Jigs vs Fixtures

People use the two words interchangeably, and on a shop floor nobody will correct you. The textbook difference is simple, though, and it helps when you're asking someone to build one.

A fixture holds and locates the part. A jig does that too, and also guides the tool.

Fixtures

A fixture clamps the workpiece in a known position so the machine, the welder or the inspector can do their job. The tool's path comes from somewhere else: the CNC program, the operator, the robot. A vise with custom soft jaws is a simple fixture. So is a plate with locating pins and toggle clamps that holds a bracket for welding.

Jigs

A jig also guides the tool. The classic example is a drill jig: a plate with hardened bushings that line the drill up over each hole, so an operator with a hand drill puts every hole in the same place on every part. Before CNC machines were everywhere, jigs did a lot of the work that programs do now. You'll still see them anywhere parts are drilled, reamed or assembled by hand.

The 3-2-1 rule

Both are designed around the same idea. A rigid part can move in six ways: along three axes and around three axes. To locate it completely you need six points of contact, laid out 3-2-1. Three points under the main face set a plane. Two points along one edge stop it rotating and sliding one way. One point on the end stops the last direction. Clamps then hold the part against those points without pushing it off them.

Most fixture problems come from breaking this rule: too many locators fighting each other on a part that isn't completely flat, or clamps that push the part away from its locators instead of into them.

Common types

• Machining fixtures: hold parts on a CNC table or in a vise, often several parts at once to cut setup time.
• Welding fixtures: hold tubes and plates in position and fight heat distortion while the joints go in.
• Assembly fixtures: line up parts that get bonded, riveted or bolted together.
• Inspection fixtures and checking gauges: hold a part in its in-car or in-machine position so it can be checked quickly, often with go/no-go features.
• Drill jigs and templates: guide drills, reamers or trim knives on parts done by hand.

When is one worth it?

A fixture costs money once and saves time on every part after that. It usually pays off when you're making more than a handful of parts, when every part has to come out the same, when a feature is hard to locate by eye, or when the setup is the slow part of the job. For a single prototype, a vise and a good operator are often cheaper. For a production run, the fixture is usually the cheapest thing in the cell.

Good fixtures also keep the operator in mind: parts that load one way only, clamps that are quick to reach, and enough clearance for chips, weld wire or a gauge.

We design and build jigs and fixtures around your part, your machine and your cycle time. Send a drawing or a sample part through the quote form.