickness of each bridge suit the height I need when I'm actually on the repair job.|
|Here's my shaper jig. It's a piece of laminated maple plywood (die board) with a big fat block onto which I can clamp the bridge. My cutter is stack of two 1" wide 1-3/4" diameter straight cutters, with a 1-3/4" bearing underneath.|
|When I clamp the blank to the block, the ends are captured by little raised stops, and the base of the jig guides the blank along the cutter bearing. (You may notice that I had to reverse the cutters from the way they were in the picture above. I had been using the shaper for another job. . .)|
|Small as it is, this shaper really spits the chips! I drag it outside and do the work in my driveway at home, to avoid messing up the shop.|
|After I cut one end, I simply reverse the blank and do the other, so I'm in no danger of tearout The jig is formed so that the cutter traces a 1.0" radius as it removes stock to form the "wings" of the bridge.|
|I have another "station" on the same jig. It holds the bridge to cut the back contour.|
|A couple of quick passes, and the bridge blank is roughly shaped to the standard Martin profile, but considerably oversize.|
|Now, most of the time I don't actually do the second shaping step, leaving it to do by hand when I'm making up a bridge. That way, I can choose one edge or the other to get a better grain and color match. As you can see, the heights vary, but the radius contours line up perfectly.|
|Here's the big deal. I'm holding a factory bridge in front of my blank (not quite lined up, though, but you get the idea). Notice that the radius cuts match the finished bridge, and that they are spaced the same distance apart. No matter what the vintage, Martin bridges always have those same 1" radius contours, and they are the same distance apart. Other features of the bridge may vary from year to year, and that's why I stop my blank at this stage. That way I can finish it out to the right contour by hand.|