Some times it's a very small ledge at the bottom of a hole. Some times it's a rib on a boss.
When things get really tight and I have a multiple cavities where I know I'll have slight differences.
I use other features to allow for good hits.
Here I measure the Diameter, take the radius from that feature "CIR4.R.ACTL"
I'm using a Ø2mm probe tip so I subtract half the tip "1mm" plus enough so that I will miss the wall but yet stay very close to it "0.125mm" and assign that to FRM_CENT1
Then I take a point after which I edit that point by placing my new "FRM_CENT1" the first position of the "THEO" and the "TARG".
This allows for the cavities and/or part to part to very and the CMM to still make good touches.
CIR4 =FEAT/CONTACT/CIRCLE,CARTESIAN,IN,LEAST_SQR
THEO/<0,0,15.9172>,<0,0,1>,10.9924,361,361
ACTL/<0.0937,-0.067,15.9172>,<0,0,1>,10.9537,361,361
TARG/<0,0,15.9172>,<0,0,1>
ANGLE VEC=<1,0,0>
DIRECTION=CCW
SHOW ADVANCED MEASUREMENT OPTIONS=NO
SHOW_CONTACT_PARAMETERS=NO
ASSIGN/FRM_CENT1 = CIR4.R.ACTL-1.125
MOVE/POINT,NORMAL,<0.9132,-0.0326,13.613>
PNT_SPLINE_SIDA1=FEAT/CONTACT/VECTOR POINT,POLAR
THEO/<
FRM_CENT1,-12.2152,15>,<0,0.9999939,0.0034907>
ACTL/<4.4646,-17.6739,14.9985>,<-0.0004155,0.9999938,0.0034907>
TARG/<
FRM_CENT1,-12.2152,15>,<0,0.9999939,0.0034907>
SNAP=YES
SHOW ADVANCED MEASUREMENT OPTIONS=NO
SHOW_CONTACT_PARAMETERS=YES
AVOIDANCE MOVE=NO,DISTANCE=0
SHOWHITS=NO
