The fourth arbor drives the escape arbor, and is driven by the center arbor. I made it from 1/8" music wire, cutting its pivots with a sharp graver, as described in a previous installment. Since this arbor is in the upper lightly-loaded part of the train, I made it with rather fine pivots -- about .040".
This arbor contains a 20-pin lantern pinion and a contrate wheel, made earlier, which drives the escape arbor. Since contrate wheels produce lateral forces, I put a thrust bearing on the side of the arbor which receives the lateral force, just to lessen train friction. Otherwise, the arbor would be pushing against the plate with the shoulder of the pivot. The shoulder friction would be small, but eliminating every bit of friction you can helps.
This pinion doesn't have the spool shape used in earlier pinions. I made it with separate shrouds or plates. This makes it possible to center-drill and drill the pinion with only one operation, without having to repeat the centering and drilling operations for the inner shroud. This is made possible by shortening the distance between the inner and outer shrouds when they are drilled.
The pinion is made from 1/2" brass in the Taig lathe. First, I drilled a hole just under 1/8", then reamed it to an eighth minus a thousandth with a "one under" reamer, as described earlier.
Then I made three slits in the brass with the Taig back-mounted parting tool; one slit on the outer end, another slit which leaves a piece 1/16" wide (forming one shroud); and yet another slit leaving yet another piece 1/16" wide (forming the other shroud). The cuts are deep enough to leave material around the pinion center hole about 3/16" thick. The photos should make this clear.
The slit separating the two shrouds is only as wide as the parting tool, and it is too narrow to permit the tiny .023" twist drill to wander when it's drilling the pin-holes through both shrouds.
After the cuts are made, I unscrewed the chuck from the lathe without disturbing the pinion brass, and moved it to the Sherline rotary table on the mill. Then I used a tiny center drill to mark the locations of the pin holes, indexing the rotary table 18 degrees for each hole. Then I drilled the .023" holes for the pinion pins, drilling through the top shroud and then through the bottom shroud in one operation. This saves time and improves accuracy.
Then I returned the chuck with its pinion to the lathe and parted off the front shroud, finishing the slit already cut between the two shrouds. Then I parted off the back shroud, also leaving a spigot on the back shroud, like the one on the front shroud. These spigots help to stabilize the pinion on the arbor. Again, view the photos for details.
I mounted the shrouds on the arbor with an interference fit. The shroud holes are .001" under the size of the arbor, but they seemed a bit too snug to go onto the arbor, so first I used a clockmakers broach to very slightly increase the hole size, at the same time taper the holes a bit. Then I put marks on the arbor with a felt-tip pen which indicated the proper places for the shrouds.
Using an alcohol lamp, I heated the shrouds, one at a time, and tapped them onto the arbor to their proper places. Once the brass has cooled, it grips the arbor tightly. (One shroud may have to be rotated a bit so that the pin holes in the shrouds line up.) Then I inserted .023" needles in the holes, holding them in place with Loctite, and trimmed them with a Dremel cutoff wheel, as described in an earlier installment.
I used the Webster depthing tool to ascertain the proper spacing between the center and fourth arbors. The location for the fourth arbor is the intersection of the Webster scribe mark, and another mark which is a vertical line which indicates the location of the vertical escape arbor. Here I drilled a .040" hole through both front and back plates, after pinning them together, as described in an earlier installment. Later, I'll use a clockmakers broach to oversize these pivot holes a bit -- until there's a proper wiggle of the arbor when the pivot is inserted into the hole.
The thrust bearing cap sits on the outside of the back plate over the fourth arbor pivot hole. It contains a threaded hole for a 0-80 stainless steel set screw, which forms and adjustable thrust bearing. It mounts to the back plate with two 0-80 screws.
The thrust bearing cap is made from 1/2" brass in the lathe. First, I centered and drilled a .040" inch hole in the brass, which is the size of the fourth arbor pivot. Then I made a slight recess in the end of the hole, using a 1/8" bit. The recess is only about 1/16" deep.
Then I removed the chuck from the lathe, leaving the brass undisturbed in the chuck, and mounted it on the Sherline rotary table in the Taig mill, where I drilled a mounting hole in the brass which is the outside diameter of a 0-80 screw. Then I rotated the table 180 degrees and drilled a second mounting hole. Using a rotary table for this job makes the holes uniformly located and on a line with the hole in the center.
Then I removed the chuck from the rotary table and re-installed it on the lathe, where I used the Taig parting tool to remove the thrust bearing cap from the mother brass. Here also, I left a small spigot around the center hole. This will leave some extra material around the hole, which will later be threaded for the 0-80 set screw which forms the thrust bearing.
I mounted the thrust bearing capon the back plate by inserting a round (smoothing) broach through the center hole in the thrust bearing capand then on through the .040" pivot hole already drilled in the back plate. This exactly centers the thrust bearing capover the pivot hole. I secured it in place with a Super glue "clamp", then used a transfer punch to mark the centers of the holes. Then they were drilled and tapped for 0-80 mounting screws. I also tapped the center of the thrust bearing capfor the 0-80 set screw which forms the thrust bearing.
I used a pilot cutter to make an oil sink in the back plate. This oil sink is located under the thrust bearing plate, and can't be seen when the bearing is assembled. You can download an article on how to make pilot cutters here.
Today's links:
Cutting slits in pinion Drilling pinion Pinion slit and drilled Arbor marked Pinion and thrust done Webster scribe mark Back of thrust bearing cap Parting off thrust bearing cap Broach mounting thrust bearing cap Thrust attached Oil sink and pilot cutter Thrust bearing cap, GIF drawing