Running a Machine Capability Study on a Deep Hole Drill
A machine capability study — also called a Cmk study — measures whether a machine can produce parts within the required tolerance. It’s a standard method for approving new machines or verifying existing ones. I run capability studies on deep hole drilling machines when setting up new jobs. The Process I follow this procedure for every machine capability study: Define the critical dimension — Select one feature to study (typically hole diameter at the mid-point) Set up the machine — Use the same tool, fixturing, and program that will be used in production Check machine condition first — Run a test bar alignment check and verify coolant pressure and spindle runout before starting Run the parts — Run 25 consecutive parts without any adjustments. No speed changes, no feed changes, no tool changes Measure every part — Use the same measurement method (air gage, bore mic, or CMM) for all 25 parts Plot the data — Create a run chart in the order the parts were produced to visualize any drift Calculate mean and standard deviation — Use the standard formula Calculate Cmk — Cmk = min((USL - mean) / 3sigma, (mean - LSL) / 3sigma) The Cmk value tells you whether the machine can hold the tolerance consistently. I also calculate Cm (process capability without centering) to see if off-center adjustment can fix a marginal Cmk. ...