Quality assurance lingo is sometimes obscure to those who approach it for the first time. Two of those terms are surely Cm/Cmk and SPC. They are both statistical terms that are used to evaluate inspection results. The first term: Cm, refers to two different parameters that assess the capability of a machine. That’s what the C and the M in this term stand for: Capability and Machine. In our case, the machine that we want to assess would be a tool, so we often hear that Cm and Cmk are capability indexes for a tool.
Once a series of torque readings have been collected from a tool, we can use the formulas to evaluate the Cm and Cmk for this tool. The Cm index would tell us how close the results are to one another and thus the tool’s repeatability. The Cmk index would tell us how close the results are to the target results and thus the tool’s accuracy.
These indexes are positive values and the higher the values, the better the repeatability and the accuracy of the tool. To have accurate Cm/Cmk values, usually about 20 to 25 rundowns are needed. This is only possible if the tool is not used continuously on a production line. Typically, Cm/Cmk tests are done before a tool is assigned to an application in the manufacturing process or after service or a repair.
How can we be sure that a tool is working correctly when it is continuously used without stopping production? This is where the SPC comes in handy. SPC stands for Statistical Process Control. By collecting only a few torque samples and feeding them every day into the SPC formulas and plotting them onto run charts, we can easily prevent a tool from going out of spec. This is because the SPC will check from trending values or values with a non-normal statistical distribution, alerting Quality Managers and allowing them to take actions quickly.
So, getting back to the title question, Cm/Cmk or SPC? It is now clear that the answer is both. Each at the appropriate time helps a manufacturing plant keep their tools under control.