Star probe configuration is widely used to check parts with grooves, complicated geometries in industries like precision machining, casting, injection moulding, etc.  But to use star probes properly is quite tricky and sometimes increases concerns on measurement accuracy. Just about a year ago, one of my clients who uses star probes all the time with PC-Dmis was concerned about one of their programs where a internal machined bore needs to be checked with a star probe with different tip diameters in 3 directions and not sure how to verify that the probe compensation is done with individual tip calibration data, not the average deviation, or the data from last tip only. So I designed the following test to prove to them that probe compensation is done correctly.

 

1, My star probe configured as (following the order of connection # in pc-dmis): 
===== TIP 1 (3X15 MM), TIP 2 (6X10 MM), TIP 3 (2X20 MM), TIP 4 (4X10 MM)
2, Use TIP 1 to set center of cal. sphere (19.05mm) as origin, then measure the equator as a CIR1 with TIP 1, and with all tips (CIR2)
3, Output CIR1 and each hit;
4, Output CIR2 and each hit;
5, Measure each hit as vector point with the corresponding tip;
6, Output each vector hit;
7, Output a constructed circle (BF) with all 4 vector hits;
8, Output a constructed circle (BFRECOMP) with all 4 vector hits;

My 4 hits are: +X, +Y, -X, -Y, and below is the results:
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===== TIP 1 (3X15 MM)

DIM LOC1= LOCATION OF CIRCLE CIR1 UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 0.000 0.100 0.100 0.000 0.000 0.000 ----#----
Y 0.000 0.100 0.100 -0.002 -0.002 0.000 ----#----
Z 0.000 0.100 0.100 0.000 0.000 0.000 ----#----
D 19.050 0.100 0.100 19.047 -0.003 0.000 ----#----

DIM LOC2= LOCATION OF POINT CIR1.HIT[1] UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 9.525 0.100 0.100 9.524 -0.001 0.000 ----#----
Y 0.000 0.100 0.100 0.000 0.000 0.000 ----#----
Z 0.000 0.100 0.100 0.000 0.000 0.000 ----#----

DIM LOC3= LOCATION OF POINT CIR1.HIT[2] UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 0.000 0.100 0.100 -0.002 -0.002 0.000 ----#----
Y 9.525 0.100 0.100 9.521 -0.004 0.000 ----#----
Z 0.000 0.100 0.100 0.000 0.000 0.000 ----#----

DIM LOC4= LOCATION OF POINT CIR1.HIT[3] UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X -9.525 0.100 0.100 -9.524 0.001 0.000 ----#----
Y 0.000 0.100 0.100 -0.001 -0.001 0.000 ----#----
Z 0.000 0.100 0.100 0.001 0.001 0.000 ----#----

DIM LOC5= LOCATION OF POINT CIR1.HIT[4] UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 0.000 0.100 0.100 0.001 0.001 0.000 ----#----
Y -9.525 0.100 0.100 -9.525 0.000 0.000 ----#----
Z 0.000 0.100 0.100 0.001 0.001 0.000 ----#----

===== TIP 1 (3X15 MM), TIP 2 (6X10 MM)
===== TIP 3 (2X20 MM), TIP 4 (4X10 MM) 

DIM LOC6= LOCATION OF CIRCLE CIR2 UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 0.000 0.100 0.100 0.001 0.001 0.000 ----#----
Y 0.000 0.100 0.100 0.001 0.001 0.000 ----#----
Z 0.000 0.100 0.100 0.001 0.001 0.000 ----#----
D 19.050 0.100 0.100 19.043 -0.007 0.000 ----#----

===== TIP 4 (4X10 MM) 

DIM LOC7= LOCATION OF POINT CIR2.HIT[1] UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 9.525 0.100 0.100 9.522 -0.003 0.000 ----#----
Y 0.000 0.100 0.100 -0.002 -0.002 0.000 ----#----
Z 0.000 0.100 0.100 0.000 0.000 0.000 ----#----

===== TIP 1 (3X15 MM)

DIM LOC8= LOCATION OF POINT CIR2.HIT[2] UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 0.000 0.100 0.100 -0.001 -0.001 0.000 ----#----
Y 9.525 0.100 0.100 9.523 -0.002 0.000 ----#----
Z 0.000 0.100 0.100 0.001 0.001 0.000 ----#----

===== TIP 2 (6X10 MM)

DIM LOC9= LOCATION OF POINT CIR2.HIT[3] UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X -9.525 0.100 0.100 -9.519 0.006 0.000 ----#----
Y 0.000 0.100 0.100 -0.001 -0.001 0.000 ----#----
Z 0.000 0.100 0.100 0.002 0.002 0.000 ----#----

===== TIP 3 (2X20 MM)


DIM LOC10= LOCATION OF POINT CIR2.HIT[4] UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 0.000 0.100 0.100 0.000 0.000 0.000 ----#----
Y -9.525 0.100 0.100 -9.521 0.004 0.000 ----#----
Z 0.000 0.100 0.100 0.000 0.000 0.000 ----#----

 

===== TIP 4 (4X10 MM) 

DIM LOC11= LOCATION OF POINT PNT1 UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 9.525 0.100 0.100 9.523 -0.002 0.000 ----#----
Y 0.000 0.100 0.100 0.001 0.001 0.000 ----#----
Z 0.000 0.100 0.100 -0.001 -0.001 0.000 ----#----

===== TIP 1 (3X15 MM)

DIM LOC12= LOCATION OF POINT PNT2 UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 0.000 0.100 0.100 0.000 0.000 0.000 ----#----
Y 9.525 0.100 0.100 9.523 -0.002 0.000 ----#----
Z 0.000 0.100 0.100 0.000 0.000 0.000 ----#----

===== TIP 2 (6X10 MM)

DIM LOC13= LOCATION OF POINT PNT3 UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X -9.525 0.100 0.100 -9.520 0.005 0.000 ----#----
Y 0.000 0.100 0.100 0.003 0.003 0.000 ----#----
Z 0.000 0.100 0.100 0.002 0.002 0.000 ----#----

===== TIP 3 (2X20 MM)

DIM LOC14= LOCATION OF POINT PNT4 UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 0.000 0.100 0.100 0.001 0.001 0.000 ----#----
Y -9.525 0.100 0.100 -9.522 0.003 0.000 ----#----
Z 0.000 0.100 0.100 0.000 0.000 0.000 ----#----

===== CONSTRUCTED (AUTO/BF)

DIM LOC15= LOCATION OF CIRCLE CC2_AUTO UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 0.000 0.100 0.100 0.001 0.001 0.000 ----#----
Y 0.000 0.100 0.100 0.000 0.000 0.000 ----#----
Z 0.000 0.100 0.100 0.000 0.000 0.000 ----#----
D 19.050 0.100 0.100 19.044 -0.006 0.000 ----#----

===== CONSTRUCTED (BFRECOMP)

DIM LOC16= LOCATION OF CIRCLE CC2_RECOMP UNITS=MM
AX NOMINAL +TOL -TOL MEAS DEV OUTTOL 
X 0.000 0.100 0.100 0.001 0.001 0.000 ----#----
Y 0.000 0.100 0.100 0.000 0.000 0.000 ----#----
Z 0.000 0.100 0.100 0.000 0.000 0.000 ----#----
D 19.050 0.100 0.100 19.044 -0.006 0.000 ----#----
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From above results, you can see the difference between two methods (using 1 tip only vs. star probes) is very small, within 6 microns, not only on the circle, but also on each individual hit.