Over the past years, manufacturing has made considerable progress. New machine tools, high-performance cutting tools, and modern manufacturing processes enable today's industries to make workpiece faster and better than ever before. Although work holding methods have also advanced considerably, the basic principles of clamping and locating are still the same.
Three important aspects to work piece clamping and holding are:
- Location. Positioning the work piece with respect to the tool. Without being under the forces of the cutting tool.
- Support. Keeping the deflection of the workpiece during machining to a minimum.
- Holding. Ensuring that the position of the work piece during machining remains stable and doesn’t move.
The relationship between locators and clamps is easily illustrated by taking a machine vice as an example. A vice contains both locating elements and clamping elements. A workpiece is pushed against the solid jaw of the vice positioning the workpiece against it. This jaw becomes the locator. To be able to clamp the workpiece we close the movable jaw. This jaw becomes the clamp.
The vice is positioned so that the locator resists the cutting forces. Directing the cutting forces into the solid jaw and vice body ensures the accuracy of the machining operation and prevents workpiece from moving.
Referencing is a dual process of positioning the workpiece relative to the work holder, and the work holder relative to the cutting tool. Referencing the work piece to the work holder on the other hand is done with locators.
To allow the work holders to perform properly they must accurately and consistently position the work piece relative to the cutting tool. To accomplish this the locators must ensure that the work piece is properly referenced, and the process is repeatable.
"Repeatability" is the ability of the work holder to consistently produce parts within tolerance limits and is directly related to the referencing capability of the tool. The location of the workpiece relative to the tool and of the tool to the cutter must be consistent.
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. Referencing the work piece to the work holder is done with locators.
Locating pin method.
The simplest and most widely applicable locating method is with the use of locating pins. There are many standardized locating pins available on the market.
With this method, locating pins are used in combination with the 3-2-1- rule. For applications where the workpieces can move due to external forces, the pin shape must be considered, and number of pins used will need to be increased to prevent the movements.
A work piece without fixturing or clamping can move in all directions. Looking at this in a simple way we can break it down into 12 directions of movement or 12 degrees of Freedom.
All 12 directions or degrees of freedom to move must be restricted to ensure proper referencing of a workpiece.
Three locators are placed under the workpiece. The 3 locators are usually positioned on the primary locating surface. This restricts axial movement downward, along the -z axis (6) and radially about the x (7 and 8) and y (9 and 10) axes. Together, the 3 locators restrict five degrees of freedom. See figure below.
The next 2 locators are normally placed on the secondary locating surface. They restrict an additional 3 degrees of freedom by arresting the axial movement along the +y axis (3) and the radial movement about the z (11 and 12) axis.
The final locator is positioned at the end of the part. It restricts the axial movement in one direction along the -x axis. Together, these 6 locators restrict a total of 9 degrees of freedom. The remaining 3 degrees of freedom (1, 4, and 5) will be restricted by the clamps.
Summary of basic 3-2-1- Rule.
In the figure below is simple example of a basic set up of a work piece.
It contains the 3 points for the primary plane, secondary plane and the tertiary plane.
3 points for primary datum plane eliminate translation along Z axis and rotation about X and Y axes
2 points for secondary datum plane eliminate translation along Y axis and rotation about Z
1 point for tertiary datum plane eliminates translation along X axis