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The 3-2-1 Locating Principle

What is it?

A fundamental concern in metalworking is locating the part to be machined, punched, bent, or stamped relative to the work platform (fixture). For example, a CNC machine tool starts its process at a specific point relative to the fixture and proceeds from there. Hence, the accuracy with which a part is machined is quite dependent on the accuracy with which it is positioned in the fixture.
Accurate locating of not just one part, but each and every part that is loaded into the fixture is crucial. Any variation in part location on the fixture adds to the dimensional tolerance that must be assigned to the finished parts.
Additionally, the method of supporting and securing the part in the fixture affects not only dimensional tolerances, but surface finishes as well. This is true because improper supporting or clamping can temporarily or permanently deform the part. Hence, techniques for supporting and clamping must be considered together with the method of locating in order to assure repeatability from part-to-part.

Locating of a part to be machined is a three-step process:
1. Supporting
2. Locating (positioning)
3. Holding (clamping)

3D

The Locating Process: Degrees of Freedom

In order to completely specify the position in space of a three-dimensional

object (such as the cube that’s shown), we refer to six coordinates:

  1. Translational position along the X-axis
  2. Translational position along the Y-axis
  3. Translational position along the Z-axis
  4. Rotational position about the X-axis
  5. Rotational position about the Y-axis
  6. Rotational position about the Z-axis

These six coordinates are known as the six degrees of freedom of a three-dimensional object. As the double-headed arrows indicate, the translational and rotational positions can vary in either direction with respect to each of the three axes.

To completely prevent movement, all six degrees of freedom must be restricted.


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.


First Plane

Geometry tells us that three points are required to define a plane. This is the "3" in 3-2-1. 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 real-world, 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 two-legged stool would certainly be unstable. A three-legged stool sits rock-solid. A four-legged 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 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.

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.

SecondPlane

Second Plane

A second plane, if it is perpendicular to the first, can be defined by two points, the "2" in 3-2-1. 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

  • 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.

ThirdPlane

Third Plane

A 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

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
  • 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).

 


Summary: Clamping Rules of Thumb

 

  • Define three reference planes on the workpiece and fixture.
  • Provide one opposing clamping force opposite each plane to restrict movement.
  • When in doubt, rely on additional cylinders to take up uncertainty.

 

Mechanical and Hydraulic Workholding

Holding a workpiece in place can be done in a variety of different manners. For those just getting started or even those with vast experience in manufacturing, choosing the right method is not always easy. What are the benefits of each method? What are the disadvantages? How do I get started? What types of components do I need? How do they work together? How do I know it is going to work? The next issue of Enerpac University will answer these questions and many more as we present a guide to mechanical and hydraulic workholding, and even offer a combination of the two that incorporates the best features of both