Double-Disc Grinding On The Move

Double-disc grinding has always offered a highly productive and accurate means for machining to-size parts with flat and parallel sides. In this grinding method, two opposed abrasive discs, each mounted on its own spindle, simultaneously grind opposite and parallel faces on workpieces traversed between them via any of several fixturing/carrier techniques. Because double-disc grinding can remove up to 1/8 inch of stock in a single pass, and multiple parts are ground on both sides simultaneously, production rates are generally 100 percent greater than those obtained with surface grinding.

Accuracies obtained are, to a large extent, application-dependent. Typically, however, double-disc grinding can hold size tolerances to as close as 0.0001 inch and flatness to within 50 microinches, while achieving surface finishes as fine as 5 rms with conventional abrasive discs. (Polishing discs typically produce surfaces of 1 to 2 rms.) The ability to achieve tight tolerances often eliminates the need for lapping or polishing work, fine grind work and secondary inspections. It also reduces part handling, scrap and rework.

Double-disc grinding has long had a home in the automotive industry, where dedicated operations can take full advantage of its high productivity rates. Today, a number of advances allow double-disc grinders to meet requirements for improved performance in these traditional applications, and they also are opening the way to productive use in new applications. Capabilities are now available for easier machine operation, control flexibility, statistical process control (SPC), faster changeovers and setup, and applications on an expanded range of materials, including ceramics and graphite.

Easier Operation

The level of technology to be found in the latest generation of double-disc grinders is consistent with other types of state-of-the-art machine tools. As with today's machining centers and turning centers, the advanced nature of double-disc grinders is best reflected in control features. For example, the latest controls include a more user-friendly operator interface, a color monitor, and a movable pendant that can be positioned in front of the machine for setup and in back of the machine, when necessary, for convenience during machine operation.

Such improvements address the concerns of the automotive industry, in particular, where machine operators frequently find themselves running a lathe one day, and a grinder the next. Plant managers in that industry rarely have the luxury of sending their operators to a two-week training school before making the transition from one machine to another.

Control Flexibility

The same technology that enhances the appeal of double-disc grinders to managers in the automotive field also heightens its appeal to users outside this traditional stronghold. It should not be surprising that the machine tool builder that is probably most strongly linked with double-disc grinding technology in this country is leading the effort to take double-disc grinding into new areas. Gardner Disc Grinders & Abrasives (South Beloit, Illinois) has been a pioneer in disc grinding technology from the time of its founding in 1905 as the Gardner Machine Company. Now a division of Western Atlas, Inc., the company is a leader in the field, producing both single- and double-disc grinding machines and abrasive wheels that are combined in process-specific designs.

To move double-disc grinding into a wider sphere, a major design objective for this company has been to build maximum flexibility into its machine controls. The controls are developed under an "open architecture" approach that allows upgrading and expansion for new applications, without incurring a cost penalty to these new users. Thus, these open controls had to be cost-competitive with standard control packages that have fixed capabilities.

These disc grinder controls incorporate an IBM-based, industrial hardened personal computer (PC), which is used to communicate to the motion control module. The PCs are compatible both with IBM-based boards and with widely-used software--including Windows-based software on which the company plans to standardize for its next generation of controls. The motion controller used, which sends command signals to the fixture/carrier servodrives to control the grinding axes, is available in versions compatible with any computer platform. Thus, no drastic programming changes are needed to go from one platform to another.

The company also can supply a variable-speed spindle control capability that can be used with either vertical or horizontal spindle machines for specialized part applications, including those in secondary automotive and job shops where the same disc grinder will be used to run more than one part.

Precision Process Control

According to Dave Forrest, Gardner's manager of electrical engineering and assembly, the most significant control enhancement over the past several years is the development of "adaptive gaging." The development is important because grinding tolerances can be as tight as several millionths of an inch. To hold them, the grinding wheel has to be constantly adjusted to compensate for abrasive wear.

"With traditional contact closure gaging," Mr. Forrest explains, "the gage reads a part to determine whether the dimensional oversize due to wheel wear has reached a pre-set limit. If it has, the gage signals the servo axes to increment the work slides by a pre-determined amount."

The problem with this procedure, according to Mr. Forrest, is that it leads to a "saw-tooth" quality effect. The first fixed infeed increment may be too great and actually lead to undersize parts--making it necessary to adjust again until in-tolerance parts are obtained. In time, additional abrasive wear will again tend to nonconforming parts, requiring still another adjustment. In this way, part-size cycles are introduced that are counter to the part-to-part consistency desired.

The new approach takes an analog output from the gage and compensates dynamically for the wheel attrition, making the necessary wheel adjustments "on the fly," rather than waiting to adjust until a pre-set limit is reached, and then by a fixed amount.

The analog signal used by the control to calculate for wheel compensation communicates more than the last part gaged and the gage measurement. It provides historical data that is used for SPC and trending analysis.

New Disc Grinding Applications

These and other advanced capabilities of the process have created new application opportunities in the automotive, compressor and other industries.

One important effort is aimed at a stronger presence in the area of disc brake rotor finishing. Although double-disc grinding accounts for a very high percentage of total production, an alternate technology has emerged as a major competitor. Some automotive suppliers are now able to turn rotors to grinding tolerances, thereby eliminating the grinding step.

The use of CBN (cubic boron nitride) wheels is expected to make grinding a viable alternative in this application. According to Rictor Lundy, sales manager, Machinery Products, at Gardner, "Compared to turning, disc grinding with CBN offers better tolerances, smoother surface finish, and cost efficiencies." Of these advantages, the improved surface finish is especially important because it determines the quality of rotor performance.

In terms of cost efficiencies with CBN disc grinding, the stiffness of the grinding machine and wear-resistance of the CBN abrasive allow more stock to be removed at reduced motor and spindle loads, resulting in lower energy costs and improved machine performance.

Another favorable cost (and productivity) factor is minimal interruption to production uptime. A CBN wheel has to be conditioned infrequently and changed less often than conventional abrasives.

For all these advantages, however, the builder recognizes that customers will be convinced of them only through comparison testing on a case-to-case basis. Tests are currently underway with one major automotive parts supplier, which plans to set up a low volume aftermarket line for disc brake rotors to compete with existing sources--many of which now supply turned rotors.

The parts supplier has set a criterion for the use of grinding--fast changeover times in the 10- to 20-minute range. From Gardner's point of view, this should present no special problems. "We think we can meet the requirement with quick-change tooling," says Mr. Lundy. "Basically, it entails only the headstock fixturing and tooling used to clamp and unclamp the rotor."

New versatility for fast change-overs is also reflected in a horizontal disc grinder now already on the floor at another major automotive parts supplier. This machine has a carrier that is configured to accommodate two different parts--both cast-iron rocker arms produced for a diesel engine manufacturer. To effect the changeover, only a secondary program has to be activated. No mechanical modifications to the tooling are required.

Productivity And Accuracy

The capability of double-disc grinding for high productivity with tight accuracies is demonstrated in a new automotive application now in the prove-out stage. Horizontal spindle, double-disc machines are used to grind both sides of two parts simultaneously by a "park and grind" method. An indexer "parks" two of the parts at a time between parallel grinding wheels, and opposite sides of the parts are ground. A servomotor is used to drive the indexer, which accelerates rapidly to park the parts, and it decelerates rapidly in removing them after grinding. To optimize surface finish and obtain necessary flatness, a "spin grinding" technique is used, by which the parts themselves rotate between the grinding wheels.

In this application, other double-disc grinders are used similarly to grind a mating part component. Because the grinding method and machines used can achieve very tight tolerances, the automotive customer believes it may be possible to eliminate the traditional practice of "classifying" the ground components--and still achieve end-user mating requirements in assembly. Parallel and flatness tolerances being obtained in testing are in the range of 25 to 40 millionths (0.000025 to 0.000040) of an inch. Two-side size tolerance is within +/-20 to 30 millionths (+/-0.000020 to 0.000030) of an inch. Along with these accuracies, the production rate is between 1000 to 1100 parts an hour.

Other New Applications

Outside the automotive field, a vertical disc grinder is used at an OEM for machining hydraulic pump components. The company also is looking to expand its applications of vertical machines in grinding compressor vanes--which are traditionally machined by spin grinding with horizontal disc grinders. Currently, a vertical disc grinder is being used for one such application at a major small-engines manufacturer.

Still another potential application outside the automotive field is the grinding of spade drills. A vertical disc grinder is now being tested in such an application against a rotary-type grinding machine. So far, one demonstrated advantage of the vertical disc grinder is superior uptime. The rotary machine can grind more parts at a time, but must be stopped for manual loading and unloading. By contrast, the vertical disc grinder never needs to be stopped, although it is also manually loaded and unloaded. Grinding is continuous, with parts always available in the grind zone.

Disc grinding machines now also are being used for grinding nontraditional materials. Glass and plastics have been disc-ground for years. A disc grinder is machining graphite components for a manufacturer in Texas. In addition, double-disc grinders increasingly are used for machining the flat surfaces of ceramic components in the computer, aerospace and other industries.