4th axis on an OM-2A, bolt it down or swap it out?

I did the pin thing on my little mill and never looked back. Made a plain aluminum subplate, bolted it down once, bored two dowel holes in it, and the rotary sits on those pins with a couple of socket head screws. Pulling it off is four screws and unplugging the cable, maybe fifteen minutes if I am being slow about it. Getting it back is just drop it on the pins, snug the screws, and one quick check on the indicator to confirm it did not move, which it never does. The trick for me was keeping the subplate thin, like half inch, so I did not give up much Z over the vise. I also drilled a few tapped holes around the edge for stops and clamps, so 4th axis work and vise work both live on the same plate now. If I were doing it again I would spend the money on a proper ground subplate instead of making my own, my homemade one is fine but the pins took a few tries to get dead parallel. As for wether it is worth swapping at all, for me yes, because without the rotary out of the way I cannot swing the door closed on longer stock, and I got tired of working around it.

I went through the same mental battle on my benchtop mill last year and ended up in a place I didn't expect: the rotary stays on, and I stopped seeing it as the thing eating my table. I picked up a small self-centering vise that bolts right to the faceplate of the rotary, so most of what used to be vise work now happens up on the fourth axis anyway. Small parts, brackets, little aluminum plates, all of it clamps fine in there, and honestly the chips clear better than they did down low near the ways.

When something genuinely long shows up, like a 10 inch bar of stock, yeah, that job waits until I have a free afternoon and the rotary comes off. But that turned out to be maybe one job in twenty. The rest of the time I stopped rearranging anything, wich fixed the real problem: I used to spend half my setup time shuffling hardware instead of making parts.

Mine comes off more than it stays on, honestly. The 4th axis work I do is maybe one job out of twenty, so I just didn't see the point of giving up that space all the time. Mine sits on a shelf next to the machine and goes on only when I need it. The annoying part on these little machines isn't the bolts, it's routing that rotary cable out through the side without it snagging when the table moves. Took me a couple tries to find a spot where it has enough slack through the whole travel. If your cable's alredy got a clean path, pulling it on and off gets way less painful.

My table is even tighter than yours and the thing that fixed it was a low-profile vise. I found a spot for the rotary near the back edge and the vise sits just forward of it, so both stay mounted all the time. Longer stock hangs out over the front, which is ugly but it works. One thing to check before you settle on a layout, mine had a flange on the rotary base that ate more depth than I planned for, so measure that first. Now the vise handle clears the rotary by about a finger width and I never think about it anymore.

Before any of these had encoders we ran dividing heads on manual mills and the old trick still works. Once you have the rotary dialed in where you want it, scribe around the base with a height scribe and center punch each bolt hole. Next time you drop it back inside those scribe lines, snug it down, give the chuck one quick sweep with an indicator and you are back in business. Takes longer to drag the thing out of the way than to set it. I also write the zero position on masking tape stuck to the column so I am not re-picking it every remount. On a table that small though, I would still pull it for anything long. No layout trick fixes a machine that barely has room for the vise alone.

Jumping in here since I'm in the same boat but a step behind. I've got a small benchtop mill and have been eyeing a rotary, but posts like this make me wonder if I even have the room for one. Question for those of you pulling yours off regularly: when you unplug it and reinstall later, does it remember its position or do you have to re-home it every time? That part is what's been holding me back more than the table space honestly.