I read about the importance of a clean true register on a lathe spindle to ensure accurate location of the chuck etc. but which part of the spindle on my ML7 is the register? Is it [a] the surface of the 31.9 mm diameter section behind the thread, [b] the axial face of the 41.1 mm diameter shoulder behind that, or [c] the outer surface of the 41.1 mm shoulder?
The actual spindle register on the ML7 is the 1.250" (31.75mm) diameter behind (and slightly larger than) the thread. As always though that is not the only important surface, the front face (facing the tailstock) of the larger collar also plays a part in locating the chuck particularly if the chuck backplate internal register is a little oversize. This is quite common on some of the pre-machined backplates. Both surfaces should run perfectly true (certainly within a couple of "tenths" and my DTI doesn't register any runout on these surfaces) if the chuck is to mount properly. If my description is not clear have a look here and it gives the various sizes (not completely accurate) of a variety of screw thread spindles:
So on this drawing for the ML7 it is diameter "B" which is not quite as drawn as it is larger than the thread diameter and the front face of diameter "A".
The thread can interfere with chuck location if it is too tight and does so on many older or worn machines. Some would argue that as there will have to be a clearance between the spindle register and the backplate internal register for it to fit then the axial face of the shoulder and the thread flanks provide the actual location. In the practical world where the thread should have enough clearance to screw on easily the two surfaces I mention are the critical areas.