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Now, I broke the tap off in my new hole. Well, there's still a bit more room. I'll just move down a bit more. . .

Most of the time, though, I was able to hold the part in place to make sure things lined right up as I drilled and tapped mounting holes.

For my base slide adjustment, I needed to transfer a measurement from the base to a clamping plate. I did it the same way I measure for guitar tuner hole spacing. I set my calipers to measure from left edge to left edge. I don't trust myself to try to measure the center of the hole.

Without actually reading the caliper, I transferred the measurement. I'd already drilled one hole in the plate, so I measured from each edge of the hole, and swung the caliper like a trammel, scribing a little line.

Then I set my caliper to half the width of my little plate, and scribed again.

From here, I just eyeballed it. I figure I can guess the center of this little space within the tolerance of my wandering drill bit when I'm working on the drill press.

Then, drilling the second hole, my clamping plate is complete.

Well, almost. I forgot those little slide mounting screws, which are neatly in the way.

Time for another design change. A quick bit of grinding will take care of that problem

Now I see why they make washers. They're useful to keep screws from bottoming out.

Oh, yes, take a look at this neat piece of work. Here's my little trio screw holes, with the broken tap in the center.

I found a big hunk of one-inch square brass that should work for my vertical adjustment.

Couldn't cut it straight by hand, and can't use the woodworking power tools on brass, so I squared it up on the lathe, which still had the four jaw chuck in place. I didn't bother to center it, because I was only squaring off the end.

Drilling and tapping brass is fun. Look at those chips. It's almost like working with mahogany. Everything seems to work so much easier and cleaner.

Working aluminum is more like machining Brazilian rosewood. It's gummy, and nasty. Just look at that big sharp stringy mass spinning around! Here, I'm drilling the aluminum base plate. It's just another piece of aluminum salvage, as is most of the rest of this tool.

Well, it only took the better part of a couple of days to put this together, but my first impression is that I waited about ten years too long to get this project done!

It's really simple - just a carriage that slides sideways, and a holder that slides up and down. Because of its mass, and the smooth, tight bearings, it runs with virtually no vibration, and cuts a really straight, smooth, true slot.

In case you're wondering, here's the label on the small bearings, McMaster-Carr part numbers 6257K18, and 6257K19. I had originally assumed I'd use a second big Thomson slide for the vertical adjustment, but when I saw these in the catalog, it was clear I could save a lot of space and weight.

This big slide is McMaster-Carr number 6109K1

So, as you can see, the big bottom slide is mounted on a 1/2" aluminum plate. It measures 15-1/4" x 7", because that's the size it was when I found it. I made an arbitrary 2-1/2" x 8" cutout under the router, which should clear most any guitar bridge.

Here's the carriage from the back side. The diagonal plates and the slide clamping plate are 1/4" thick anodized black aluminum scrap. Most all the rest was made from 1/2" thick aluminum salvage. I buy most of my aluminum from a local metal salvage yard, so I don't know (or much care) about alloy, or precise dimension.

If I hold up the vertical slide, you can see the two different length linear bearings. I tried to turn the accident of a mistaken order into lemonade by working the vertical stop screw adjustment in an asymmetrical orientation.
I'll be able to insert measured spacers under the stop screw so I can plunge to precise depths. After a few trial cuts, I'm ready to give this tool a real test in the shop.
Oh, yes, on the bottom, I've stuck a layer of 1/8" leather for to keep the tool from skating around and to protect the guitar's finish. As usual, I thought of a few more holes to drill after I already had the leather stuck there with contact cement.
Here's the tool in place, doing its job. I simply clamp it there with four mini Quick Grip clamps. As you can see, there's lots of clearance around the cutter for great visibility.
The four clamps act as support feet, holding the guitar firmly in place on my bench.
This is a tiny 3/4 size Martin 5-18. The guitar is so small that the tool doesn't fit below the bridge, but it's no problem to use it the other way around. Sure enough, the sheer mass of the tool makes it a snap to rout a clean, full depth slot with no vibration or wandering even with a 1/8" shank bit. I'll be using 3/16" shank end mills most of the time, though, for extra rigidity.

Let me reiterate once again. I'm no expert on tooling, and only I wanted to present this little piece just to give an idea of how I "get by" in metal working. If I had the skill and the mill, I'm sure I could make adequate slides and other devices which would bring the cost of this kind of tool into a reasonable range. Fact is, though, it fit me the way it is. In another year (or perhaps in another life) maybe I'll get into more serious machining, because it's a great thrill to put together a really good tool I've wanted for a special job!


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