Methodology / Tolerance capability

Tolerance capability — definition and counting

The tolerance band that can be held in stable batch delivery, not the best single-part result.

Unit

mm

Formula

Band corresponding to Cpk >= 1.33 across 30 consecutive parts from one batch

Why suppliers disagree

• Single part versus batch: the former is often an order of magnitude better
• Measurement conditions: a temperature-controlled room versus the shop floor
• Fixturing: the same machine with a different fixture gives a different result
• Measurement point definition: who measures, how many points, maximum or average
• Reducers are quoted as backlash in arcmin, structural parts as dimensional tolerance in mm; the two are not interchangeable

How to collect it on site

• Ask for the first-article inspection report and check the instrument and temperature
• Ask for a process capability report (Cpk / Ppk), not just a single part
• Measure three parts on site with your own instruments
• Ask how tool wear is compensated

The three most common misreadings

1. Confusing machine accuracy with part accuracy, the most common substitution
2. Quoting a batch order from a single-part report; disputes are guaranteed
3. Omitting the datum; the same part against different datums can differ by a factor of two
4. Quoting transmission accuracy as if it were machining tolerance