I found out that Harmonic Drive make some really nice integrated drive/bearing/servo assemblies that are pretty much a 4th-axis waiting to happen. Lots of torque capacity, a large crossed-roller bearing and an integrated servo drive with a hollow shaft for through-spindle work. All very nice, and extremely expensive new. They are even expensive on eBay, but if you set up a watch you can occasionally find a bargain. I was in no hurry, and eventually picked one up for $250 from a seller in the US. It wasn't quite that simple, I had to get it shipped to a friend in Richmond, CA, then he stripped off a huge and heavy bracket and sent it on the slow (and cheap) boat to me. I ended up paying both California VAT and UK VAT on it, but it still saved a few $100 on the original quoted shipping price.
The drive I got was an older FHA-25B drive. This turned out to be a happy accident, as the FHA-xxB drives use Hall sensors for commutation and conventional quadrature encoders. The later FHA-xxC drives use a proprietary serial encoder for feedback and commutation, and only really work with the dedicated drives. If you choose to follow this route, look for the B-series actuators.
The harmonic drive is pretty-much ready to go as-is, it just needs a bracket. I decided to use cast iron.
I designed a bracket in Inventor, and then used the excellent CAM in Fusion 360 to machine a pattern.
Machining took quite some time. I used a some pre-used SikaBlock M970 that I had lying about. In the process I made quite a mound of pretty green petals.
And then at the end had a fairly good pattern in the wrong colour to send to the iron foundry.
One thing that I decided early on about this 4th-axis is that it would use the same spindle-nose as my lathe, so that I can use the chucks, face-plates and collet adaptors that fit that, and potentially transfer work directly from one to the other. A not unimportant consideration here is just how tedious it is to centre work in the 4-jaw chuck in a dividing head. Even a CNC one is tedious, I hate to imagine what it would be like twiddling a handle.
My lathe is a D1-4 nose so I set about making that while waiting for the foundry. I used some EN24 / 817M40 (having bought half a pallet of bar-ends on eBay). The D1-4 nose has 3 locking cams (the D1-5 to D1-20 have 6). This leads to some difficulty as the harmonic drive has 8 mounting holes round the register and this was a bad fit to the 3-fold symmetry of the spindle nose. It took a bit of fiddling in CAD but by deciding to retain the locking cams in an unconventional way (there are no centrifugal forces to counteract) I managed to find a way to squeeze in 5 mounting screws. I also did a CAD investigation of how to manage a D1-5, but that ended up with a two-piece nose with mounting bolts buried inside.
First I bored out a recess to match the register on the dividing head.
Then I drilled and deeply countersunk the mounting-bolt holes on the mill. At the same time I drilled and finish-bored the holes that take the three camlock locking studs.
I then machined a dummy register to match that on the harmonic drive, mounted the nose on that, and completed the machining.
A trial fit on the harmonic drive proved that I hadn't messed up my units or something silly.
There was then something of a hiatus waiting for the castings. During this time I was looking around for a suitable drive. I got in touch with the chaps from the STMBL project who have an open-source drive almost ideal for the actuator (it is a 200V class servo, I will probably be running it on rectified UK mains). Luckily one of them was due to visit London Hackspace the next week, so I popped in too, with a few motors, including the harmonic drive, and was lucky enough to go home with a beta-sample of the V4.0 drive
Eventually the castings came back. I had 4 cast. One for me, one spare, and two for two other folk who expressed an interest. They came in at £60 each.
First the base to the as-cast front face (the mould parting face, to pretty flat)
Then the front face square to the base.
For making the bore/seat for the harmonic drive I needed to ensure that the bore was true to the front reference face, so I squared the part on the mill with a dial indicator for perpendicular.
I then had to decide where in the casting the centre of the hole was. This was, of necessity a rather approximate process as the hole was not round, and the surface not smooth. But I minimised the blur on my coaxial indicator and bored through with my automatic boring head:
The other diameters are bigger, so I had to make a rather Heath Robinson setup with one of the extension bars to enable back-boring. This looked a bit implausible, but actually worked surprisingly well.
Once the bores were done, I could drill and tap the mounting holes. This could have been done from the chuck-side with through-holes, but I decided to do it the hard way, which required the purchase of a long-series drill and the manufacture of a tap extension:
The only thing remaining was to machine the location grooves in the base to align the head with the table slots. This was actually a problem that exercised my imagination, as the slots need to be exactly aligned under the mounting bore. Here is what I did, I would be interested in other ideas.
First, I trued the base of the casting to the X axis of the mill:
Then picked up the middle of the bore with my coaxial indicator in the vertical head.
I then made a reference slot with a 5mm cutter in an area that would be removed by the alignment key slot. This was made to fit a piece of brass with a hole bored as exactly as I could manage in the middle.
I then switched to the horizontal head and picked up the hole in the piece of brass with my coaxial indicator. I thus found that the axes of my horizontal and vertical spindles are not absolutely exactly coincident, there seems to be a 0.15mm offset. Or I made a 0.15mm error in my work...
Alignment slots and cut, a coat of paint, and the mechanical work is done, time now to figure out the drivers and HAL connections. Once that is done I can bore the holes for the camlock cams. these are specified at a specific angle from the camlock stud holes, so it makes sense to wait until the head is powered to machine those.