Optical Table CNC Lathe - Spindle Modifications

The spindle I have came from Setco. It is a surplus unit that pretty clearly was a secondary spindle in a CNC lathe. It has a heavy cast iron body.

I checked the radial runout with a 0.002 mm indicator as we as a Kaman capacitive displacement sensor. Both indicate a runout less than 5 microns. I checked both instruments against my laser interferometer.

It came with a trough hole and a 5/8-16 left handed thread. I wanted a right handed thread, so I cut that over the existing thread:

This allowed me to use a 5/8 bolt to which I just mounted a standard v belt sheave. I mounted another sheave right onto the motor. This worked well....

... until I set the lathe up to cut threads. At that point the slip between the sheaves and the v-belt had to be dealt with, because the encoder was mounted directly on the motor shaft. So I replaced the sheaves with timing pulleys.

And then threading worked well too. However, one big problem with this arrangement is that I could not use the trough hole for rod stock and always had to lop off a piece short enough to fit into the chuck. That is very annoying. Even if I could, though, the hole is smaller than I like, so lets take corrective action.

First, I take apart the back of the spindle to see how much 'meat' the shaft has where it goes through the bearings.

This is very nice - the bearings are quite big. The shaft is about 1.85" in diameter. I figure that if I use 3 1/4-20 screws on a 38mm diameter, I can open the bore as much as 30mm - Yes, I mix units all the time. My caliper is dual unit too....

Next, I make a drill template in Fusion 360 and print it on a 3D printer

Then I can proceed to mark the drill positions. This has to be done carefully. The front face should be fully supported so that the impact from the hammer on the transfer punch can travel to the floor without brinelling the bearings:

Then the holes can be drilled and tapped. The shaft is also made from cast iron, so I use a sheet of paper to try to keep the dust away from the back plate.

Then, just repeat the process on the pulley, and the through-hole spindle mod is ready for bar stock of up to about 0.5"