Milling scrolls at high speed and accuracy.

For all their benefits in auto and home air conditioning systems, scroll-type refrigerant compressors challenge the ingenuity of metalworking manufacturers to achieve the levels of machining accuracy required. The scroll design offers the advantages of fewer moving parts, greater operating efficiency, quieter performance, and use of environmentally safe, non-fluorocarbon refrigerants compared with conventional piston-type compressors.

Scroll compressors, however, place a burden on machining accuracy. The part consists of a fixed scroll and an orbiting scroll to compress the refrigerant at three or more points of contact and discharge it through a center hole. Proper operation requires that height and profile dimensions, as well as surface finish, meet extremely tight tolerances.

To achieve these requirements on a high-production basis, the European operations of Ex-Cell-O designed an entirely new high-speed milling machine, designated the XHS, which incorporates an innovative rotary C axis. The machine produces a more accurate scroll profile than could previously be generated by conventional machining centers with linear X and Y axes only.

The rotary-axis spindle is directly driven by a stepless servo-motor with ten times the positioning sensitivity of a conventional linear-axis spindle. John Fletcher, Ex-Cell-O's North American sales manager for special products, explains that if you looked at a scroll produced by a conventional machine, you would see "glitches" along the form contour. These disappear in parts produced by the XHS milling machine because the C axis eliminates profile errors as the linear X axis changes direction.

The contour required for the scroll compressors, known as an "involute", is defined with reference to a base circle. "You could look at it as the curve described by a point on a thread kept taut as it is unwound from a central core," says Mr Fletcher. "As the string unwinds, the distance from each successive point around the central core to the end of the string becomes progressively greater."

Using the C axis to generate this shape by a process known as "spline interpolation," the Ex-Cell-O high-speed machine achieves a finished form accurate to |+ or -~20 microns. The tolerance standard used, identified as the "wrap profile", defines the dimensional deviation between the involute shape actually achieved and the theoretical, or "true" involute. In operation, the Ex-Cell-O machine has even bettered that standard, consistently producing wrap profiles within a tolerance band of |+ or -~15 microns, Mr Fletcher notes.

A second critical scroll dimension, also affecting a compression contact point, is the "wrap height"--the distance, at any point, from bottom to top of the scroll walls. Here, the machine is designed to achieve a tolerance of |+ or -~20 microns. Again, however, says Mr Fletcher, demonstrated capability consistently falls within a narrower 75% portion of the total tolerance band.

A critical requirement for proper functioning of the scroll compressor is a high-quality surface finish. To achieve this standard over the entire involute form, a constant feedrate must be maintained throughout the milling cycle. The C axis meets this requirement at the highest possible feedrate allowed by its maximum rate of angular acceleration and the tool geometry. See exhibit B.

"At constant rates of up to 10 meters a minute (393 ipm), we deliver what we believe are the fastest C axis feedrates in the world," says Mr Fletcher. "At the same time, we get consistently superior surface finish. Our standard for surface smoothness is one micron, but our demonstrated capability is 0.46 micron."

The cast aluminum automotive compressor components are being machined using polycrystalline diamond (PCD) inserts; the cast-iron home compressor components with tools using polycrystalline cubic boron nitride (PCBN) inserts.

A major challenge in the scroll work is minimizing tool deflection, because parts are very flexible and tend to lean away from the cutter, which can result in profile errors. An effective way to combat deflection is to run the cutters at extremely high speeds, says Mr Fletcher. "That's one reason we use the PCD and PCBN tooling. We can take full advantage of the high-speed spindles, which run at 40,000 rpm for the profiling operation, and still provide excellent tool life."

The XHS machine can be built with one, two, or three spindles to provide high-volume production options. Multiple spindles can be used either to run additional parts for increased production, or to perform sequential operations on the same part. Other features of the machine include a polymer-concrete machine bed, antifriction circulating ball bearing ways, and a Siemens Sinumerik 880 CNC controller for very fast block processing and spline interpolation for generating the involute curve.

The machines are being used in the US, Japan, Korea, France, and Germany. Ex-Cell-O is researching applications in other industries at its R&D laboratory in Eislingen/Fils, Germany.

Standards a hit on CD-ROM disk

"Everything you always wanted to know about quality standards" might be the title of a CD-ROM disk from Information Handling Services, Englewood, CO.

The disk gives users access to quality-related standards published by a variety of industry and government organizations. Included is the complete text--with scanned images of figures, tables, equations--of quality guidelines from ASME, AWS, EIA, IEC, IPC, and NATO; quality-related MIL specs; and the complete set of ASQC standards, including the Q90 (ISO 9000) series.

Users can search the CD-ROM Quality Standards Service (the disk's real title) in several ways. By entering a search term, users can locate each reference to the term, anywhere it appears. They can also search by document number and subject, or browse through tables of contents to find the exact paragraph of interest.

The service also incorporates a "bookmark" feature that lets users return to frequently accessed data without completing a new search. To ensure up-to-date information, purchasers of the disk will receive regular updates, probably every six months.

For information from Information Handling Services, Englewood, CO, circle 314 or call 800/241-7824.