How to Perform a Capability Study (Cm/Cmk)

One Cm/Cmk-studie Machine capability (Cm/Cmk) is the natural first step in any technical verification. It answers the critical question: Is the machine itself capable of producing within tolerance under ideal conditions? Does this machine have the technical precision required to meet the tolerance?

“Here is your step‑by‑step guide to succeeding with the study.

Step 1: Preparation (Create ideal conditions)

The purpose of Cm/Cmk is to isolate the machine’s performance. Therefore, we want to eliminate as many external sources of disturbance as possible:

• Material: Use material from the same batch or melt.

• Operator: Use the same operator throughout the entire test. It is important to plan the study together with the relevant stakeholders to ensure alignment.

• Settings: Set the machine to the nominal value (the target value) and make… NO …adjustments during the course of the test.

• Environment Make sure the machine is warmed up and that the conditions are stable.

Step 2: Sampling (n=50)

The standard for a machine capability study is to produce and measure… 50 consecutive partsYou can use fewer or more, but try never to go below 25 parts to maintain a sufficient statistical basis.

• Traceability: Number the parts or arrange them in the order they were produced if you want to be able to analyze trends over time.

• Measurements Measure the parts with a calibrated measuring instrument that has sufficient resolution (preferably 10 times better than the tolerance). If you are unsure about the measurement uncertainty, you should first perform a… MSA (Measurement Systems Analysis).

Step 3: Data entry in Digicap

When you have your values, it’s time for the analysis:

1. Open Digicap-app.

2. Select ”NEW STUDY” …and fill in the basic information (Part number, Machine, Date).

3. Select Machine Capability (Cm/Cmk) …as the study type

4. Enter your tolerance limitsLSL and USL).

5. Enter your measurement values (you can easily paste them directly from Excel).

Step 4: Analyze the results

When you press... Run Analysis …you will see three important components to review:

• Cm (Potential): Shows the machine’s variation. If the Cm value is high (e.g., > 1.67), the machine is sufficiently accurate on its own.

• Cmk (Outcome): Shows how well the machine is centered. If Cm is high but Cmk is low, the machine is ‘shooting off‑center’ (the setup is incorrect).

• The normal distribution curve: Look at the shape of the curve. Is it uniform and well‑formed? If the bars and the curve do not align, the cause should be investigated immediately.

• Tips: Do you want to compare the results from several flows that produce the same characteristic? Use the function… OVERLAY …in Digicap to overlay the curves.

Step 5: Interpretation and Decisions

In industry (e.g., the automotive industry), the following threshold values for Cm/Cmk are commonly used:

• Cmk ≥ 1.67: Approved machine. It is stable and ready for production.

• 1.33 ≤ Cmk < 1.67: Uncertain zone. Often requires more frequent checks or fine‑tuning.

• Cmk < 1.33: Rejected. The machine is not stable enough for this tolerance. You must either reduce the variation (repair the machine) or discuss a tolerance change with… …the customer or the design department..

Next step: Cp/Cpk and Pp/Ppk

When the machine is approved (Cm/Cmk), you move on to:

1. Cp/Cpk: Performed during ongoing production to see how the process behaves over time with all natural variations (different shifts, material batches, etc.).

2. Pp/Ppk: Used to measure the actual performance over a longer period (e.g., an entire week or month).

Good luck with your capability study!
Besök digicap.app för att utföra dina beräkningar snabbt och säkert enligt gällande industristandard.