The Locating Process: 321 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 321 principle, socalled 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 321 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 321 method.
First Plane Geometry tells us that three points are required to define a plane. This is the "3" in 321. So, three specific points are used to define the first plane. Fewer than three points cannot define a plane, and in the real world dimensional tolerances mean that four or more points will not be coplanar. A realworld, less than ideally perfect part placed on four or more reference points will, in fact, rest on only three of the points due to its less than perfect surface. Different parts may rest on different combinations of three points, resulting in variation between finished parts. A stool can be used to illustrate this concept. A twolegged stool would certainly be unstable. A threelegged stool sits rocksolid. A fourlegged stool is often found to rock. In the illustration, a three dimensional part, represented by a cube, is placed on a datum plane defined by three support points. The part’s six degrees of freedom have
TIPS • Use the largest surface of the part for the first ("primary") reference plane. • Position the three support points as far apart as possible. • If more than three support points are required to prevent deflection, make the additional points adjustable. 
now been reduced to three. It can still move along the X or Y axes, and it can still be rotated about the Z axis. (The part cannot move along the Z axis because it is held against the plane by clamping force.
Second Plane A second plane, if it is perpendicular to the first, can be defined by two points, the "2" in 321. The part is now constrained to one degree of freedom: movement along the Y axis. (The part cannot move along the X or Z axes because it is held against the planes by clamping force.)
TIPS • Try to choose the second largest surface of the part for the two support points of the second plane. • If more than two support points are required to prevent deflection, make the additional points adjustable. 
