Centerless Grinding is a machining process that uses abrasive cutting to remove material from a work piece. Centerless grinding differs from centered grinding operations in that no spindle or fixture is used to locate and secure the work piece; the work piece is secured between two rotary grinding wheels and the speed of their rotation relative to each other determines the rate at which material is removed from the work piece.
Centerless grinding is typically used in preference to other grinding processes for operations where many parts must be processed in a short time.

In Centerless grinding, the work piece is held between two grinding wheels, rotating in the same direction at different speeds, and a work holding platform. One wheel, known as the grinding wheel (stationary wheel in the diagram), is on a fixed axis and rotates such that the force applied to the work piece is directed downward, against the work holding platform. This wheel usually performs the grinding action by having a higher linear speed than the work piece at the point of contact. The other wheel, known as the regulating wheel (moving wheel in the diagram), is movable. This wheel is positioned to apply lateral pressure to the work piece, and usually has either a very rough or rubber-bonded abrasive to trap the work piece.

The speed of the two wheels relative to each other provides the grinding action and determines the rate at which material is removed from the work piece. During operation the work piece turns with the Regulating wheel, with the same linear velocity at the point of contact and (ideally) no slipping. The grinding wheel turns faster, slipping past the surface of the work piece at the point of contact and removing chips of material as it passes.

There are three forms of Centerless grinding, differentiated primarily by the method used to feed the work piece through the machine.

In through-feed Centerless grinding, the work piece is fed through the grinding wheels completely, entering on one side and exiting on the opposite. The regulating wheel in through-feed grinding is canted away from the plane of the grinding wheel in such a way as to provide a lateral force component, feeding the work piece through between the two wheels. Through-feed grinding can be very efficient because it does not require a separate feed mechanism; however, it can only be used for parts with a simple cylindrical shape.

In end-feed Centerless grinding, the work piece is fed axially into the machine on one side and comes to rest against an end stop; the grinding operation is performed, and then the work piece is fed in the opposite direction to exit the machine. End-feed grinding is best for tapered work pieces. 

In-feed Centerless grinding is used to grind work pieces with relatively complex shapes, such as an hourglass shape. Before the process begins, the work piece is loaded manually into the grinding machine and the regulating wheel moved into place. The complexity of the part shapes and grinding wheel shapes required to grind them accurately prevent the work piece from being fed axially through the machine.


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