I use a Starrett N0. 25-511 TDI mounted in a TTS holder used as the #1 tool
for datum reference . I set the Starrett TDI up on the surface plate and measure
the height with a height gage at defection of 0.05" and enter this value in the
tool table as tool #1. see: http://tinyurl.com/cvv4nd I then measure the height
of all my other tools and enter them in the tool table.

Setting up a datum is easy. Set the Starrett TDI as Tool #1 in Mach 3 on the
Tormach then bring the tool #1 down to the surface used as a datum reference
shuttle in the reading on the TDI to 0.05" and then zero the Z-axis.
The surface used as a reference as shown here is the top of the 0.5" roll pin on
placed a 45* angle fixture. See: http://tinyurl.com/b888mp In this case I set
the datum for both the front surface of my part and also the bottom surface. I
recently switched from the Starrett TDI No. 25-211 with a 0.025" range (shown in
the second photo) to the much wider range Starrett TDI N0. 25-511 with a 0.2"
range. By setting up at 0.05", I have a 0.150" safety cushion when coming down
to the reference surface with tool #1. This is an easy thing to do using the
shuttle jog controller when one gets close to the 0.05" reference switch from
quick movement using the outer ring to 0.0001" increments using the inner finger
knob on the shuttle. With this TDI tool #1 there is no danger of damage to the
part. The Starrett TDI is an AGD type gage and has a replaceable 4-48 threaded
end. I use an flat elephant foot for landing on either the top of a round
surface such as the 0.5" roll pin or a plain old flat surface. The Starrett TDI
TTS tool #1 has worked extremely well for me for the last couple of years. For
finding a side reference surface I use a plain old Starrett edge finder.

Don Clement
Running Springs, California