What Types of Workpiece Holding Systems Are Available and What Are They Used For?
Different principles are often combined. A zero-point system can carry a vice, magnetic plate or special fixture. Pneumatic or hydraulic actuation can operate a vice or another workholding fixture. When making a selection, separate three questions: what locates the workpiece, what holds it and how the clamping process is actuated and monitored.
Machine and Self-Centring Vices
A vice holds the part between jaws and is suitable for rectangular, compact or pre-machined stock. A mechanical vice offers simplicity and flexibility. A self-centring vice locates the part relative to its centre, which is particularly useful for multi-axis machining and when access is needed from several sides. Replaceable soft jaws can be machined to match the geometry of a specific part.
Zero-Point Systems and Interchangeable Pallets
A zero-point system creates a repeatable reference between the machine table and a pallet, fixture or vice. Its main benefit is not the direct clamping of the workpiece, but the rapid and accurate exchange of the complete workholding assembly. The next part can be prepared outside the machine’s working area.
Magnetic Workholding
A magnetic plate holds a ferromagnetic workpiece across its surface. It provides good access to the top face and reduces obstructing contours. Suitability depends on the material, contact area, part thickness and direction of the applied forces. It is not enough to establish that the material is “steel”. The specific surface condition, geometry and machining process must also be checked.
Vacuum Workholding
A vacuum plate uses a pressure differential and is suitable for flat, non-magnetic parts such as aluminium, plastic or composite plates. The holding force depends on the effective area and the vacuum level achieved. Porous materials, uneven surfaces, leaks, a decreasing effective area during machining or high lateral forces can all cause problems.
Tombstones, Clamping Pyramids and Multiple-Part Workholding
Tombstones and clamping pyramids increase the usable capacity of the machine’s working area and allow several parts to be held for one cycle. They are particularly useful on horizontal machining centres and in multi-axis machining. The advantage is that more parts can be machined each time the machine is opened. The design must still account for collisions, tool reach, chip evacuation, assembly weight and even load distribution.
Custom Fixtures
A dedicated or modular fixture is appropriate when standard jaws and plates cannot locate, support or expose the part reliably. With complex castings, forgings and thin-walled parts, correct support can be more important than the clamping force itself. A custom fixture is generally justified only when the time savings and process stability outweigh the cost of design, manufacture, measurement and maintenance.
Pneumatic and Hydraulic Actuation
Pneumatic and hydraulic systems are not locating methods in themselves. They provide and control the clamping force. Pneumatic actuation is generally fast and clean, while hydraulics can deliver higher forces in a compact space. For both systems, it is necessary to define the operating pressure, required force, state following a loss of pressure, position sensing and a safe sequence of steps in the CNC program.
