I replaced the AC spindle motor on my Unimat in lathe configuation with a large stepper motor so that I could control it with my computer. This will enable digital threading as well as rotary machining and engraving. I also plan to use the CNC controlled spindle as a fourth axis for my DIY 3 axis mill.
In the above picture, I rubbed a crayon across the engraved parts of this acetyl rod. I warmed it with a heat gun, then sanded and polished off the excess crayon.
Notice that the mount for the cutting tool is 3D printed. My 3D printer is also DIY, and is my own design built from scratch. I went through two iterations on the design and I still need to make a small change in height. It would take me a long time to make this part out of metal in a a machine shop, and if I had made the first and second parts, I would be disappointed to find out that I still needed modifications. By utilizing rapid prototyping, I draw the parts and print them quickly. I modified the design to make it stiffer since it is made of ABS instead of aluminum. I made a third revision to decrease the overall height by 0.070 inches to match the spindle and cutting tool heights. Making a quick change to the drawing, exporting to STL format, and printing over again is quick and easy compared to modifying or machining a new part. So far the system seems to work perfectly well for my purposes.
So far I am using Mach3 in mill configuration. I simply made the X axis the lathe axis, Z the cross slide, and Y the spindle. I measured the diameter of my part to determine the circumference, which is the length traveled in one revolution. Since I know that the stepper motor takes 200 steps per revolution, taking in to account the pulley ratio, I can determine steps per mm around the periphery of the object that I am machining.
I am looking forward to digitally threading a metal rod to see if I can make it work. The conventional solution involves attachment of a power feed with a known thread pitch. I should be able to choose an arbitrary pitch since my lathe spindle and axis motion are both under CNC control.
So far, so good!