AGMA 02FTM3-2002 pdf free.Application of Statistical Stability and Capability for Gear Cutting Machine Acceptance Criteria.
The criteria for stability may vary depending on the customers specifications. There may be changes in the number of points that determine a trend or a run, i.e six [6J instead of seven [7], or the number of subgroups specified to be within the middle third of the chart, etc. As stated earlier, agreement with the customer should be reached and understood before any trial begins.
If the process is shown to be out of control, the assignable cause or causes must be identified and eliminated. The process for troubleshooting and correcting the assignable causes in the gear manufacturing process is a subject in itself and will not be addressed in this paper.
The important point is that the process must be in control and stable before determining the process capability.
Capability
Process capability is a measure of how well the process output meets the specified requirements (tolerance]. Every process can be classified as falling into one of four cases as shown in Table 2. The preferred situation is to have a Case 1 condition where the process is both in control and capable. As often occurs in the case of machine qualifications, a customer will allow a Case 3 condition where the process may not be in control, but capable. For the most part, it is not a case of the process being out of control, but the fact that you do not know if the process is in control or out of control. This is generally due to the fact that not enough parts are available from the customer to perform a true stability study. If this is the case, only a test for capability is conducted at the supplier facility.
The capability indices that are used today are Cp,Pp and Cpk, Ppk. Cp and Pp are indices of process variation that are only relative to a specification. Cpk and Ppk are indices that combine process variation and process centering [location] relativeto a specification. As you will see below, the equation for Cp and Pp is the same, except for the method of calculation used for the standard deviation in the equation. As with Cp and Pp, Cpk and Ppk also have the same equation except for the method of calculation for the standard deviation.AGMA 02FTM3 pdf download.