I'm (still) a total rookie and I'm pretty sure I'm just being a numpty and missing some obvious method, so apologies if this is a daft question, but...
I have a lathe of the mini-lathe type: my chucks don't screw on to a central thread, they are bolted on to a flat plate (with a central register) at three places around the rim.
I have recently acquired a backplate for another piece of equipment, which I want to turn a register onto to match that on my lathe - so I can use my lathe chucks on it. The backplate is (and needs to stay) threaded and registered to fit a Myford spindle nose: it seems the makers assume that a Myford lathe will also be available to fit it to... Because it has this fitting, the back of the backplate is quite small diameter compared to the front. (It's nowhere near as glossy as
http://www.chronos.ltd.uk/acatalog/myfback.jpgbut it's basically that shape, if you see what I mean.)
Since any inaccuracy I inflict on the backplate register will affect everything I ever do using it, I am rather concerned to get this right! I understand that what's really important is keeping the register I put on the outside front concentric and parallel with the register that exists on the inside back. However, I'm really struggling to mount the plate concentrically enough on my lathe.
I tried holding the outside of the smaller diameter part of the backplate in a 4-jaw chuck... this doesn't seem to be the answer, as I struggled for ages and I can't seem to get it concentric
and parallel at the same time. Part of the problem is that the "neck" of the backplate is heavily radiused and that really restricts the way I can grip it. My 4-jaw doesn't seem to open large enough to comfortably grip the outside of the plate. I also can't figure out to indicate off the backplate's existing register while I do this, because the front's in the way from one side and the chuck from the other, so my measurement is doubful in any case.
I've also failed at the "clamp to the faceplate" approach, because the only place I can see to hold it is around the outer rim which I want to turn.
Therefore I thought I'd seek the collective wisdom of the Mad Modders! I'd appreciate opinions on whether I should...
1 - try the 4-jaw again and see if I can improve my admittedly weak 4-jaw skills to the point where I can successfully mount this thing (I can't imagine how)
2 - Build a temporary, chucked, adaptor: chuck a chunk of spare metal in the 3-jaw, turn it to the size of the backplate's register, and fit the backplate to that (will it be rigid enough? I don't think I can get a 12tpi thread out of my metric lathe, either. Ceases to be concentric as soon as I dismount it.)
3 - Build a less temporary, taper fitting, adaptor: like plan 2 but turn the chunk of spare metal to MT3 at one end first, turn it round and put the taper in the lathe spindle, then turn the Myford-type register on the front. (Rather daunting. Would I need to harden this? Would it need a space to fit a drive dog? Still the threading problem. But at least it's reusable.)
4 - Buy some pre-existing adaptor with the right thread (but I can't find one - maybe it has a proper name I don't know)
Advice would be greatly appreciated. (apart from "buy a real Myford" ;-) that'll just make me scream)
Thanks very much in advance!
- Lost of London