Cpk for 3D data - Capability study from CMM data- Need Help

B

Benoitb

#1
I have to prepare a capability study from CMM data. The CMM program is made from a GD&T drawing. The CMM report gives me the following information :
Nominal data (in X, Y and Z axis, so Xn, Yn and Zn),
Actual result (Xa, Ya and Za);
Tolerance from the GD&T (for example a position tolerance of 2mm when compared to datum A, B and C) (we interpret this as being a sphere of 2mm around the nominal point in space);
Tolerance overall (I understand there sometimes is a "bonus" scheme with GD&T based on maximum material conditions);
Deviation from nominal (which is the length of a vector between the nominal point and the measured point in space);
Resulting error, which is the out of tolerance value.

My problem is that ALL data related to deviation from nominal is of positive sign. I will never be able to make a decision with this kind of data because it doesn't tell me if it is over or under the nominal. The mean I get is not the target, since the target is Zero. It's not a one sided spec because I can never possibly get a negative deviation.
 

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