An inexpensive alternative to the commercially available tools
By Greg A. Saville
I enjoy making the tools of our trade almost as much as rebuilding and repairing the clocks we use them on. This article describes how I built my own depthing tool as shown in the accompanying photograph. Lately, I have found myself working on a couple of projects that really needed one of these tools to do the job right. However, the occasion just had not come up often enough for me to justify the high cost of purchasing a commercially built one. My last project finally persuaded me to take the time to cobble one up. This one can be built for very little cost, depending on how well stocked your junk box is.
For the base frame, I used some 1/2" thick Plexiglas hinged together with some inexpensive commercial box hinges. The pivot runners can be made from drill rod or any suitably sized rod from your stock or junk box. A few more screws, some rubber bands and a couple hours of your time are all it takes to put one of these in your own workshop.
None of the dimensions are really critical which helps make construction easier. See figure 1 for the general layout. While I give approximate dimensions for the one I made, feel free to improvise to suit your own needs. What IS critical is to make the holes for the runners absolutely true and perfectly parallel with each other. Fortunately, this is not too difficult using the technique I describe. For the frame, I used two pieces of 3 1/2" X 5 1/4" X 1/2" Plexiglas. If you don't have any of this handy, you might be able to find some scrap pieces at a local industrial plastics supplier at low cost. I picked up a dozen or so scraps at a flea market for only a couple of dollars. Before cutting to size and squaring up, I taped two pieces together and then cut them both out at the same time to be sure they're identical. First cut to overall size and then cut out a 2 1/2" X 3" rectangle to make the basic "U" shape with a coping or scroll saw. While still taped together, use a belt sander for final shaping and to smooth out the rough edges from the saw.
To ensure accurate results with your depthing tool, it's most important that the holes for the pivot runners are drilled precisely in line with each other and exactly square. The method I describe here is very simple, yet ensures the holes for the pivot runners will align perfectly. Carefully measure, mark out and then deeply center punch where you want to drill the holes in the arms. Then mount one of the base arms on the center punched marks between centers on your lathe as shown in figure 2. Clamp the base to your cross slide to maintain its position for the drilling operation. You will likely need to place some shims between the arm and the cross slide to position the piece securely and at the proper height. I found a small pad of paper worked nicely and was easy to position accurately by using as many sheets of paper as you need for your setup. Release the tailstock center, back off the carriage and confirm your piece is held securely in position. Now replace the headstock center with a drill just slightly undersize of the rods you're going to use. Carefully drill through one side then replace the drill with a "D" reamer made from a piece of your rod and carefully ream the hole true. When finished, remove, swap end for end, re-center, re-clamp and then drill and ream the other side. Repeat for the other frame half. Test your work for accuracy by passing a piece of rod through the holes and verifying they line up exactly. Aligning on centers like this guarantees that your holes are perfectly in line. Next you need to drill and tap holes for some set screws to hold the runners in place. For simplicity, you could just use some 1" screws. To obviate the need for a screwdriver, I cut the heads off the screws and made some simple plastic knobs for easy finger tightening. I wasn't too sure how well the threads would hold up in the plastic, so I used (relatively small) 6-32 threads. In case they stripped out, I could re-drill and tap the next larger size. However, after a fair amount of use, I've found it really doesn't take much force at all to hold the runners securely in place. With reasonable care, these threads should last a long time.
Hinge the two frame pieces together with a couple of small brass hinges. Use a scrap piece of plastic to test for the right size drill for the hinge screws. The plastic won't give as much as wood does when you screw the wood screws in, so it's important to get a good fit so the hinges will stay snug, yet not so tight that you risk breaking the heads off as you screw them in. Position the hinges and then use tape again to hold everything in alignment while you drill the screw holes and mount the hinges.
I contemplated long and hard trying to decide how I was going make the depth positioning adjuster. I thought about compressing a spring between the frame pieces and using a nut and bolt to hold the desired position, but couldn't figure out a reasonable way to make it adjustable. Besides, it would have required a (difficult to make) oval shaped slot in the frame arms for the adjuster. I ended up taking the simple way out by using a couple of rubber bands to provide the closing force and then drilled and tapped a hole in one of the frame pieces for an 8-32 X 1-1/2" machine screw to provide the positioning adjustment. I turned up a little knurled knob for the adjuster screw to allow easy fingertip adjustment. It also helps to round off the tip of the screw to provide a nice sliding bearing surface for smooth adjustment action. While not as esthetically pleasing as I would have liked had I used some other method with a spring, the rubber bands actually work out quite nicely and don't look all that bad after all.
The pivot runners can be turned up on your lathe in almost no time at all. Start with 4 pieces of 1/8" drill rod, each about 3-1/2 inches long. Face and square all the ends off. Using a small center drill, cut a female pivot in one end of each of the 4 rods. Machine the other ends to male center points by setting your compound slide to 45 degrees or just use a hand file. If you find you need some larger runners (like for mainspring arbors), you can turn some adapter pieces out of 1/2" rod with larger pivots and centers that slip snugly over the rods when needed.
While my depthing tool may not be as durable as the all-metal commercial versions, with careful assembly and use it can be just as accurate and should provide lasting utility.
(Photo and figure captions)
Photograph 1 -- An easy to build, yet fully functional depthing tool.
Figure 1 -- Layout drawing. Exact dimensions are not critical.
Figure 2 -- Drilling and reaming accurate holes for the runners.