"yourname" wrote in message
....

When turning, you want the cutting edge to be right on the
centerline.
What I do is (with the machine off) pinch a small scale (ruler)
in between the bit and the work piece until it just stays without
falling.
If the top of the scale tips toward you your cutter is low. If it
tips away
your bit is too high.

you would want it very slightly high, so that under load it is near
centerline, but never low, or so I was taught

You are absolutely right, and were taught correctly. The phrase you use
however (very slightly high) is also known as an RCH. Some think that
this is the smallest measurement that can be made. ;-}

This is also "book correct" and, like so many things, is not really
practical to achieve 100% of the time. This actually attempts to address
the fact that under the force the cutting, the work will "raise itself"
while being worked. The obvious part of this becomes "more correct" with
smaller, more flexible material and cuts further out from the spindle.
Diameters of the stock and heavy cuts come into play - the heavier the
cut the more force attempting to "raise the work."

The less obvious part of this is that a cut that starts several inches
from the spindle or Collette will allow much more "lift" in the material
which will be less and less as the cut approaches where the material is
being gripped in the spindle. As this happens your cutting tool will get
"higher" until, if you started high it will become high on the work once
again.

Start "dead on center" if you can - or a thousandth or two low... Being
Highwill cut an RCH from where it should, but it cuts. Being Low cuts
only at the start and then does not cut - it rubs...

JHbs