The Grizzly G0602 is a great lathe, but it doesn’t have gears to allow cutting left-hand threads. It also lacks a neutral, so the leadscrew always turns unnecessarily. This simple mod can remedy both these inadequacies.
This page will serve as my build log for the reverse. I got the idea from this thread. (My posts are under “Isotope”.
First I had to figure out what gears to use. I purchased 3 different ones from Stock Drive Products/Sterling Instrument.
This was my first guess, a standard 40-tooth gear, 14.5-degree, diametrical pitch 16. Seems like this would work.
The second gear I ordered was also a standard gear with DP=16, but with a contact angle of 20-degrees. You can see this actually meshes slightly better.
The third gear in the order was a metric gear with module 1.5, 20-degree angle. By calculation I had already figured this was the correct gear, and you can see it meshes perfectly with the grizzly gears. So I ordered a second one of these.
I decided to use 1/4″ aluminum plate for the tumbler, because I had some lying around. With 0.25″ between the lathe and the gears, the hubs ended up being a little too wide, so I turned them down 0.21″. Here’s my clever improvised mandrel using a small (ER8) collet extension that happened to have a 10mm shaft and threaded end. A piece of rubber compressed by a screw held the gear just enough so I could take light passes:
A gear is supposed to mesh precisely at it’s pitch radius, so it’s a simple matter of adding radii together to determine how far apart two gears should be. The radius of the gears I purchased is 30*1.5 / 2 = 22.5mm. The radius of the 60-tooth grizzly gear is 45mm. (I worked in millimeters to make the math easier, and converted to inches later.)
The tricky part is that all three gears should mesh and be close, but the 60-tooth gear should not interfere with the second 30-tooth gear. So I made the constraint that the outer radii of these two gears should be at least 2mm apart. I then put my math degree to use and solved for x and y. After converting back to inches, here is the diagram for hole spacing in the plate:
The hole at (0,0) is 0.544″; the other two holes are 0.390″ (10mm = 0.393″).
You can see I turned down the shoulder bolt so a washer and the 1/4″ plate can fit. I didn’t measure the diameter, but slowly turned it down til the plate just slid on without binding. The bolt still works as-is, so I could revert back to the original design if this doesn’t work.
Next I turned grooves in some 10mm drill rod, and put a slight chamfer on the opposite end before parting off. I pressed these into the aluminum plate and assembled the gear train. Here’s what it looks like: