"Projects" at georgesbasement.com

11. Making Main Gear Screws for No.2 Drills.
These screws are rarely found missing or broken on either of the later models of Millers Falls No.2 drills, whether they be the LRRCW variety (where the screw has virtually no loading) or even the two-pinion drills, where the main gear screw handles the forces between both the main pinion and the idler pinion. That said, I have gotten an inquiry, and so I made a half-dozen screws anyway. Not a simple task, because the screws have to fit flush against the integral shaft upon which the main gear rotates, and the tapped hole wasn't counterbored by Millers Falls, necessitating relieving the chased threads, which is what Millers Falls did, and in turn making a setup to slot the head, which had to be solidly supported because of its fragile waist.  These screws have a No.10 diameter but 28 threads per inch instead of the standard 32 threads per inch.
Form tool in action
The material that I chose is non-magnetic Type 303 free-machining steel, which looks a lot like chrome-plated steel, but which doesn't rust.  It has a nasty reputation for work-hardening dramatically when machined with dull tools, because its austenitic face-centered cubic structure isn't stable and transforms to hard body-centered martensite upon severe deformation. It's OK if the martensite forms in the chips, but not OK when the martensite forms in the machined surface.

Therefore, I was a little apprehensive about making the oval profile of the heads of these screws by using a form tool instead of making a spherical-turning attachment for the lathe. As seen at left, there is nearly a half inch of active cutting edge at work while the form tool is doing its job.  I found that I had to run the lathe at its slowest back-gear speed and apply a lot of force to the cross-feed screw of the carriage, but there were no complaints.  I did have to adjust the bit holder to minimize its overhang in order to keep it from chattering.
Profile of the oval-head screw
I only had to sharpen the tip of the cutter once during the making of the six screws that I made, and I was able to preserve its outline by grinding only the top of the cutter. As seen in the inset, the form tool could make nearly the entire top of the head of the screw.  After this step I sawed off the screw at the neck and faced it off while held in a 3/16th inch collet. I had forgotten to relieve the threads of this one screw, which I did later with a rattail file. All the rest were relieved by undercutting their heads with a small radius tool before parting off with the form tool.
Slotting setup in Atlas milling machine
The slotting saw that I used is 0.052 inch thick but has a 9/16th inch hole, so I had to make the arbor seen at left. The arbor has three main components: The main body, which is 0.499 inch to slide into the 0.500 inch hole of the end-mill holder that fits the No.2 Morse taper hole in the spindle of the Atlas milling machine; a cup to fit over the 0.513 inch extension of the main body so I didn't have to thread outside of the extension; and a 5/16-24 Allen head cap screw to clamp the slotting saw onto the arbor. I added a small steel washer as an expendable wear plate.

The screw being slotted is a dummy that I used to demonstrate the setup for this early picture; it is held in a slotted collet that is clamped into the smaller Vee on the back side of the Vee block, which is clamped vertically in alignment with the 0.375 inch thick steel block standing inside the Tee slot of the main table of the milling machine. This block helps to keep the Vee block from rotating.
Collet function
Making the slotted collet
The two insets above show how the collet functions and how it was made from a 3/8 inch steel round by drilling it axially with a 3/16th inch hole, cross drilling, filling the hole with a piece of 3/16th inch rod, and then sawing the slot. The 3/16 inch rod minimized internal bur formation; I pulled it out, of course.  Not shown is the dimple that I made to accommodate the clamp screw of the Vee block.
Last cut during the slotting of one of the finished screws
The screws shown at right, below, are for sale at $12 each, shipping included to the lower 48.
Finished product