Inspection Jig Design Basics

A jig answers a question so make the question explicit

Before any jig is designed, write the question down: is this bore within tolerance, is this bracket square to this face, will this assembly fit its mating part. A jig built to check everything checks nothing quickly, and it will be abandoned on the shop floor within a month. Pick the characteristics that actually cause failures downstream and build for those.

The jig inherits the drawing datum scheme

This is the single rule that separates a jig that works from one that produces confident nonsense. If the drawing controls a feature relative to datums A, B and C, the jig must hold the part on A, B and C. Hold it somewhere else and you are measuring a different quantity that happens to have the same name.

Choose between a verdict and a reading

A go/no-go jig gives an answer that cannot be argued with and takes seconds. A jig with an indicator tells you where in the tolerance band the part sits, which is what you need if you are watching a process drift towards a limit. Go/no-go for sorting, readings for control. Deciding which you need before design is cheaper than converting afterwards.

Keep the jig's own error small

Every jig contributes its own uncertainty to the measurement. As a working rule that contribution should be a small fraction of the tolerance being judged, and the exact fraction is a commercial decision as much as a technical one, because it drives what the jig costs. Agree it explicitly rather than leaving it implied.

Correlate against known parts

A new jig is not trusted until it has been shown to discriminate. Run known good parts and known bad parts through it, including parts that are marginal, and confirm the jig sorts them the way the measurement equipment does. Keep those samples. They are how the jig gets re-verified later.

Plan for re-verification from the start

Jigs wear, get dropped and drift. Build in a reference feature that can be checked against a gauge block or a master, record the jig's reference dimensions on its drawing, and set a re-check interval. A jig nobody verifies is a jig nobody should trust.