The kit that I used for my CNC mill was a 3-axis design, but before I even purchased it I had plans for adding a 4th axis. Rather than buying a complete cnc rotary table I decided to convert a manual rotary table to save money and give more flexibility.
The basis for my 4th axis is a Grizzly H5685 4" rotary table. It's a decent rotary table for the price, but also has some shortcomings that needed to be overcome:
flexible table mounting
removable shaft collar and handle
standard shaft size
compatible with 3 and 4 jaw chucks
4 bolt shaft bushing mount
Only one bearing used in the entire table (???)
Luckily I was able over overcome all of the cons with some simple changes:
The original configuration just had the gear and shaft collar rubbing directly on the shaft bushing:
The shaft collar was a "set screw" type and sat slightly out of alignment with the shaft when tightened. This caused backlash when it didn't rub, and excessive friction when it did rub. I was also worried about excessive wear over time. I added thrust bearings to the worm gear shaft on both sides:
The thrust bearings completely eliminated the backlash and made the shaft spin very smoothly. I'm also not worried about wear at all anymore. I held the new bearings in place with a proper clamp style collar. Adding the thrust bearing near the worm gear pushed it forward about 1/4". Ordinarily that would be an issue, but I planned on mounting a 1/4" bracket beneath the bushing which would cancel out the movement and preserve the worm gear placement.
The table itself was held on one side by the face of the frame, and on the other side by a radial bearing (the one and only bearing). This didn't seem like much of a problem with proper lubrication (due to the large surface area), but the only thing holding the table in place was a single screw. If the screw was tight, it caused a lot of friction and if it was too loose then it would vibrate off. To solve this I applied some locktite blue so that the tension could be set more accurately without fearing that it might loosen on its own.
The conversion was pretty straightforward. I used a 3"x3"x0.25" square aluminum tube for the frame and then machined mounting holes for the stepper on one side and the rotary table on the other:
The stepper is the same 380oz*in NEMA 23 size motor that is used on the other 3 axes:
The NEMA 23 side of the frame has 4 threaded holes (M5), but the rotary table side had much higher tolerance holes to allow for proper alignment. The stepper has a 1/4" shaft and the rotary table has a 10mm shaft, so I used a Ruland spider/jaw coupling to connect the two. The shaft coming from the rotary table was slightly too long, so I cut approximately 1/2" off with a hack saw. Once assembled the frame is extremely stiff and has very good alignment with the stepper. The coupling works perfectly and the soft spider makes it nice and quiet without introducing backlash.
The rotary table is then attached to a frame made of two parallel pieces of 1575 extrusion and some aluminum angle end caps. The rotary table sits on a piece of aluminum plate to keep the extrusions from wanting to twist (since the table isn't quite wide enough). It's fastened using 5/16" carriage bolts. The tailstock is mounted on a sled made from 1/4" aluminum and two bearing pads with ratcheting brakes. This allows it to easily be adjusted to the workpiece length while maintaining proper alignment.
The chuck is a 3 1/4" 4-jaw chuck also from Grizzly. You'd think that would limit my maximum stock size to ~3" diameter... but that's just the limit of what the chuck can hold on to. The center of the axis is around 2 1/2" away from the rails which is really the limiting factor. So... if you cut a nice 1" radius handle at the end of your workpiece (using the vertical clamp) then the chuck can grab it easily and you can actually work with 5" diameter stock.
The entire assembly can be placed on the CNC table and clamped down. Alignment is critical, but I'll be drilling some alignment holes so that I always put it in the same location. This will allow me to save the X and Z location of the 4th axis center and only have to worry about where to cut in the Y axis. This is very similar to the vertical clamp, where the location is also known in two dimensions. Originally I wanted to mount my 4th axis along the X axis (to take advantage of the longest length, right?), but I only have <4 inches of clearance. This wasn't enough for the table and supporting hardware to clear the gantry and I didn't want to put holes in my table. But luckily the router is cantilevered out far enough in the +X direction that it can hang just past the center of the A axis.