Jigs and fixtures: purpose and 3-2-1 principle

A jig is a work-holding device that holds and locates the work and also guides the cutting tool (by drill bushes), e.g. a drilling jig. A fixture is a work-holding device that holds and locates the work but does not guide the tool; the tool is set relative to it by set blocks or feeler gauges, e.g. a milling fixture.

Differences

Purposes and advantages

1. Eliminates marking out, trial cuts and individual setting; gives interchangeability.
2. Higher production and shorter cycle time; semi-skilled labour is enough.
3. Consistent accuracy, repeatability and lower scrap.
4. Safe operation and reduced operator fatigue.
5. Allows several operations in the same setup, and permits the use of automatic machines.

They are economical only if the batch size is large enough to repay the tooling cost.

3-2-1 principle of location

When each direction is counted separately, a free body in space has 12 degrees of freedom: translation in +/- x, y and z (6), and rotation clockwise and anticlockwise about x, y and z (6). The 3-2-1 principle uses only six locating points to restrict nine of them; the clamps restrict the remaining three.

         z
         |   Plane 1: 3 points on base (XY)
         |        . . .   (restrain -z translation and
         |                 +/- rotation about x and y: 5)
         +----- y   Plane 2: 2 points on side
        /              (restrain -y translation and
       x                +/- rotation about z: 3)
             Plane 3: 1 point on end face
                       (restrain -x translation: 1)

1. Three points on the base (primary plane) fix the height and the two tilts: 5 freedoms.
2. Two points on a side (secondary plane) fix the side position and the rotation about the vertical axis: 3 freedoms.
3. One point on an end face (tertiary plane) fixes the last translation: 1 freedom.

The 9 freedoms are restricted by the locators, and the remaining 3 (translation in +x, +y and +z, away from the locators) are removed by the clamps. For round work, a V-block with a flat or a locating pin is used instead. A locating surface should be a machined face (not a rough casting surface), and the points should be spread as far apart as possible.