A fixture holds a part in a known position so an operation can be done to it repeatably. A jig does that and additionally guides the tool. The distinction matters because it decides who is responsible for accuracy: with a fixture, the machine is; with a jig, the jig is. Both live or die on the same principle, which is locating the part before clamping it.
Locate first, then clamp
A part must be constrained against every degree of freedom it has, and the standard scheme uses six points: three on a primary face, two on a secondary, one on a tertiary. Clamps then hold the part against those locators. The universal fixture error is using a clamp as a locator, which means the part's position depends on how hard someone tightened a handle.
Support without over constraining
A thin or flexible part will bend under clamping and under cutting force, so it needs support beneath the working area. Supports must be adjustable or spring loaded so they touch without lifting the part off its locators. Adding a seventh rigid locating point over constrains the part and makes its position depend on which points happen to touch first.
Jigs, bushings and wear
A drill jig guides the cutter through a hardened bushing so hole position depends on the jig rather than the operator. Bushings wear, so they are replaceable and are checked on a schedule. The same applies to any locating surface: a fixture that has been in use for years and never inspected is quietly producing parts to a different dimension than it did when it was made.
Practical things that decide whether it gets used
Loading must be quick and obvious, and it must be impossible to load the part the wrong way round, which is what a deliberate asymmetric locator is for. Chips and swarf must have somewhere to go, since a chip under a locator moves the part. And the fixture must be light enough for one person to handle if it will be lifted on and off a machine.
Questions people ask about assembly fixture
What is the difference between a jig and a fixture?
A fixture holds the part in a known position. A jig also guides the cutting tool, usually through a hardened bushing, so accuracy comes from the jig rather than the machine.
What is the three two one principle?
Locating a part with three points on a primary face, two on a secondary and one on a tertiary, which fully constrains it without over constraining it.
Why should a clamp never be a locator?
Because the part's position would then depend on clamping force, so every operator and every tightening produces a slightly different result.
How often should fixtures be checked?
On a schedule, because locating surfaces and drill bushings wear. A fixture in daily use for years will not hold what it held when it was made unless it is inspected.