Yes, I know, a few of you here have said this won't work. But no one stepped up to make me grids, either, so...I mad a jig that's just gotta be better than the cam-timing degree wheel magnetically stuck to my lathe chuck - I'm lucky to get half a degree accuracy with that; which is the important parameter here, all the holes could be a bit off center and not affect anything much, as long as they are in a circle and nearly concentric with the middle - spacing is much more important for this lens.
Here it is.
http://youtu.be/dLRHJM6Vtzk
I found out (again, duh) that a 1/4" drill doesn't make quarter inch holes. They come out a bit oversize in my case - and yes, I tried shimming and other foolery, but that piece just wasn't going to work out, so I made another. But a "D" size is a few mils too small, so with that, and a 1/4" ball driven through the hole, I managed a tight fit on the precisely .250" motor shaft, in my 1.25" OD by 1.75" long 6061 t6 stock. After a little foolery to get the thing chucked up again with perfect hole concentricity (well, about 2 tenths), I was able to machine the rest of it in a couple chuckings and fiddlings. I added a setscrew for the motor shaft, not sure why - it's a very tight fit unless the aluminum is really hot - and 4 more to capture and center the grid-end, which for this design is in the range of 3/4" OD by 1.25" cross section (starts out as a square cross section, the last step is hand rounding that after all the holes exist).
According to various sources, I can expect around 3% of a step accuracy in position with a common stepper. This was one of the nicer ones in my collection, so hopefully it's better, but that would be good enough anyway (for now). This is a 1.8 degree/step motor, I'm full-stepping it as I only really want 45 degree hole spacing just now. It's rated at 4v, 1.1 amp, and I'm running it at about half that (limitations of the wall wart that also drives the arduino circuit). I built a total-overkill driver, with 4 IRL-1104 fets, and a current limiting resistor (Dale, 1% 6 ohm, 25w). Like I said, utter overkill. And no worries - the wire is an adequate fuse should something go wrong with fets of this spec...and they are easily driven straight off the arduino digital outputs with the stepper library. What's shown in the movie is just a demo - I will add some software to parse inputs from a PC for commanding it, maybe add a pushbutton or switch or two if I want stand alone manual uise, this isn't rocket surgery after all, and a PC control is probably overkill for this use...but I'll do that anyway, since it's not that hard to add. It'll be more trouble to make the USB-B and wart jack square holes in the right places in the aluminum box this is going into.
I haven't made any grids with this yet, but it's sized to duplicate the one we are now setting new records with, but make it more accurate than I did with that degree wheel and the lathe toolpost drill. The proof will be in the pudding...on which I'll report when I have something to report.