The Locating Process: 3-2-1 Method

We have two objectives when mounting a part in a fixture for machining:

1. Accurately position the part at the desired coordinates.

2. Restrict all six degrees of freedom so that the part cannot move.

A widely used method of accomplishing these two objectives uses the 3-2-1 principle, so-called because it entails three steps that employ three, then two, then one fixed points of known location. Since that adds up to six fixed points, it’s also known as the six point method.

In the three steps of the 3-2-1 method, three mutually perpendicular planes, called datum planes, are introduced, one at each step. These three planes define the workpiece position, and together with opposing clamping forces fully constrain the part. Let’s take a look at the details of the 3-2-1 method.

1. Six Points Location of a Rectangular Block Considering the six degree of freedom of a rectangular block as shown in Figure It is made to rest on several points on the jig body.

2. Provide a rest to workpiece on three points on the bottom x-y surface.

3. This will stop the movement along z-axis, rotation with respect to x-axis and y-axis.

4. Supporting it on the three points is considered as better support then one point or two points.

5. Rest the workpiece on two points of side surface (x-z), this will fix the movement of workpiece along y-axis and rotation with respect to z-axis.

6. Provide a support at one point of the adjacent surface (y-z) that will fix other remaining free movements.

7. This principle of location of fixing points on the workpiece is also named as 3-2-1 principle of fixture design as number of points selected at different faces of the workpiece are 3, 2 and 1 respectively.