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Advancing the art of piston turning.

A fully automated manufacturing system built by Giddings & Lewis Integrated Automation for the Automobile Associated Works (VAZ), in Togliatti, Russia, promises to be the standard of the future in automotive manufacture.

The system, which employs advanced machining technology, does finish machining of three different pistons for gasoline engines while accommodating variations within each piston type. System output is 630 parts/hour at 100% efficiency. Piston material is aluminum with 11% to 13% silicon content. Machining tolerances held throughout the system adhere to "single class" requirements, eliminating selective assembly.

All the metalcutting machines and a portion of the support equipment were designed and built by Giddings & Lewis which also was responsible for purchased equipment such as gantry loaders, parts washers, blue-steel automation, and a temperature-controlled coolant system.

Homing on the dome

A key feature of the machining process is that all machines use the piston's dome surface and upper ring lands for location and clamping. Traditionally, a nonfunctional surface (such as a counterbore) is machined into the part as a locating surface. Using the dome and ring lands for location creates three major benefits:

* No non-value-added machining.

* Elimination of tolerance stackup or accumulation from the nonfunctional surface.

* Use of piston-clamping methods that minimize stress and deflection.

The system consists of two parallel lines, each with three basic sections: a transfer machine for roughing operations, three identical CNC finish-turning and grooving machines, and a transfer machine that finish bores the wrist-pin hole. The parallel lines can be set up to run different models, if desired.

The first machine is automatically loaded by a vertical-storage part feeder. Incoming parts have been qualified at the foundry with a rough-machined OD. Using the same locating spots as the foundry qualifying machine, the cast combustion chamber in the dome face is then probed to establish the datum point for machining of the dome face. The piston OD is also semi-finish turned in this CNC station. Remaining stations in this machine counterbore the skirt, finish skirt length, rough bore the wrist pin hole, groove the wrist pin bores, broach the oil slots, and drill the smoke holes.

Parts are then automatically unloaded, elevated to a dual-lane, blue-steel storage unit, then to a custom gripper/loader and auto-loaded onto a pallet conveyor that transports them to one of three identical CNC turning machines that finish turn the elliptical OD, and rough, finish, and chamfer the ring grooves. The elliptical OD-turning operations are three times faster than cam-type turning machines due to a patented tool feed unit developed by Giddings & Lewis.

A unique locating and clamping procedure allows these machines to finish the entire piston OD in a single chucking. Under light pressure, the tailstock pushes the piston against a faceplate that contains four round carbide tips extending 0.003" above the faceplate's locating surface. Next, the chuck jaws close under light pressure to center the piston on the faceplate.

The tailstock then applies higher pressure to embed the carbide tips in the dome surface. The part is driven by the collet chuck for grooving and semifinish turning. Then the chuck jaws are retracted, and the entire OD is finished. The embedded carbide tips supply enough driving force for this lighter turning operation. The diamond cutting tools typically produce 20,000 to 30,000 pieces before requiring replacement.

Beyond the productivity improvement, the Giddings & Lewis tool-feed allows shapes to be programmed in the CNC system, eliminating the expense of developing and producing cams. Changes in turned piston shape are as easy as swapping floppy discs. At the end of this second section of the line, overhead gantry units automatically pick up the pistons and load them into the first station of the last machine for finish boring of the wrist pin hole and wire brushing of the piston-ring grooves and smoke holes.

This boring station has a one-piece base, filled with a cement composite material to optimize vibration damping. High-precision, low-profile feed units mounted on the base each carry a special hydrodynamic spindle.

A pump-up type fixture locates and clamps the piston against the dome face, and a low-force chuck centers it. A retractable locator establishes the correct radial position of the wrist pin bore prior to clamping. With the dome face up, chips generated by the boring operation fall away from the locating surfaces, minimizing the chances of mislocation due to chip interference.

The diamond boring tools produce as many as 100,000 pieces before replacement. When a new tool is mounted, a semiautomatic adjustment system allows fast fine-adjustment of the tool. Each time the size-adjust button is depressed, the tool moves 0.000 008".

The combination of machine stability, long tool life, and fine adjustment has eliminated the need for in-process gaging and automatic tool-wear compensation. Wrist-pin bore tolerances of less than 0.0001 " can be held under normal production conditions.

The exceptional machining tolerances are enhanced by a temperature-controlled coolant system. Refrigeration units maintain coolant temperatures in both the finish-turning and boring machines to minimize the effects of ambient temperature swings in the plant.

With manufacturing accuracy limits continuing to tighten, it's no surprise that systems such as this are in demand, says Giddings & Lewis. The firm has several totally automated piston-manufacturing systems installed around the world.
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Article Type:Product Announcement
Date:Mar 1, 1992
Words:871
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