The Fourth axis outlined here has the following attributes:

1. 40:1 Worm drive for good torque with a small stepper motor
2. Near zero backlash in indexer mode
3. Converting from Indexer mode to Lathe mode takes a few seconds

The photos below probably tells the whole build process, but here are a few details.


The unit is based on a Taig Headstock.

The lathe mode motor belt is engaged all the time. I used a couple of XL pulleys and belt
simply because I had them. You can use two V pulleys and a V belt.

When the unit is in Indexer mode, the lathe motor is not powered, and since the indexer max speed
is no more than 15 RPM, the lathe motor just takes a free ride.

The real key to the whole design is the way the stepper motor is mounted.

Take a 4" length of 3/8" Drill rod and mill 2.25" down to 1/2 diameter.
Drill and tap two 10-32 holes in the flat section to match the stepper mounting holes.
You can see this in the "Indexer mode" photo.

Mount a 3/16" plate on the rear T slot of the Headstock.
Mount a 3/4 x 3/4 x 1-1/4 Aluminum block with a 3/8" reamed hole on the plate as shown.

The 3/8 shaft is simply inserted into the reamed hole, and the worm applies even pressure
on the worm gear due to the weight of the motor on the other side. I also added a spring
to prevent things from moving.

Since the worm is free to move up and down, it always remains in contact with the gear,
virtually eliminating backlash.

To change from indexer mode to lathe mode, just swing the stepper away.
In the photo "Lathe Mode" I have used a block of wood, but I plan to use an S hook on the worm end
of the stepper shaft to keep the worm disengaged.

Feel free to ask any questions.