Why aren’t specification (spec) limits displayed on control charts?
Control charts are intended to monitor the random variation in a process, which should not be affected by the specification limits (tolerance). Displaying spec limits can drive tampering and adjustments to a process in order to achieve the limits, which causes the process to go “out of control.” If the process is not capable of achieving the limits, then changes to the process mean or a reduction in standard deviation should be addressed. Control charts are measures of process stability only.
