BLM 3750 User Guide Appendix A: Tool Testing Guidelines
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11.2 Cm-Cmk Test
The Cm-Cmk test is used to test the accuracy and repeatability of a power tool or a torque wrench at a
specific calibration. The result is a value that summarizes the dispersion of a specific number of values
measured (Cm) and the position with respect to the control limits (Cmk).
The Cm measures the repeatability of the tools, while the Cmk measures also the average of the tool
respect to the target value. Therefore the Cmk value cannot be higher than the Cm value.
The following figure summarizes the possible scenario of Cm and Cmk results for a test on a tool:
See APPENDIX B: Processes and Statistics for the details of the formulas used to calculate the Cm and
Cmk indexes.
Cmk low
Cmk low
Cmk high
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