Steve: We two CNC Lathes that run small batches. These batches may be of different material and configurations which require setups between runs. There is a lot of variation between runs (due to setups) and very little variation within runs (variation in lathe).
I have a similar problem, but unable to use any SPC that I know of. We take a batch of material, mold it, cure it in an oven under clamped pressure, grind it, scorch it, then take 5 pcs/batch (batch = approx 3000 parts) and do various physical tests on it. SPC is not done on the process, but the end result of these operations.
So for example, the density of the part is measured per batch as a subgroup. This charateristic is affected by every operation above beginning with the material batch, potentially processed on 1 of 3 presses, through 1 of 6 ovens, 4 clamping stations,etc. It is not possible to keep these separate and plot individually. Each control chart subgroup is from a different batch of material and off a multitude of processes and equipment.
Limits are very tight from Rbar, which makes the charts look terrible from subgroup to subgroup variation, but still very controlled compared to the specifications. This is hard to explain to vistors that do not understand the complexity of the problem.
To me, it seems that these could be considered different setups, processes, or population just as you would expect a mean shift for a different lot of steel in a fineblank press.
When things appear to go out of control, there is not much you can do to react to the problem. There is no adjustment at this point in the process because the parts are already made. Because this happens very frequently and because of the huge number of influencing factors, you can't investigate everything and record the reaction as required for OUC conditions. Some real assignable causes can be determined after a week or two if your lucky, others are just ellusive despite DOE after DOE. I have began to think that this level of variation is normal, little more can be done to improve it, and that a lot of my problem is the use of control charts that shouldn't be applied to this situation. I think standard Xbar & R charts are inappropriate here. Anyone agree or am I out in left field?
Thanks,
Chemman
