Harold,

Any common DTI will suffice. It should be held in the spindle of the
machine and inserted into the bore of the rotary table. Do not rotate the
table, that won't tell you anything aside from the eccentricity of the
bushing in the table (which should be 0). The spindle of the machine is
rotated with the indicator in intimate contact with the bushing. The table
and saddle are then adjusted until you get an equal reading on all four
sides, or a constant reading, regardless of what it is. At that point,
you should mark your table and saddle (wax pencil) so you have a reference
point at which your dials will read 0-0, and each of them should be set such
that you know which direction has eliminated backlash. My style is to
always set the dials so they are reading in the right hand direction. That
way you never have to remember which way you set the backlash. Only under
very unusual circumstances do I set the backlash differently.

This reminds of a question that I have been trying to formulate. It
first stood in my way with a plate that I "freed" from a rough cut piece
of metal. The stock was too thin and big to be suitable for my usual
approach to squaring in a vise, so I clamped it on the table (with
plywood underneath of course). I arbitrarily set the dials to zero, and
settled for allowing half the nominal size of the endmill to establish
the dimensions. That was more than good enough, because only relative
hole positions were important on the part.

For the right and front edges, it might have been (or seemed??) easier
to switch to what you have termed left hand backlash. Is there a
correct way to do that? As it was, for each cut that went against
backlash, I backed up, overshot, and then stopped at the new position.

I suppose the same problem would arise if one wanted right hand backlash
but somehow needed to edge find on the right side of something. So
far, the best I know to do is backup, overshoot, and re-approach in the
backlash-safe direction. For an RT, I assume this will be necessary on
one side in each direction??

Please feel free to answer with something along the lines of "keep
trying to formulate that question".

You made reference to getting by with small table. How does one know
what is small relative to a given part size?

Thanks!

Bill