What is the Difference between a Jig and Fixture?
If you work in a machining environment, then you have probably heard the terms “jig” and “fixture.” You probably also know that both jigs and fixtures have something to do with holding workpieces for machining operations.
But what exactly are jigs and fixtures, and what is the distinction between them?
In the broadest of terms, the distinction is as follows:
A jig forms a guide for the tool(s) used in a machining operation.
A fixture secures a workpiece for a machining operation.
Types of jigs:
Jigs must be specially designed to suit the machining operation and the geometry of the workpiece.
An open jig, also called a plate jig, attaches to workpiece without completely enclosing it. A plate jig might be square or doughnut-shaped, and affixed over the workpiece with clamps.
A diameter jig is used for cylindrical workpieces. It encloses a workpiece in a V-shaped groove. A diameter jig may have a cover portion that contains guide holes for a drill operation.
Some jigs, like leaf jigs and box jigs, completely enclose the workpiece.
The range of potential shapes for machining jigs is nearly endless. The above descriptions are but a few examples.
Jigs and Clamping Force
Jigs should exert a clamping force which is adequate for the operation, but not excessive. If the clamping force is insufficient, the workpiece may shift while it is being machined. This will result in a finished product that is out of spec.
On the other hand, too much clamping force can distort a workpiece. To get an idea of this principle, hold a playing card on its edge in your hand, suspended between your fingertips and thumb. If you squeeze the card too hard, it will bow upward or downward. Excessive jig clamping force can affect a workpiece in the same manner. The result is, once again, a defective finished product.
Fixtures
A fixture secures a workpiece during a machining operation, but it doesn’t have any locating devices for the cutting tool(s) that are used to shape the workpiece. Fixtures are often bolted to the table surface of vertical and horizontal mills. Distinctive tombstone fixtures, for example, are common in horizontal milling operations.
Fixtures are by nature somewhat more generic than jigs---which are most often customized for a specific operation in a mass production environment. Nevertheless, both dedicated jigs and dedicated fixtures may be necessary for a given operation, depending on its complexity and the geometry of the workpiece involved.
