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.
Hope this helps
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