FLAT-LAPPING and FINE-GRINDING MACHINES

Machine Shop Guide - September 1999-2000 Guide to: Flat-lapping and fine-grinding machines
Reprinted with permission of the Publisher.

P. R. Hoffman
   The seven series of dual-plate lapping and polishing machines includes a dozen models in all. There's the PR-1 Series for small work-pieces; the PR-2 for very small ones; and the computer - controlled 1500, 1900, 3100 and 4800 for large ones, up to 17.8 in. diameter Details follow on the PR-1 and latest machine, the 3100.

In the three-model (32T, 50T and 66T) PR-1 Series, workpieces are placed in carriers driven in planetary motion between two stationary plates. Ring and center gears are driven counter-clockwise by a 1-hp, variable-speed, permanent magnet dc motor acting through a gear train. The top plate floats on the work-pieces and is restrained from rotating by a dogging arm that pivots down and engages two plate posts. Screw adjustment positions bottom-plate height relative to ring and center gears. Ring and center gears and plates are changed to accommodate the size and number of carriers. Plate sizes, ID x OD, range from 11 x 14 in. for the 13-carrier 32T to 6.8 x 14.3 in. for the five-carrier 66T which accept workpieces up to 2.1 and 5in. diameter

and as thin as 0.003 and 0.008 in., respectively. A selector switch activates the lap cycle, timer and the optional air gage, digital LVDT gage and slurry-delivery system. Lap time can be preset and expressed in 0.1-min increments up to 99.9 min. The machines can operate under manual, time or gage control. The gages approximate part thickness - to +- 0.0005 (air) and +- 0.0002 or better (LVDT) -- and stop operation when final thicknesses are reached. The slurry is supplied from a 5-gal container on a remote cart. A variable-speed peristaltic pump and mixer allow adjustable delivery and mixing rates.

With the computer-controlled machines, the bottom plate is fixed, carriers orbit the center sun gear at controlled speed, and the top plate rotates at twice the speed of the planet-carrier orbit speed. The 3100 (photo) is a five-carrier, sprocket-type (pin-drive) machine with 10.5 x 31.4-in. plates and brush-less variable-speed drive: 5-hp (top plate), 1-hp (ring sprocket), 0.75-hp (center sprocket). It accepts workpieces up to 10.9-in. diameter and as thin as 0.010 in., provides programmable soft starts, precision load ramping, and soft-stop sequencing to keep work-pieces from adhering to the top plate. Programmable reversal of carrier rotation automatically corrects for plate flatness, minimizing plate conditioning.

As many as 24 programs with 14 steps each can be stored. The controller governs all process operations, including sprocket speeds, top-plate speed and direction, and acceleration and deceleration ramping. Speed changes can be coordinated to avoid abrupt changes and workpiece shock. Top-plate force, 15 to 530 lb, is set through the controller using a feedback circuit that monitors part pressure. Changes are gradual, through a stepper motor, which also raises and lowers the top plate. Abrasive slurry is fed through the top plate by a peristaltic pump set to desired flow rate and switched on and off by the program. Programming is via panel display and keypad, the panel also displaying program parameters.

PR Hoffman's:   Index  Site-Map  More NEWS about PR Hoffman