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Number of points

dcoss
I was wondering if anyone has any sort of rule-of-thumb about how many points to take on diameters. I know that varies for size and/or maybe what kind of tolerance on the feature. I mean, I know how many hits I usually take on certain features and what works for me, but is there any sort of stuff out there that talks about recommended numbers of hits for features? Just wanted peoples thoughts on this...


  • One good practice is to study different methods on a ring gauge, number of hits, clockwise, anticlockwise, pre hit / retracts, touch speed's, probe size, touch trigger, scanning etc. Then check the results against the calibrated ring size.

    The problem with using a ring gauge is that it doesn't take into account the process used to create the circle. If the circle is produced using a sand casting you will use way more touches than if it was made using a grinding operation.


  • The problem with using a ring gauge is that it doesn't take into account the process used to create the circle. If the circle is produced using a sand casting you will use way more touches than if it was made using a grinding operation.

    Agreed Wink Form error is a killer. I hate castings;(


  • Agreed Wink Form error is a killer. I hate castings;(

    CMM's are designed for perfect parts...
  • Just SCAN everything!!!

    I have the space x number of hits per feature spec. Let me see if I can dig it up.
  • in reply to Neil Challinor
    Neil, I was not aware that you cant use a UPR filter on a partial arc. If I'm scanning a circle that I must scan as two separate halves how would I apply a UPR in this case? Do I need to construct a circle from the two halves and apply the filter then?
  • in reply to Neil Challinor
    Yes, combine all the hits of the two halves in a constructed BFRE circle and then apply the filter to that. Something like this,,, 

    CIR1       =FEAT/CONTACT/CIRCLE/ADAPTIVE_CIRCLE_SCAN,CARTESIAN,IN,LEAST_SQR
            THEO/<35.291,32.983,-0.55>,<0,0,1>,6
            ACTL/<35.291,32.983,-0.55>,<0,0,1>,6
            TARG/<35.291,32.983,-0.55>,<0,0,1>
            START ANG=0,END ANG=180
            ANGLE VEC=<1,0,0>
            DIRECTION=CCW
            SHOW FEATURE PARAMETERS=NO
            SHOW CONTACT PARAMETERS=NO
CIR2       =FEAT/CONTACT/CIRCLE/ADAPTIVE_CIRCLE_SCAN,CARTESIAN,IN,LEAST_SQR
            THEO/<35.291,32.983,-0.55>,<0,0,1>,6
            ACTL/<35.291,32.983,-0.55>,<0,0,1>,6
            TARG/<35.291,32.983,-0.55>,<0,0,1>
            START ANG=180,END ANG=360
            ANGLE VEC=<1,0,0>
            DIRECTION=CCW
            SHOW FEATURE PARAMETERS=NO
            SHOW CONTACT PARAMETERS=NO
            WORKPLANE/ZPLUS
CIR3       =FEAT/CIRCLE,CARTESIAN,IN,LEAST_SQR,NO
            THEO/<35.291,32.983,-5.55>,<0,0,1>,6
            ACTL/<35.291,32.983,-5.55>,<0,0,1>,6
            CONSTR/CIRCLE,BFRE,2D,CIR1.HIT[..],CIR2.HIT[..],,
            OUTLIER_REMOVAL/OFF,3
            FILTER/OFF,UPR=0
SCN3       =FEAT/SET,CARTESIAN
            THEO/<35.291,32.983,-5.55>,<0,0,1>
            ACTL/<35.291,32.983,-5.55>,<0,0,1>
            CONSTR/SET,FILTER,GAUSSIAN,POLAR,CIR3,
            FILTER/ON,WIDTH=1,UPR=50,OUTLIER_REMOVAL/ON,3​
  • in reply to Liambo734
    Interesting topic.
    Thanks for that guide, was not aware of it. it gives me weight to discuss with my technician who doesn't always agree on the number of points Slight smile
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