Wednesday, 21 June 2006

Robot Assembly (2/4)


Handling device selection for a particular part depends on the size and geometry of a part, as well as the rate at which the part is required.  Each handling device has its own performance characteristics.  This means that it is suitable for dealing with a limited range of parts.  Small to medium sized parts, with features that can be seen in silhouette, can be handled by the most common of devices, the vibratory bowl feeder.  Parts with no useful features for orientation purposes, or parts with adverse physical properties, are expensive to feed automatically and require special automatic feeding devices.  These types of parts need to be re-designed to reduce their cost for automatic feeding.  There are many properties of a part that would prevent it from being handled by vibratory bowl feeders, such as flexibility and stickiness.  Parts with adverse properties such as these, and larger parts, must be handled by other feeding devices like magazine systems or pallet transfer systems.

The multi-part linear vibratory linear feeder can deliver different parts to a robot assembly station.  It consists of two straight and parallel vibratory orientating tracks on a common drive unit.  The rejected parts fall into return tracks and are brought to the start of the orientating tracks by a reciprocating elevator.  The tracks can be CNC machined from a database of designs that are identified by an automated handling code for a particular part.  Only the orientating tracks are replaced to changeover this multi-part feeder to handle other part types.  The vibratory drive unit and reciprocating elevator are completely re-usable and the cost of these devices is divided between the different part types.  The orientating track for this multi-part feeder is straight and it is much less expensive to produce than the curved orientating track of a vibratory bowl feeder.  Applications of this feeder are limited to parts which require orientating devices simple enough to be produced in one set-up on a horizontal machining centre.

Gravity feed track magazines are simply short lengths of track which are loaded manually on-line or off-line.  During off-line loading, a full magazine is substituted for a magazine when it becomes empty.  These magazines are specifically designed for the particular type of part type and cannot easily be re-used for different types of parts.  Although most of the gravity feed track magazine is special-purpose, the cost of these devices is relatively low.  They are useful far feeding large parts and they provide an economic alternative to palletisation.  Parts that are to be handled by this type of device must be stackable, for vertical magazines, and not susceptible to damage when the part is slid into position by the pusher.

The pallet transfer system consists of a walking beam transfer device to load a paternoster, an unload paternoster, and pallets.  Full pallets are elevated by the load paternoster and transferred to the robot working zone by the walking beam transfer device.  Parts are picked from the pallet and the pallets are then indexed to present a new pallet of parts to the robot.  Empty pallets are offloaded from the walking beam by an unload paternoster that produces a stack of empty pallets.  Virtually all of the pallet transfer system is general-purpose, with only the vacuum-formed part retainers being specific to a particular component.  Pallets are loaded by standard means.  Filling of the pallets at the point of manufacture is a very economic way of loading parts, although the cycle time of most manufacturing operations makes it difficult to use this method of loading.  Parts are positively held in position on the pallet by ensuring that they are sandwiched between the underside of one pallet and the top of the one beneath.

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