Page 392 - CarrLaneCatalog_2019ed-c.pdf

LOCATING AND CLAMPING PRINCIPLES

Locating and clamping are the critical functions of any workholder.
More engineering and machining principles can be found at
carrlane.com under “Engineering Resources”.

BASIC PRINCIPLES OF LOCATING
To perform properly, workholders must accurately and consistently
position the workpiece relative to the cutting tool, part after part.
To accomplish this, the locators must ensure that the workpiece is
properly referenced and the process is repeatable.
Figure 1. The twelve degrees of freedom.
Referencing and Repeatability
“Referencing” is a dual process of positioning the workpiece The twelve degrees of freedom in Figure 1 all relate to the central
relative to the workholder, and the workholder relative to the axes of the workpiece. Notice the six axial degrees of freedom
cutting tool. Referencing the workholder to the cutting tool and six radial degrees of freedom. The axial degrees of freedom
is performed by the guiding or setting devices. With drill jigs, permit straight-line movement in both directions along the
referencing is accomplished using drill bushings. With fixtures, three principal axes, shown as x, y, and z. The radial degrees of
referencing is accomplished using fixture keys, feeler gages, and/ freedom permit rotational movement, in both clockwise and
or probes. Referencing the workpiece to the workholder, on the counterclockwise radial directions, around the same three axes.
other hand, is done with locators.
Locators restrict a workpiece’s movement, so they must be strong
If a part is incorrectly placed in a workholder, proper location enough to maintain the position of the workpiece and to resist the
of the workpiece is not achieved and the part will be machined cutting forces.
incorrectly. Likewise, if a cutter is improperly positioned relative
to the fixture, the machined detail is also improperly located. In Locators, not clamps, must hold the workpiece against the cutting
workholding, referencing both the workpiece and the cutter must forces. Locators provide a positive stop for the workpiece. Placed
be considered and simultaneously maintained. against the stop, the workpiece cannot move. Clamps, on the other
hand, rely only upon friction between the clamp and the clamped
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“Repeatability” is the ability of the workholder to consistently surface to hold the workpiece. Sufficient force could move the
produce parts within tolerance limits, and is directly related to the workpiece. Clamps are only intended to hold the workpiece against
referencing capability of the tool. The location of the workpiece the locators.
relative to the tool, and of the tool to the cutter, must be consistent.
To maintain repeatability, the workholder must be designed to
accommodate the workpiece’s locating surfaces.

The ideal locating point on a workpiece is a machined surface.
Machined surfaces permit location from a consistent reference
point. Cast, forged, sheared, or sawed surfaces can vary greatly
from part to part, and will affect the accuracy of the location.

The Mechanics of Locating
The direction and magnitude of machining forces exerted during
Figure 2. Cutting forces in a milling operation should be directed into
the operation are vital considerations in fixture layout. A workpiece
the solid jaw and base of the vise.
free in space can move in an infinite number of directions. This
motion can be broken down into twelve directional movements, In Figure 2, the milling forces generated on a workpiece when
or “degrees of freedom.” All twelve degrees of freedom must be properly clamped in a vise tend to push the workpiece down
restricted to ensure proper referencing of a workpiece. and toward the solid jaw. The clamping action of the movable

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