SPC only considers specifications such as parallelity, perpendicularity, etc. as unilateral.

For unilateral tolerances such as Perpendicularity, parallelity, etc. calculate only Cpk/Ppk, since our target here is 0 and there can't be any reading less than 0. Cp/Pp is of no significance here, since the specification can only be more than 0. Therefore we want to check only shift(location) of the process.

Cpk formula would be USL-Xdbar/3s

For specifications such as 20 +0.02, Cp/Pp, Cpk/Ppk both should be calculated, since there is a possibility of specification being less than 20.

Cp = USL-LSL/6s

Cpk= Minimum of : USL-Xdbar/3s or Xdbar-LSL/3s

For processes where batch production happens, such as heat treatment, pressing, IMR (XMR) chart can be used. Subgroup size is considered as 2 here for calculating and plotting MR (Moving Range).

For specifications such as Dia 10H6, Cp/Pp, Cpk/Ppk both should be calculated. Eg. in case of 10H6, LSL is 10.000 and USL is 10.009. But the results can be less than 10.000 as well. Cp helps us to identify ratio of tolerance to process variation i.e. USL-LSL/6s. In this case, it can happen that the ratio is good .i.e. Variation is less than tolerance. But the results might be nonconforming (Out of specification), hence we need to calculate Cpk/Ppk to understand shift (Location) of the process.