🚀 go-pugleaf

I have a real-time SPC system in place on several CNC machines.  The
runs are relatively short (100 to 2000 parts in one run) and between 5
and 15 different part numbers are produced on each CNC during the month.
The parts produced on each machine vary month to month, a part produced
on one machine this month may be made on a different machine the next
time it is made.  Most of the characteristics we are checking have tight
tolerances, 50% of the characteristics have a tolerance less than
0.0006" and 75% of the tolerances are 0.001" or less.

Currently I'm producing a report that shows the capability of each CNC
by tolerance.  I do this by subtracting the measurement from the nominal
and using the result to calculate mean, standard deviation, Cp and Cpk.
For example, for one CNC the results are:

     Tolerance     Mean     Std Dev     Cp     Cpk
      0.0002     0.00003    0.00005    0.67    0.48
      0.0003     0.00001    0.00005    0.95    0.88
      0.0004    -0.00001    0.00006    1.06    1.03
      0.0005     0              0.00008    1.11    1.10
      0.0006     0              0.00004    2.24    2.23
      0.0007     0.00004    0.00005    2.22    1.96
      etc.

Is there a way to establish one index for each machine that would
reflect the overall machine performance, say over the tolerance range
0.0002" to 0.001".  I'd like to have one number that would reflect the
performance that we could track monthly and use to compare the
variability and performance of one machine to another.

Regards,
Roger Duffy
rduffy@asqnet.org