Vacuum clamping devices and grippers

         Vacuum clamping devices operate using the atmospheric pressure for clamping the workpieces. The principle of operation is shown schematically in figure below. Vacuum clamping devices are particularly good for clamping workpieces in the form of thin plates. To achieve a uniform multipoint clamping of such workpieces, the locating plate of the fixture has a dense network of holes on it. In this case, clamping needs no sealing and entails no bulging or warping of the plate workpieces. Experiments revealed that the effective area of holes exceeds the total area of these holes by the quantity which is 0.5-0.8 of the locating plate’s non-hole area. The larger values of the correction factor hold for the larger Ra values of the plate surface. The workpiece is mounted on the locating plate of the clamping fixture. The work is clamped to the fixture’s plate with the atmospheric pressure upon the evacuation of the air from fixture’s cavity (vacuum chamber). 

Vacuum clamping devices are applicable to workpieces from different materials and with a flat or curvilinear locating surface. Usually the clamping force is enough to do surface finishing and finish-machining. Stops are regular features in fixtures to take up the shearing force. With these clamping devices, the workpiece surface can be both processed and unprocessed. In the latter case, it should be smooth enough that is free of visible peaks and valleys. The surface of the fixture’s locating element should be finished to Ra=0.63-0.32 microns and have a straightness to within 0.02/300mm.
Unclamping the workpiece needs bringing the vacuum chamber in communication with the atmosphere. To reduce the time of operation and consumption of energy, it is feasible the cavity cube space will be minimum.
Another vacuum clamping version uses rubber seal cord to assure air tightness. The sealing cord surrounds the vacuum chamber and the workpiece rest directly on it.  The size of the cord and the groove are important for the normal operation of the fixture. Upon the contact of the work with the locating plate surface, the cord fills in the groove leaving 5 to 10% freeboard above the groove for deformation.
Vacuum grippers operate exactly the same with the vacuum clamping devices. The gripper is usually mounted on a pick and place mechanism or on a robotic arm, and is used in packaging and with high speed assembling machinery. The gripper must have low weight and holds strongly the workpiece(s). In order to assure strong clamping we need to test the operation under high acceleration and deceleration movements (simulate robot movements). 

Advantages of vacuum clamping

• Many sides of the workpiece may be machined in one setup.
• No distortion of workpiece because (atmospheric) pressure is applied to the whole surface.
• No marks from clamping, as common with mechanical clamping devices.
• Vibrations are damped because the workpiece is completely supported.
• High repeatability, therefore high precision.
• Quick clamping and release.
• Less setup time, higher quality.