Depthing tool

The process of setting the distance between gear centers is called "depthing."  The pitch circles of two meshing gears should be tangent -- deviation from this ideal can cause problems.  Gary Mahony has some nice animations of what various problems can occur.

So how does one set the distance between a pair of actual gears, rather than on the plans?  George Daniels says (in his fabulous book, Watchmaking) that one should never rely on calculations to depth a pair of hand-made gears.  One should instead use a gear depthing tool.  Such things can be purchased, or made.  For instance, the tool made by Jesse Donaldson is pretty close to what I envisioned, though John Moran's tool is perhaps a bit more standard.

I decided to make a gear depthing tool, though I wanted one that can fit a pair of 8" diameter gears.  So it has to be big.  On the other hand, as a first pass, I was not too concerned with overall parallelism of the two axes.  We'll see how well this works, and will modify if needed.

In order to cut the slot between the two jaws, I set up a fence and used a router.  This worked OK, though it took a few passes to get the router aligned correctly.  I made each pass about 1/8" deeper than the previous one, until the bit finally just cut through.

I made both of the jaws on the same piece of wood, so it was easy to cut the second slot once the fence was in the right place.  This ensured that both slots were aligned at the same distance from the outer edges.  Once cutting the jaws apart, the inner edges weren't exactly even, so I planed both jaws so they were the same size, and then screwed them together.  Since the jaws are soft pine, I made sure to drill pilot holes first, and I sighted the opposite edges to ensure that the slots were exactly parallel.

Each of the four arbors is cut from 1/4" brass rod in the lathe.  I started by facing off the end, and then drilling a 45 degree bevel into the end with the tailstock drill.  After this, I drilled the center with a 5/64" bit to receive a pivot.

After that, I turned down the the shank of the arbor to the appropriate diameter to receive 8-32 threads.  This took a few tries to get right, since I cut the first arbor too narrow and the second one wasn't all a uniform diameter.

I tapered the non-bearing end to a point, and then threaded the entire length of the rod at SAE 8-32.  This took a few tries -- I messed up two rods by cutting them too narrow, but finally ended up getting the right diameter for my die.  That's somewhere between 0.148" to 0.156", which is a bit off from the specified outer diameter of the threads.

Threading on a pair of wingnuts and washers attaches the bearing to the frame.  Adding a second bearing allows a gear to be supported between the two bearings.  The picture below shows an escape wheel mounted in the frame. 

After two more bearings, the tool is completed.  Here it is holding an escape wheel and an anchor.