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Adventures in metalwork, or how I made my
"Bridge Mill"
© Frank Ford, 12/13/99; Photos by FF, 12/99

In the last two years I've written hundreds of pages about instrument repair, and I've really gotten used to taking photos of my work in progress. So, as I put together this bridge routing fixture, I couldn't resist composing a small documentary of the work.

I do all my luthier type activity in the back room at Gryphon Stringed Instruments, but when it comes to tooling, I usually work in my garage at home, where I have a variety of power tools that aren't compatible with a retail environment. This is one of those "at-home-in-spare-time-jobs."

I present this material more as a view of how I go about tooling, rather than a detailed "how-to" article, for a number of reasons. First, I hope to illustrate how I, a woodworker, manage to make simple tools without being a machinist. Second, I chose an expensive way to go because of my limitations (time, tools, and machine skill) which I don't necessarily recommend. Third, I hope to encourage other wood-bound luthiers to consider working out their fantasies in tooling.

For many years I've wanted a very solid, precision routing fixture for guitar bridge saddles. I can't count the number of saddle slots I've routed in new bridges, or in old ones for intonation correction or pickup installation. I finally retired my 20 year old Dremel, and for the last year have been fighting the inherent flexibility of the new models, which cause the bit to chatter and wander under load. I've always hated my saddle routing fixture almost as much as I dislike the commercial acrylic ones. I rout a lot of saddle slots, and I figured the cost of the tool wasn't much of an issue, compared to the hassles of setup. So, this time I was determined to go first class all the way.

Here's my power unit. It's a regular Bosch laminate trimmer, one I bought when I first decided to retire the old Dremel. I never used it, because it's a bit heavy to use with my old fixture, and the starting torque makes the thing jump almost out of my hand.

I got out my McMaster-Carr catalog, and ordered up a precision linear slide unit. I figure there's no chance I'd be able to align any kind of bearings this well, so I spent the dough (over $400.00) for the best unit I could find. This baby is rated at 600 lb. working load.

The carriage runs on "recirculating" ball bearing races, with 1/2" hardened steel rails. It's really something. There's absolutely no side play at all, and it glides effortlessly. At that price, it had better!

On the next page, I spied this little ball bearing unit. I figured it would be just the ticket for a vertical slide, but being so small, I ordered two, for good lateral stability. Oops! I made a small mistake in my order, so I got two different lengths. Well, I'll just use them anyway, because I really want to get this project done right away. These little slides are each rated at over 200 lb. (And each cost over $200.00, dang it!)

The little slides have built-in end stops, but the big one doesn't. So, to keep the slide from running off the end as I work on this project, I made a couple of aluminum stop clamps. They'll be my stops for the final tool.

I just cut them out of 1/2" aluminum on my bandsaw after drilling the 1/2" diameter hole. Notice I have it set up as a clamping device, not a setscrew. I don't want to scratch those precious rails.

I've been a woodworker all my life, and have done relatively little metal and machine work. One thing I do know is that it's important to keep your bench neat. <g>

Recently, I bought a used Taiwanese metal lathe, which I've used from time to time on tooling projects. It's my only real metal working machine tool, and I'm still a novice when it comes to working with it. Here, I've chucked a chunk of 1/2" aluminum and I've bored a hole to receive the turned end of the router. This was the only way I could think of to get a round, perpendicular hole. I just counted on the faces of the chuck jaws to be square enough to align the work piece.

Rather than count on my ability to measure, I just tried the router in the hole until I got a nice fit.

I drilled and counterbored a hole in the piece, before cutting it to shape as a "clamp" to hold the router. Many years ago I learned that I could cut aluminum on my wood cutting bandsaw, because the cutting speed for aluminum is the same as that for wood.

Wow! I don't know why I hadn't thought of it myself, but someone just told me I could also use my table saw for aluminum. Those chips are nasty, but it works reasonably well.

So here's my first bit of joinery. I trued up the butt joint as well as I could, using my little 1"x42" belt sander. Then I drilled and tapped one hole to screw the two parts together. With the parts screwed together, I could drill the other two holes through both parts. That's how I aligned everything on this project. Every time I try to measure and drill, I screw up and parts don't fit.

A little "sculpting" with the sander made my router clamp complete. I tried drawing and planning this tool on paper, but gave up almost immediately. I just can't work that way. Every time I try to visualize how things will fit, I find I'm way off. I seem to get better results by modifying the design to fit how things come out, if that makes any sense.

Because this unit will slide up and down, I was worried that the bit might not be parallel to the sliding action. I chucked in a piece of 1/4" drill rod, rotated it by hand, and measured it height from the cast iron table of my saw.

I remembered I'd bought this dial indicator for another project (don't ask!) and found it worked a bit better for measuring. I'm off by about .010" per inch of length. I suspect that'll be close enough for routing bridges, but if it's not, I can later shim the bearings under this slide.

I needed a piece with a slot to make "fore and aft" adjustments once the jig was mounted on a guitar. Not having a better way, I decided to cut the slot on my bandsaw.

Cleaning up the slot with the sander, I got a decent, but not all that straight slot. As with all the parts of this tool, I don't really need precision in my work. I'm counting on the ball bearing slides

OK, a 1/4-20 bolt slides up and down in the slot.

The next few photos illustrate how I join parts. I'm not going to show each and every part or operation on this project, because I'd have hundreds more photographs to process. Also, I'm not suggesting that you make this tool. I hope you might grab an idea or two about the process, though. Here, I'm just checking the end for "squareness."
I really like those hex socket cap screws. Some years ago, I bought an assortment of 1/4-20 and 10-24 screws, along with their associated taps and counterbores. Great to have 'em around. I've drilled four holes, and counterbored them to receive the screw heads. Next, I drilled through the holes to spot the centers of the piece to which this will be bolted.
I then drilled the proper hole in the mating piece, and tapped the holes by hand. If the tap isn't dead on straight, things still fit together OK, and the bolts will flex a bit as the two parts draw together.
Not being able to make a truly flat surface on the end, I simply hollowed it a bit on my sander, just the way I would if it were a piece of maple. I don't know if machinists ever do this kind of thing, but it works for me. . .
So, as the two pieces are bolted up, this one will bear more or less on the outside edges, and I'll get reasonable support, without "rocking."
Naturally, the end didn't line up well, but I fixed that!

So, that's pretty much how I did the joinery.
I needed to tap some little 6-32 holes to mount the small slides, so I went at it fearlessly. Oops, got one in the wrong place, a little too much toward the outside. Well, I can always move down a bit, and start again.



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