CNC lathe conversion, page 3

X axis, cross slide

Adding a ball screw to a cross slide is always hard, there is simply not enough room under it for any reasonable size screw. I have seen a couple conversions that added a ball screw alongside the slide, on the tailstock side. I considered lots of options and finally decided I'd simply use the original leadscrew, at least to start with.

Since this is a new machine the screw is not worn so backlash should be minimal. I also think on a lathe we rarely need to make cuts that involve direction change- we usually cut on the infeed. If backlash becomes a problem I think my first solution will be to use an epoxy nut, like I did for my Sherline Mill

The next issue is how to arrange the drive. I would prefer to have a direct drive, with the stepper right on the end of the screw, but in this case that would have left the motor way out in front of the machine and in the way, so I decided to go with a belt drive. The photo shows the mount for the motor and the belt. The upper pulley was machined exactly like the original handwheel so it fit directly on the screw.

Spindle Sensor

To do any threading requires a spindle sensor to give Mach an index position. Mach only needs one pulse per rev of the spindle. Usually such a pickup is added on the spindle, outside the headstock. In this machine the headstock is a big empty cast iron box, so I decided to mount the pickup inside it. This will keep it completely out of the way, and with no way for chips or anything else to hang up.

Instead of a wheel with a hole I made a simple piece of aluminum angle to act as a shutter. I tapped a small hole into the spindle and screwed the angle on. Obviously this kind of tab would never work outside the headstock- it would grab every chip and wire on the machine.

For a sensor I used an electronic limit switch from Makerbot. When I ordered them they were under $3 each. Similar sensors are sold by PMDX and CNC4PC

I mounted the sensor on a piece of aluminum angle, and used some plastic standoffs to adjust the height of the board to get the shutter positioned correctly to the sensor.

Drive motor

This lathe comes with a single speed AC motor. It has an idler shaft and belts to get 6 speeds. I wanted variable speed and computer control of the speed, so I switched the motor to a 3 phase 1/2hp unit I found at a surplus store. To that I added a VFD from DealersElectric. I've ordered several VFDs from them and always had great service and good prices. this unit is for 120 volt input and 240 volt, 3 phase output. It cost less that $100.

I had to make a new mounting plate for the motor which I made from some 6" channel iron I had in my shop. The motor can swing for changing belts.