An inspection or checking fixture holds a part in the same way the assembly holds it and provides a reference against which features can be checked quickly, often with simple gauges rather than a coordinate measuring machine. Done properly it turns a twenty minute inspection into a twenty second one; done casually it produces confident, repeatable, wrong answers.
It must replicate the datum scheme
The fixture's locators must correspond to the drawing's datums, in the same order of precedence. If the drawing says the part is located on a face then two holes, the fixture must do exactly that. A fixture that locates on convenient surfaces instead measures the part against something the assembly does not care about, and will pass parts that do not fit.
What a fixture can prove
Whether features are within tolerance of the reference, using pins, gauges, indicators or attribute go and no go checks. Whether a formed or welded assembly has come out to shape. Whether a trimmed part's profile is within limits. It is very good at answering yes or no quickly and repeatably across many parts, which is what production needs.
What it cannot
It does not produce variable data unless it carries indicators, so it tells you a part passed and not by how much, which means process drift is invisible until parts start failing. And it cannot check anything nobody designed it to check, so a drawing revision that adds a feature makes the fixture incomplete rather than wrong, which is harder to notice.
Calibration and the fixture's own life
A fixture is a gauge and must be calibrated and re verified on a schedule, because locators wear, pins wear and bushings wear. A fixture that has been used daily for years without checking is quietly producing a different answer than it did when it was made. It should also carry an identification number and a calibration record like any other measuring equipment.
Questions people ask about inspection fixture
What is an inspection fixture?
A fixture that holds a part on its drawing datums and provides a reference for checking features quickly, often with simple gauges rather than a coordinate measuring machine.
Must it match the drawing datums?
Yes, including the order of precedence. A fixture that locates on convenient surfaces measures against something the assembly does not care about.
Does a fixture replace a coordinate measuring machine?
For fast attribute checks in production, largely. It does not produce variable data unless it carries indicators, so process drift can be invisible.
Do fixtures need calibrating?
Yes. Locators, pins and bushings wear, so a fixture is calibrated and re verified on a schedule like any other measuring equipment.