There are numerous control pan examples available, and this example (used with permission from DMAICTools.com ) shows a three step assembly operation in a furniture manufacturing plant (click here for the control plan example):
Here are a few important comments about this control plan example, illustrating effective and last-resort control methods –
Process Step #1 – Error Proofed, a.k.a. Reliable Process Control
In the first process step the assembly operator is positioning parts in fixtures for assembly. Rather than relying on inspection (which is never 100% effective), the manufacturing engineers have designed the assembly fixture such that parts cannot be placed incorrectly for assembly. An error-proofed process is one that makes a defect impossible to create – this is the best form of process control!
Characteristic #2D – Screw Depth Monitoring – Weak Process Control
Characteristic #2D focuses on a screw being sub-flush after assembly. In the case of this control plan, a subgroup of five samples is pulled hourly and checked with a go/no-go functional gage. The results are then plotted on a p-chart, which is a percent-defectives SPC (statistical process control chart). This is a weak process control step and would produce a high risk-priority-number on a PFMEA.
Unless the underlying process capability for screw depth is outstanding (six sigma = Cpk of 2.0 or better), a sample of 5 pieces per hour would potentially let small quantities of defects escape the process let a process for days on end without detection. Even if a problem is detected within the 5 samples, the reaction plan only requires that production since the last inspection be sorted 100% (only one hour worth of production). The best control in this case would be a depth sensor on the screwdriver, to stop the process and not let the screwdriver retract if the screw was not sub-flush.