Head: Three-ball "plug" positions parts accurately

Manufacturing Engineering, Jul 2000

Prototype precision Locator, made by SDI (Collins,OH), is designed to accurately fixture, cast, forged or previously machined parts. For example, it might be used with a part that needs a hold pattern drilled that must be accurately positioned relative to an existing hole.

The locator, said to be more accurate than commonly used plugs or expanding mandrels, consists of three balls, 120 apart, positioned next against a larger diameter spring-loaded ball. All are contained in a metal tube.

In operation, the locator mounts on a work table or tombstone in the machining center, or lathe with a pull-back chuck. It could also be used in an automated assembly application. When an operator fits a workpiece hole over the locator's tube, the three balls are compressed and the part is centered around the hole.

Before machining the part can be secured with other clamps, or the SDI unit can act as a clamp. Currently, units can handle hole sizes from 0.250 to 4" with a center large ball. With a center tapered nose plug, size is said to be virtually unlimited. The unit can center a part with an accuracy of 0.0002".

When the unit functions as a clamp, hydraulic or pneumatic pressure acting on the larger ball secures the smaller balls against the workpiece to hold it in place. When combined with a sensor, it can be used as a gage. The amount the small balls travel when fit into a hole can translate to the hole's diameter or geometry (bellmouth, taper, etc.) Circle 331.