Can someone provide the formula / sheet for unequal bilateral tolerance. Is it different from the one we use for equal bilateral tolerance ? pl. clarify.
Thanks in advance
When the two sides of the tolerance are unequal, use the smaller one for Cpk calculations.
Also, you specify the type of tolerance, which is one of the least important aspects of an SPC chart (which I assume you are looking for, since you posted in the SPC forum.) We need to know the process you have an the type of characteristic - length, diameter, form (roundness, parallelism). The distribution of the process - not the tolerance - is the most key question to answer. Tolerance only comes into play in SPC if you have a continuous uniform distribution. It also relates to capability - not control.
The Cp/Cpk indices do not take off-center targets into consideration. They are based solely on the specifications, not the target. You simply calculate both the upper (Cpku) and lower (Cpkl) values and use the smaller of the two as Cpk.
In your situation, the Cpm index may be more relevant as it takes the actual target value into consideration.
There are also a lot of discussions on the relative merits and failings of capability indices in general if you search the cove.