Some questions about Cmk and Cpk

[Hennry]

Hello Marc,
Sorry I´m new at this.I´m looking but not finding a general statement how to right doing the capability of machine (CNC lathe). I heard that the machine have to be in controlled conditions. What does it mean? May I use automatic compensation of cutting tool (wear of tool) when this function is available? I think during machining of batch of 50 pieces for study?
What is different between Cmk and Cpk? Can anyone gives me any answer or the right method how to do capability of machine.
Thank for your help.

[wmarhel]

Originally Posted by Hennry

What is different between Cmk and Cpk? Can anyone gives me any answer or the right method how to do capability of machine.
Thank for your help.

Before jumping into a process capability study, perform a Gage R&R. If you aren't getting reliable measurements than any data you collect is suspect, and irregardless of the capability index you use, the results are suspect.

Second, run some control charts to determine whether your system is stable.

If memory serves me correct, there isn't a huge difference between Cpk and Cmk as they both take the target into consideration.

Wayne

[Robert O'Brien]

Hello Hennry,

Just wanted to add some thoughts to Wayne’s comments. It is important to understand that your calculated Cpk results are going to be invalid unless your process is in a state of statistical control. This is why it is important to do the control charts (following the gage R&R of course). Once you eliminate any special or assignable causes on your control charts, you should only see natural or inherent variation on the chart. It is then ok to perform a Cpk study.

Regards,
Rob O’Brien