Can any one explain to me how using datums on could increase my positional error? Below is the same feature true positioned three ways, first is with datums off (.112), next datums on using MMC (.130) and finally datums on at RFS (.177). If PC-Dmis is best fitting the datums to simulate a hard gage check how can my true position error be greater by using datums @ MMC? Current alignment is -E- -C- -M-, bonus does not apply to hole because it is a threaded hole.

DIM LOC119= TRUE POSITION OF CIRCLE E_CIRCLE3 UNITS=MM ,$

GRAPH=OFF TEXT=OFF MULT=10.00 OUTPUT=BOTH DEV PERPEN CENTERLINE=OFF DISPLAY=DIAMETER

AX NOMINAL +TOL -TOL BONUS MEAS DEV OUTTOL

X 400.800 400.826 0.026

Y -86.500 -86.450 0.049

DF 10.500 0.100 0.100 10.592 0.092 0.000 --------#

TP RFS 0.800 0.000 0.112 0.000 -#-------

END OF DIMENSION LOC119

DIM LOC120= TRUE POSITION OF CIRCLE E_CIRCLE3 UNITS=MM ,$

GRAPH=OFF TEXT=OFF MULT=50.00 OUTPUT=BOTH DEV PERPEN CENTERLINE=ON DISPLAY=DIAMETER

AX NOMINAL +TOL -TOL BONUS MEAS DEV OUTTOL

X 400.800 400.738 -0.063

Y -86.500 -86.517 -0.017

DF 10.500 0.100 0.100 10.592 0.092 0.000 --------#

D1 PLANE DATUM_E_DCC AT RFS

D2 12.763 0.020 0.020 0.014 12.757 CIRCLE DATUM_C_DCC AT MMC

D3 82.626 0.020 0.020 0.026 82.632 CIRCLE DATUM_M_BORE AT MMC

TP RFS 0.800 0.000 0.130 0.000 -#-------

END OF DIMENSION LOC120

DIM LOC121= TRUE POSITION OF CIRCLE E_CIRCLE3 UNITS=MM ,$

GRAPH=OFF TEXT=OFF MULT=50.00 OUTPUT=BOTH DEV PERPEN CENTERLINE=OFF DISPLAY=DIAMETER

AX NOMINAL +TOL -TOL BONUS MEAS DEV OUTTOL

X 400.800 400.717 -0.083

Y -86.500 -86.530 -0.031

DF 10.500 0.100 0.100 10.592 0.092 0.000 --------#

D1 PLANE DATUM_E_DCC AT RFS

D2 12.763 0.020 0.020 12.757 CIRCLE DATUM_C_DCC AT RFS

D3 82.626 0.020 0.020 82.632 CIRCLE DATUM_M_BORE AT RFS

TP RFS 0.800 0.000 0.177 0.000 -#-------

END OF DIMENSION LOC121

Regards

Terry

DIM LOC119= TRUE POSITION OF CIRCLE E_CIRCLE3 UNITS=MM ,$

GRAPH=OFF TEXT=OFF MULT=10.00 OUTPUT=BOTH DEV PERPEN CENTERLINE=OFF DISPLAY=DIAMETER

AX NOMINAL +TOL -TOL BONUS MEAS DEV OUTTOL

X 400.800 400.826 0.026

Y -86.500 -86.450 0.049

DF 10.500 0.100 0.100 10.592 0.092 0.000 --------#

TP RFS 0.800 0.000 0.112 0.000 -#-------

END OF DIMENSION LOC119

DIM LOC120= TRUE POSITION OF CIRCLE E_CIRCLE3 UNITS=MM ,$

GRAPH=OFF TEXT=OFF MULT=50.00 OUTPUT=BOTH DEV PERPEN CENTERLINE=ON DISPLAY=DIAMETER

AX NOMINAL +TOL -TOL BONUS MEAS DEV OUTTOL

X 400.800 400.738 -0.063

Y -86.500 -86.517 -0.017

DF 10.500 0.100 0.100 10.592 0.092 0.000 --------#

D1 PLANE DATUM_E_DCC AT RFS

D2 12.763 0.020 0.020 0.014 12.757 CIRCLE DATUM_C_DCC AT MMC

D3 82.626 0.020 0.020 0.026 82.632 CIRCLE DATUM_M_BORE AT MMC

TP RFS 0.800 0.000 0.130 0.000 -#-------

END OF DIMENSION LOC120

DIM LOC121= TRUE POSITION OF CIRCLE E_CIRCLE3 UNITS=MM ,$

GRAPH=OFF TEXT=OFF MULT=50.00 OUTPUT=BOTH DEV PERPEN CENTERLINE=OFF DISPLAY=DIAMETER

AX NOMINAL +TOL -TOL BONUS MEAS DEV OUTTOL

X 400.800 400.717 -0.083

Y -86.500 -86.530 -0.031

DF 10.500 0.100 0.100 10.592 0.092 0.000 --------#

D1 PLANE DATUM_E_DCC AT RFS

D2 12.763 0.020 0.020 12.757 CIRCLE DATUM_C_DCC AT RFS

D3 82.626 0.020 0.020 82.632 CIRCLE DATUM_M_BORE AT RFS

TP RFS 0.800 0.000 0.177 0.000 -#-------

END OF DIMENSION LOC121

Regards

Terry

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