Today's grinders make big cuts without forfeiting accuracy or precision.
By Russell Kaiser
Landis Grinding Systems
Edited by Charles Bates
Green grinding involves high speeds and heavy stock removal to grind parts, such as this crankshaft, in the as-cast condition without prior machining.
According to Landis Gardner engineers, green grinding and finish grinding (at speeds up to 160 m/sec) are possible thanks to advancements in CBN wheels, hydrostatic/hydrodynamic spindle bearings, sensor/feedback systems, and linear-motor drives.
Landis Gardner's linear-motor wheelfeed unit features fewer moving parts than conventional systems, which contributes to the speed, response accuracy, and longevity of the unit.
Today's open-architecture CNCs, such as the Landis 6400, provide speed and accuracy necessary for submicron, multi-axis contour grinding.
Don't believe the rumors that hard turning and other advanced machining processes are crowding out grinding. Technological advancements have not only improved grinding's accuracy and precision but have also increased its metal-removal rates tremendously. For instance, some of today's grinders can hog out more than 8 mm of material within 5 sec — at a rate of 160 m/sec — from raw cast iron crankshafts in an as-cast condition (green grinding). This is done without sacrificing accuracy or precision.
Technologies that make green grinding possible on crankshafts are also spreading to other part applications. Automakers are now testing modern grinders for roughing camlobes from steel barstock blanks with machined grooves that delineate the main journal and lobe diameters. So rather than losing ground to other operations, grinding directly from raw stock has the potential to compete with turn-broaching, milling, and other large-chip machining processes.
Although advancements in grinding technology are redefining the rulesof-thumb for specifying machining process, grinding's principal advantage is still its ability to create geometric features to single-digit micron tolerances and surface finishes to submicron specifications. Today's turning and other machining technologies have difficulty holding those tolerances and creating such finishes, especially on high-production lines.
Of course, grinding is no different than other machining methods in that surface finish and dimensional tolerances suffer as the material-removal rate increases. Grinding, however, retains its advantage in holding tight tolerances and leaving fine finishes throughout its new range of cutting speeds. Surface finish might be rougher and tolerances looser in a green-roughing operation, but they are typically better than those produced by a turning or milling operation. And the more accurate the roughing operation is, the easier it is to hold tight tolerances at high speeds in subsequent finishing operations.
Moreover, grinding yields these results with less wear and tear on most high-production lines because wheels do not dull as quickly as cutting tools. In similar situations, hardturning operations typically have difficulty with tool wear, and tool management becomes an issue. The problem is so significant that some automakers have contracted third parties to manage their tools — to plan the tooling, stock the toolrooms, and embed monitoring systemsin the machines. These cutting tool managers often fight to stretch tool life to last one shift by working on chip-load balance. A frequent wheel change for a cam grinder, on the other hand, is once a month, and a wheel made of cubic boron nitride (CBN) grinds as many as 60,000 green crankpins.
A grinding wheel's ability to hold a form also eliminates indexing errors that degrade crankshaft-pin finishing operations. When turning is the manufacturing method, a secondary operation is necessary to make undercuts, which can lead to washout if the indexer is off in the previous operation. A grinding wheel, on the other hand, can cut a pin's diameter and the undercut in one operation. For example, on one job, a Landis 3L CNC orbital crankpin grinder roughs pin diameters, sidewalls, and undercuts on a four-cylinder crankshaft in a little more than a minute. Stock removal is 10 mm from the pins' diameters and 3 mm from each pin's face.
Another advantage of grinding is that it is a more forgiving process. It tolerates more variation in the incoming raw stock and cuts much faster in certain applications than hard turning.
CBN, a technological nexus
A confluence of technology is responsible for the newfound competitiveness of grinding, but the link is CBN. Builders design their most-capable machines around this grindingwheel material, and more end users are also designing their processes around it. Consequently, the superabrasive continues to drive the development of faster, stiffer, and more advanced machines and processes.
Although CBN wheels are not really new, the high-speed limit on the latest solid-steel-cored grinding wheels is. Bursting speeds are above 250 m/sec for vitrified CBN and diamond versions and between 500 and 600 m/sec, or twice the speed of sound, for electroplated CBN versions. Grinders that exploit faster surface speeds reduce the workload on each wheel grain, which boosts specific-material-removal rates.
The job itself is the constraining factor on cutting speed, often limiting it to about 160 m/sec for vitrified wheels and 200 m/sec for electroplated wheels. One reason is that the greater power from faster wheel speeds sometimes generates heat that causes distortion, unwanted stress, or other problems. Another reason is that vibration or balance becomes much harder and more expensive to control at extremely high spindle speeds.
To support high material-removal rates without sacrificing accuracy and to withstand higher-than-normal forces in CBN grinding, grinder OEMs increase the stiffness on some of the already stiffest machinesin the metalworking industry. They do so without adding exorbitant cost by deploying computeraided design techniques before building the machines. Such computer analyses let designers try many more iterations on a design in the same or less time than in the past. The result is not only a superior machine but also an optimum design for production, which means a better machine at less cost.
Simple, stiff grinders
Designers simplify machines to eliminate sources of error and enhance precision. Reducing the number of parts used means fewer joints and greater stiffness. Consequently, designers are always looking for new technology that simplifies linear and rotational-drive components.
For rotational drives, the most important development is direct-drive spindles. A notable case in point is grinding and headstock spindles in cylindrical processes. Conventional designs transmit power with belts and pulleys, which require-auxiliary equipment such as jackshafts, brackets, and belt-tensioning devices. Direct drives on these spindles eliminate many elements that add cost, create greater potential for error and failure, and induce noise and vibration that limit dimensional accuracy and surface finish.
For simplifying linear drives, some grinding-machine builders are using linear motors for manipulating wheelfeed systems on camlobe and crankpin grinders. Unlike ballscrews, linear motors drive a machine's moving elements with a magnetic field and receive feedback via linear encoders or lasers. Replacing mechanical powertrains with this tighter drive eliminates backlash, friction, stick-slip, and wear. The result is fast acceleration and high accuracy, even on highprecision contours. And fewer parts mean less chance of failure, greater reliability, and less maintenance.
Although linear motors have been around for at least a decade, this technology is now practical for grinding precision forms thanks to new digital drives and fast controllers. Profitable application of linear motors is like that of most other technologies — the key is matching them to the right application by considering the costperformance tradeoff.
The best matches are usually those that demand high acceleration, speed, and accuracy simultaneously. Good examples are applications that traditionally "eat" ballscrews and that require micron-level accuracy while contouring. For this reason, linear motors are now crucial to a CNC crankpin grinder's ability to hold pin roundness between 1 and 2 µ on high-production lines.
Linear motors on wheelfeeds also speed grinding camshaft-lobe contours and improve process accuracy and repeatability. In fact, these motors excel at complex re-entrant contours (concave forms on the cam's flank), which are more common for smooth performance and fuel efficiency in diesel and high-performance gasoline engines. The difficulty in grinding these lies in putting a concave shape in the cam's form, with the center of the circle being outside, rather than inside, the lobe. Linear motors make for faster, more precise, and controllable wheelhead movement, which is required to generate the negative radius of curvature.
Another way builders stiffen grinders is by replacing conventional rolling-element bearings with hydrostatic designs for linear and rotating applications, such as spindles, ways, and leadscrews. The rule-of-thumb for Landis engineers is to specify hydrostatic bearings in places where conventional ball bearings wear quickly. A good example is wheel spindles.
Engineers use hydrostatic bearings whenever high accuracy on part geometry is critical, such as on crankshaft-support bearings. In this application, hydrostatic bearings deliver 10-millionths-of-an-inch rotating accuracy throughout the life of grinders on high-production lines.
Hydrostatic bearings hold such accuracies because they don't wear. An oil film separates their sliding elements, so that they ride on a smooth fluid and never touch one another. Besides eliminating a source of wear and vibration propagation, the pressure exerted by the film centers spindle bearings automatically. The result is an extremely stiff design with nearly no radial-error motion and no mechanism that can go into a gray failure mode (in which the bearing continues to perform its function, but degrades enough to adversely affect quality).
When most manufacturing engineers hear the terms "PC-based controls" and "open architecture," communications and fancy software immediately spring to mind. In grinding, however, these terms also relate to greater precision. The fast Pentium processors inside these CNCs and the digital-signal processors (DSPs) distributed at the axes enhance program processing and execution speeds, including those for following contours smoothly at high speeds. Faster computing speed allows processing in smaller increments for precise digital control to tenths of a micron.
Such precise control ensures efficient dressing of vitrified-bond CBN wheels. Today, programmable 2-axis slides move dressers in 2D space, generating any concave, convex, or multi-radius shape. Producing cumbersome and costly profile bars for each shape is a thing of the past.
The open architecture of PCs and Windows NT operating systems also let builders and users quickly upgrade or implement new technologies. For instance, it's now quite simple to adopt sensor technology for real-time position feedback for precision work. The PC platform lets
builders exploit both new sensors and other technologies simply by inserting an interface card.
As grinding technology progresses, however, CNCs need more capacity for receiving and processing feedback to monitor the process and make adjustments on-the-fly to enhance the precision of the machine's mechanics. CNCs must collect information with many types of sensors — encoders and pressure and temperature sensors — stationed throughout the machine.
Some of the more-advanced sensors are load and acoustic emission (AE). AE sensors detect contact between the grinding wheel and dresser. This is vital in CBN-wheel dressing to ensure that the process removes only a few microns of the expensive material. AE sensors detect contact between the grinding wheel and workpiece, so Landis researchers are investigating ways to use such data to conduct quantitative analyses of the grinding process as it proceeds.
Load sensors measure bearing pressure for monitoring forces on grinding wheels and power drawn by the process. Such power monitoring helps control part quality by tracking how close consumption is to a threshold known to exceed acceptable tolerances. Should the spindle exceed a limit, the CNC adjusts the feedrates to maintain the proper level or signal an operator for human intervention.
Mr. Kaiser is vice president of engineering at Landis, Waynesboro, Pa.