## CMK Machine Capability Index: Proving a Machine Can Hold Tolerance CMK (Maschinenfaehigkeitsindex in German) is the machine capability index used across the automotive and precision manufacturing industries to prove that a **machine alone** - independent of material, operator and method - can hold the specification limits. If you supply machined parts, injection moulding, stamping or precision assemblies, CMK is the number your customers will ask for. ### Cm vs Cmk - **Cm = (USL - LSL) / 6σ** - the machine potential capability based on the short-term spread of the machine output. - **Cmk = min[(USL - μ) / 3σ, (μ - LSL) / 3σ]** - the machine actual capability, including how well the machine is centred on target. Cm and Cmk use the short-term standard deviation of a continuous run of parts, so they isolate the machine from long-term drift, tool wear and operator influence. ### The 50-Piece Machine Capability Study The classic automotive method is simple: 1. Set up the machine under normal production conditions. 2. Run it and collect **50 consecutive parts** without adjustment. 3. Measure the critical characteristic (in process order) and record the values. 4. Calculate the mean, short-term σ, Cm and Cmk. A Cmk of **1.67 or higher** is the typical customer requirement - it gives enough margin for tool wear, material variation and future shift. Between 1.33 and 1.67 is considered conditional; below 1.33 the machine needs adjustment, repair or replacement. ### When to Run a CMK Study - **New machine acceptance** (buy-off) before the machine enters production - **After maintenance, tooling change or repair** - **After moving or re-levelling equipment** - **Customer PPAP and supplier qualification** requirements - **Periodic re-validation** of critical machines (often annually or per customer contract) ### How to Use the Free CMK Template Our free editable CMK template follows the automotive 50-piece practice: 1. Enter USL, LSL, target, unit, machine ID, operator and date. 2. Type the 50 consecutive measurements into column B. 3. The mean, short-term standard deviation, **Cm and Cmk calculate automatically**. 4. Check the built-in capability judgment (Excellent / Capable / Not Capable) and the Cm target check. The workbook uses the QCANT professional layout with all sheets protected, so your team can enter data without accidentally breaking formulas or formatting. ### CMK vs CPK vs PPK - Quick Guide - **CMK** - machine only, short-term, 50 pieces: "Can this machine hold tolerance?" - **CPK** - process short-term capability from subgroups: "Is the controlled process capable?" - **PPK** - process long-term performance from all data: "Is the running process actually meeting specification?" Use the right index for the right question, and your capability evidence will be clear, defensible and accepted by customers. ### Download the Free CMK Template Running machine capability studies for your production lines? **Register a free account** on QCANT to download the editable CMK template and our full quality toolkit - FMEA, 8D, CPK/Control Chart, GRR, PPK and more. Registered users can preview inspection case reports and subscribe to quality insights for free. If you need independent verification of machine or process capability at a Chinese supplier, our engineers can run the study on site and provide a formal report. Email **cs@QCANT.com** - we respond within one business day.