The Evolution of Finishing Machines

The automotive industry's efforts to reduce the weight, and thereby improve the fuel economy, of its passenger cars have been well-publicized. But not all of the weight-shaving changes are as "glamorous" as the use of ultrahigh-strength steels, the use of aluminum for major body panels, or the substitution of engineering plastics for metals. Many of the less publicized modifications are under the hood. Such is the case with Delco's new lightweight compressor for small-car air conditioners.

The Delco Air Conditioning Division of General Motors (formerly a part of GM's Frigidaire Division) has more automotive air conditioning compressors in service than any other manufacturer. Its compressor for six-cylinder vehicles is used not only by GM but also by customers as diverse as Rolls Royce and Deere & Co.

Among the most highly stressed components in any automotive air-conditioning compressor are the valve reeds. These deceptively simple-looking devices (photo at right) must be flexible enough to provide minimum resistance to the desired gas flow, flat enough to provide a gas-tight seal to prevent reverse gas flow, and yet strong and fatigue resistant enough to withstand the stresses of high-speed compressor operation. And these compressor-valve reeds are expected to have a minimum life of 66-million cycles under full load.

Delco has made over 450 million reeds since its six-cylinder compressor went into full production in 1961. Production techniques, while modified somewhat over the years, are relatively straightforward; the disk-like reeds are punched out of spring steel (Re 52-54), stress relieved, and then finished to remove die marks and any stress risers. As a rule, reeds for the six-cylinder compressor have been finished in conventional barrel tumbling machines.

Changes Bring Changes

Development of the new four-cylinder auto compressor, however, led to a review of the deburring and finishing techniques. Among the processes evaluated were thermal deburring, high-pressure-water deburring, ball burnishing, extrusion deburring, photochemical etching, and chemical deburring.

Some compressor components posed special problems, and the valve reeds were in this "problem" category. To be certain that the new, lightweight compressor (17.3 lb compared with up to 35 lb for some six-cylinder models) would meet high-performance specs, it was built to operate at speeds up to 7000 rpm. This, in turn, called for a redesign of the reeds.

A thorough testing program in Delco's product engineering department resulted in the evolution of this reed from a four-spoke, 0.013" thick disk to the present two-spoke, 0.015" thick reed, which is known as the R-4 reed. During these tests, it became obvious that a full radius was needed on the edges of the 0.191" to 0.195" diameter center hole; it also became obvious that this wasn't going to be easy to do.

The combination of the small central hole and the high material hardness (Re 51-53 at this stage) resulted in 90- to 96-hour cycles in conventional vibratory finishing machines. Because of these excessively long cycles, Delco investigated several alternative finishing methods, settling finally on centrifugal barrel finishing and deburring.

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