Understanding the principles of
location is essential for achieving precision in machining. The term location
refers to the process of establishing the correct relative position of a
workpiece with respect to the cutting tool. This positioning is critical to
ensure accuracy and efficiency in manufacturing. When deciding on a method for
locating a workpiece, several factors must be considered, including the shape
of the workpiece and any features that may obstruct tool movement. Here, we
explore the fundamental principles of location and the various methods used to
secure workpieces effectively.
Degrees of Freedom
A workpiece free in space can
move in an infinite number of directions. For analysis, this motion can be
broken down into twelve directional movements, or degrees of freedom. These
consist of six axial degrees of freedom and six radial degrees of freedom:
Axial Degrees of Freedom: Allow
straight-line movement in both directions along the three principal axes (X, Y,
and Z).
Radial Degrees of Freedom: Permit
rotational movement in both clockwise and counterclockwise directions around
the same three axes.
For accurate machining, the
workpiece must be placed and held in the correct position and orientation in
the fixture (or jig), which is itself appropriately located and fixed with
respect to the cutting tool (part of the machine tool) and the machine tool
(such as a lathe or milling machine used to cut metal into the desired shape).
It is crucial to ensure that the workpiece, once fixed or clamped, does not
move at all. By properly locating, supporting, and clamping the blank, all
degrees of freedom can be effectively arrested.
Key Principles of Location
1. 3-2-1 Principle
The 3-2-1 principle is a widely
adopted method that effectively restricts the 12 degrees of freedom of a
workpiece. This technique employs a sequential arrangement of locators:
✔ Three pins are placed at the base
to restrict five motions, including rotations around two axes (XX and YY) and
downward movement along the Z axis.
✔ Two additional pins are inserted
in a plane perpendicular to the first three to restrict rotation about the Z
axis and axial movement along the X axis.
✔ One final pin is added to
restrict further degrees of freedom.
While this method provides a robust framework for positioning, it can result in limited accessibility for loading and unloading the workpiece. Thus, external clamping forces are often utilized to secure the remaining degrees of freedom. The 3-2-1 principle is particularly effective for square or rectangular parts, where pin-type locators enhance accuracy due to their minimal contact area.
2. Mutually Perpendicular Planes
Achieving ideal location involves
positioning the workpiece at six distinct points across three mutually
perpendicular planes. While alternative arrangements exist, they are generally
less effective and can lead to inaccuracies in machining.
3. Principle of Least Point
For effective location within any
plane, it is crucial to use no more than the necessary number of points. While
extra locators can be beneficial for finished surfaces, they should only be
included if they serve a purpose and do not compromise the integrity of the
workpiece.
4. Principle of Extreme Position
To maximize stability, locating
points on a workpiece surface should be positioned as far apart as possible.
This distribution enhances rigidity and minimizes potential movement during
machining operations.
Locating Methods
The method of locating a
workpiece varies based on its surface characteristics. Below, we detail the
most common locating techniques.
1. Locating from Plane Surfaces
The machine table typically
serves as the primary reference for locating flat workpieces. For effective
support, a minimum of three locators must be employed, although additional
supports may be required based on the work piece’s size and strength. This
setup ensures that the workpiece is securely positioned relative to the
machine's feed movements.
2. Locating from Circular
Surfaces
For circular workpieces, the
focus shifts from flat surfaces to aligning the axis of the circular component
with the machine table. Conical location is a prevalent method in this
scenario, where conical locators are used to secure un-machined cylinders,
particularly in castings and forgings. Additionally, the V method is often
employed for round workpieces, with a recommended V angle of 90 degrees for
optimal security while minimizing location errors.
3. Concentric Locating
Concentric locators establish the
position of a workpiece based on its axis, which may not always be centered.
The most common method involves placing a locating pin in a hole, ensuring
precise alignment and stability during machining.
Mastering the principles of
location is fundamental for achieving accuracy in machining operations. By
understanding and applying methods like the 3-2-1 principle, mutually
perpendicular planes, and various locating techniques, manufacturers can
significantly enhance their machining processes. As technology advances, these
foundational concepts will continue to play a vital role in the future of
precision manufacturing.