Revision Date: 2005-09-06
| The major difference between the two approaches relates to the purpose for doing the study. In the past, most companies were concerned only with getting good parts. Being able to sort parts (good from bad) was the requirement for the gage. Now most companies require their suppliers to have controlled, capable processes. If you are going to control a process, you need to be able to detect changes in the process (out-of-control conditions). Therefore, the recommended approach uses process variation based on the parts used to do the study. GAGEpack, gage management software, allows you to also estimate the process variation based on a control chart for the process measurement being studied.
The push to improve processes by improving the capability ratios is a complicating factor. This reduces the variability of the process. As a result, many gages do not pass the R&R % required. Often, the alternative comes down to purchasing a "better" measurement system (often a very expensive gage). In some cases, better systems do not currently exist. In any event, there is some question as to the value of improving the measurement system. In cases where Cpk > 2 (or whatever number you like), the customer is not likely to notice any improvement and the cost of producing (measuring in particular) increases. (Side Note: a frequently used major criteria for selecting improvement project is: Will the customer notice the improvement? Will it make a difference to the customer?) In this case, one might want to consider a policy of using tolerances for situations where Cpk is greater than some value, with the stated objective of applying their improvement efforts to situations where the process is not capable and the potential for improvement is greater. | |
