What does prehit and retract numbers mean?

RobOntario said:

I would like to get detail about prehit and retract. For example: PREHIT/1.5 (mm) RETRACT/1.5 (mm).  Is this 1.5mm from center of the tip top to the surface ( For example we are measuring a surface) , or 1.5mm from edge of the tip to the surface?

It depends on your controller, Per PC-DMIS help file: 

Use the Prehit / Retract box to define a distance value away from the part or calibration tool. The speed of PC-DMIS decreases to the defined touch speed while within this distance. It remains at the touch speed until the hit is taken and the distance is reached again. At that point, PC-DMIS returns to the defined move speed.

Some controllers do not retract on their own. In these cases, PC-DMIS issues the move to do the retract, and the distance is based on ball surface to the part's theoretical hit location. If the controller does do the retract, the distance may be calculated either from the ball surface or the ball center to either the theoretical or measured hit location, depending on the controller.

docs.hexagonmi.com/.../index.htm

Thank you.  How to determine what kind of movement my controller has?  Anything I could rely on from the software or I have to guess the number from common sense based on what I get from measurement ?

We have a very busy CMM ( 4 years old Global S with 2019.2), it checks over 100 parts a day with short programs under 5 mins mostly. My job is to maintain the programs and keep them running efficiently.  I try to reduce some prehit and retract to improve the speed without having too much stops or unexpected hits 

set a routine to end after a known retract value that you can measure... IE 0.250" retract SHOULD fit a 0.250 gage block between probe and part.

If it doesn't, see if a gage block will fit with a retract of 0.250 + probe radius.  Boom you've confirmed if it's relative to contact surface or center of probe.