3-2-1 Principle of Fixture Design
For a fixture designer, the major portion of design time is spent deciding how to locate the work piece in the fixture.
You know that any free body has a total of twelve degrees of freedom as below:
6 translational degrees of freedom: +X, -X, +Y, -Y, +Z, -Z
And 6 rotational degrees of freedom:
- Clockwise around X axis (CROT-X)
- Anticlockwise around X axis (ACROT-X)
- Clockwise around Y axis (CROT-Y)
- Anticlockwise around Y axis (ACROT-Y)
- Clockwise around Z axis (CROT-Z)
- Anticlockwise around Z axis (ACROT-Z)
You must fix all the 12 degrees of freedom except the three transitional degrees of freedom (-X, -Y and -Z) in order to locate the work piece in the fixture. So, 9 degrees of freedom of the work piece need to be fixed.
But, how? By using the 3-2-1 method as shoon below:
Rest the work piece on three non-collinear points of the bottom surface (XY), and you will be able to fix the +Z, CROT-X, ACROT-X, CROT-Y and ACROT-Y degrees of freedom.
Now, rest the work piece at two points of side surface (XZ), and you will be able to fix the +Y and ACROT-Z degrees of freedom.
Now, rest the work piece at one point of the adjacent surface (YZ), and you will be able to fix the +X and CROT-Z degrees of freedom.
So, you can successfully fixate 9 required degrees of freedom by using the 3-2-1 principle of fixture design.