My Lathe has a 4 teeth per inch (tpi) lead screw, and the chuck thread is 1 1/4″ 9tpi BSF.
According to the screw threading data, I can cut this thread using a 40 tooth drive gear on the spindle, an idler and a 90 tooth gear on the lead screw.
There are other combinations that would work, but this is the simplest.
A full set of change gears for my lathe looks something like the following:
20, 25, 30, 35, 38, 40, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 127.
Unfortunately, I am missing most of the gears for my lathe below 55 teeth, as well as the 127 tooth gear.
Other 4 tpi lead screw lathes, such as the Cromwell 3 1/5″ lathe, also include a second 80, a 110 and 120 tooth gears for metric and imperial threads, and then 31, 39, 43, 48, 53, 59, 66, 73 and 81 for cutting B.A. Threads.
I am not overly worried about cutting the B.A. threads as I have a set of B.A. taps and dies, so can safely ignore all of the gears that are specific to those threads, however I could do with making up the missing ones from my basic set – 20, 25, 30, 35, 38? 40, 45, 50 and 127.
My 60 tooth gear could also do with being replaced as it has a couple of missing teeth.
Back in July 2014 I bought a set of involute gear cutters off ebay, so that I could make any of the missing gears that I needed, as finding suitable 16dp gears on the second hand market could take a while.
Since I currently have a requirement to make a few chuck back plates and adapters, I thought now was as good a time as any to start with a 40 tooth gear so I can cut that 9tpi thread.
A friend of mine kindly gave me some pieces of aluminium a while back and a slice off one of these makes a good starting point.
First I cut off a 10mm slice on the bandsaw and then fly-cut the sawn face to make it smooth.
Next I drilled the hole in the centre out to 19mm (3/4″ is 19.01mm) and milled a keyway with a 3mm cutter (1/8″ is 3.175mm).
Next I made a mandrel out of some steel rod so I could hold the blank in the lathe and also on the rotary table.
I chucked the blank on the mandrel in the lathe and reduced the outside diameter to 66.675mm (2.625″), this being the crest circle diameter for a 40tooth 16dp gear.
Next I transferred the blank to the rotary table on the milling machine, using a 2 morse taper ER32 collet chuck and a 20mm collet to hold the mandrel.
I set up the dividing attachment using the plate that includes a 20 hole circle and set the arms for 5 holes – my rotary table has a 90:1 reduction worm, so for each tooth I will need to rotate 2 turns and 5 holes on a 20 hole plate to give me 40 teeth in total.
Cutter #3 (for 35-54 teeth) is mounted on an arbor and then set to the centre height of the blank in the rotary table, at which point I lock the Z axis.
I locate the edge of the blank, dial in 1mm of cut and lock the Y axis.
It is then simply a matter of making a cut on the X axis and then advancing the rotary table to the next position.
Advance another 1mm and start on round two.
Same again for round 3
The final depth of cut is 1/8″ or 3.175mm, so a finishing cut was taken at this depth
Finally I returned the blank to the lathe and knocked off the sharp corners with a file and here is the completed gear.
I have checked with some of my other gears and this gear meshes well.