C Chart
A C chart is a statistical process control (SPC) count chart that monitors the number of nonconformities per unit when each inspection unit has a constant opportunity for defects. It plots defect counts over time against control limits derived from the average defect rate, helping detect special-cause variation such as tool wear or process drift. It is not a CNC axis or geometric tolerance.
In a CNC cell or millwork line, inspectors regularly count the total defects on each identical unit—such as surface scratches, edge chips, or tool chatter marks—at intervals like every hour or shift. These counts are plotted on a C chart against the average defect rate and control limits. This method works only when each sample has the same inspection area or part size; if that changes, a u chart is more appropriate. The chart is used to catch gradual process drift, such as a dull cutter, poor adhesive application, or fixture damage, before defect rates become rejectable. A stable C chart supports SPC reaction rules: if a point exceeds limits or shows nonrandom patterns, the team investigates machine condition, tool wear, material variation, or operator method rather than adjusting blindly. This practical application ensures quality issues are addressed at the source.
When should a C chart be used instead of a p chart?
Use a C chart when counting defects per unit on equal-sized units; use a p chart when tracking the fraction of defective units in a sample.
What is the statistical basis of the C chart?
It is based on the average count of nonconformities, with center line and control limits derived from that count under constant-opportunity conditions.
What shop-floor signals does a C chart help detect?
It helps identify special-cause spikes such as a damaged cutter, edgebander glue instability, clamp slippage, chipped tooling, or a new operator setup that increases the number of visible defects per part.