Putting it in Place
Workholding involves three steps: positioning, supporting, and clamping. In this chapter, we will examine specifics of the first step, positioning, in hydraulic workholding systems.
Positioning may seem a mundane matter, but a metalworking setup begun with insufficient attention to proper positioning is destined to poor results inconsistent finished parts and a high scrap rate. Before a machine tool can perform an operation on a part, it must “know” the location and orientation of the part. The accuracy of positioning significantly affects the accuracy of results.
A part (three-dimensional object) is said to have six degrees of freedom: translation and rotation with respect to each of three axes. Workholding objectives are to position the workpiece to specific coordinates and then restrain it from moving. As detailed in chapter 2, a set of three mutually perpendicular reference planes can be uniquely defined by three reference points in a first plane, two in a second, and one in a third. This fact gives rise to what is known as the 3-2-1 locating method.
When positioning a part by the 3-2-1 method, three widely spaced points on the largest defined surface of the part are positioned against the first plane, which is defined by fixed locators on the fixture. The part is then translated on that plane until two points on a second plane of the part meet a second reference plane on the fixture. Finally, the part is translated along both of those planes until one point on a third orthogonal plane of the part meets the third orthogonal reference plane on the fixture.
Positioning against the first reference plane takes place during loading when the appropriate face of the part is placed against the fixture. The part can be moved to each of the other reference planes in sequence or simultaneously. This is the only time the part should be moved on the fixture.
When clamps have been applied, the part is ready for machining. All tool movement will be based on a reference point in space (relative to the machine or fixture) that is “known” to the machine. Clearly, the more accurately each part is positioned on the fixture, the more accurate and repeatable the results.