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.
Piston end-attachments, called “end effectors” or “contact bolts, “ protect the piston and the part during the pushing operation. Mounting versatility is provided by a selection of mounting blocks, also referred to as mounting brackets. Flange nuts, also known as jam nuts, secure threaded cylinders in place.
Selection of a positioning cylinder is dependent on several factors:
- The cylinder must overcome the weight of the part and friction between part and fixture. Whether weight or friction predominates depends on whether the machining center is vertical or horizontal. Additionally, support and locating elements may add friction.
- Product literature provides maximum rated force for each cylinder offered. Selection is then narrowed by examining force vs. hydraulic pressure data for each cylinder.
- Bigger is not necessarily better. Oversize cylinders may damage parts, and they create crowding on the fixture.
- When selecting cylinder stroke length, observe the 80% rule: the actual stroke in the application should not exceed 80% of the cylinder’s maximum stroke.
Order of Activation
The geometries of the fixture and workpiece may dictate that the positioning moves be carried out in some sequence. Chapters 1 and 3 explained control valves that are used to accomplishing sequential operation of cylinders. The three most commonly used are:
- Directional control valves are multiport valves that can be used to direct hydraulic fluid to cylinders in sequence.
- Sequence valves are activated by a sensed pressure. When one part of the hydraulic circuit reaches a preset pressure, the sequence valve opens to permit hydraulic fluid to flow to another part of the circuit.
- Flow control valves control the operating speed of hydraulic components. Cylinder travel speeds must be matched to the nature of the workpiece, but adjustment of travel speeds is also a way to accomplish sequencing.
Parallel vs. Series Plumbing
The cylinders in a hydraulic circuit can be supplied with fluid in either parallel or series fashion. Parallel feed is the method of choice. When supplied in parallel, all cylinders receive pressure and flow simultaneously, so there are no inequities of pressure or delays in activation other than those intentionally incorporated in the circuit design. On the other hand, a series arrangement takes the least space and may be the best choice for a tight fixture.
Tubing vs. Manifold
Whether to supply hydraulic fluid to cylinders and other components by means of external plumbing (rigid tubing) or a machined manifold is a classic tradeoff of upfront cost vs. long-term cost. Tubing has the attraction of lowest upfront cost. A drawback is the extra cluttering of the fixture by the tubing and fittings. In addition to taking up space on the fixture, the external plumbing tends to collect chips
A manifold approach results in a “cleaner” fixture with less chip-catching hardware. Additionally, manifold-mounted components can be easily removed and replaced. Clearly, the length of a production run is a major factor in choosing between external plumbing or a manifold..
Why Use Hydraulic Positioning?
Properly implemented, hydraulic positioning provides these benefits:
- Increased safety
- Vastly reduced setup time per part
- Accuracy and repeatability
- Less scrapped parts