loose fit

Hi,

Sorry for this lathe newbie question. This the the first time I'm
trying to turn something on a CNC'ed Sherline lathe.

I need to drill a just a mite over 2 mm hole into the face of the
part. So I attact a drill chuck onto something which I think is
called a "taper" which then fit into the tailstock. Turning the
handwheel on the tailstock feeds a 2mm twist drill bit into the face
of the part, producing the 2 mm hole. I have 2+ concerns:


1. There is nothing to secure firmly the "taper" into the tailstock,
and while the "taper" do not rotate when the twist drill is fed in,
withdrawing it frequently result in the "taper" coming loose. There
is a thumbscrew on the tailstock with somekind of "rubberseal", but
turning it in fulling don't seem to result in anything. It appears to
me to be more of a decoration piece?

2. I need to get the hole just a mite over 2 mm (for a "loose fit?")
but the next bigger twist drill is way too large. How do I enlarge
the 2mm hole for a "loose fit"?

3. I need to turn the outer diameter down, with CNC control. How do I
zero the Z-axis?

I find using the radius as the z-axis to be logical, but I understand
that for lathe, the diameter is taken to be z-axis? Not very sure how
to handle that from a programming point of view.

Thanks in advance to all those who respond.

Peter