The 3-2-1 Locating Principle
Third PlaneA third plane, if it is perpendicular to both of the first two planes, can be defined by one point, the "1" in
3-2-1. The part is now entirely constrained. It cannot move along or rotate about the X, Y, or Z axes. (Remember that the part is held against each of the three planes by clamping force.)
| TIPS • If more than one support point is required to prevent deflection, make the additional points adjustable. |
Machined is Best
It’s best if the workpiece surfaces intended to contact the support points are machined. Cast surfaces are usable, but then tend to create tolerance problems. Machined holes are the number one choice. Two machined holes can be used to eliminate all but one degree of freedom.
Repeatability
Repeatability is a paramount concern. It means that each part should be nearly identical to each other part coming off the same fixture. "Nearly identical" actually has a specific, quantifiable meaning: key dimensions of all parts produced on the same fixture must fall within defined tolerances.
In order to stay within tolerances, it’s best if the workpiece surfaces intended to contact the support points are machined. Cast surfaces are usable, but they tend to create tolerance problems. Machined holes are the number one choice, because two machined holes can be used to eliminate all but one degree of freedom.
The next best feature to use for consistent locating is machined surfaces at right angles. The "machined surface" desired here can simply be a tab or small flat specifically included for locating purposes.
Repeatability is achieved through application of the 3-2-1 method to the three aspects of the locating process:
1. Supporting
2. Locating (positioning)
3. Holding
Supporting
• The part is supported from "below" with reference to the Z axis. (Introduction of the first reference plane.)
• Gravity positions the part against the support locator.
• Provide adjustable auxiliary support points as needed to prevent deflection.
• Vertical location with respect to the fixture is now established.
Locating
The part is positioned horizontally, in the X-Y plane. (Introduction of the second and third reference planes.)
Holding
• The part is clamped against fixed locators. Therefore, a known and unvarying position of the part is maintained with respect to the X, Y, and Z axes.
• One strap clamp or clamp arm will restrict the workpiece in three dimensions due to friction between the workpiece and clamp. However, friction alone may not be enough to prevent the workpiece from moving during machining.
• Cutting forces will not move a properly supported and clamped part.
Locating
• The part is positioned horizontally, in the X-Y plane. (Introduction of the second and third reference planes).
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