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Next-generation grinding

Next-generation grinding

Streamlining specific production processes leads to innovative grinding methods and machine designs that compliment them.

Streamlining specific production processes leads to innovative grinding methods and machine designs that compliment them.

Combining CPC grinding with HSG and CBN wheels on a machine designed for concave cam grinding can reduce machining time by more than 60% over conventional grinding.

CPC grinding on a Schaudt CR41 eliminates the manual loading of crankshafts to grind their crankpins. It also improves the accuracy of angular positioning.

Unlike conventional crankshaft pin grinding, the Schaudt CR41 rotates crankshafts concentrically around the center axis of the main bearing. The lift-bearing is ground in a continuous path-controlled operation by interpolating the X axis (grinding wheel) and the C axis (workpiece).

By studying specific automotive applications, researchers have devised a number of innovative grinding methods that not only improve these parts and processes, but may also impact future grinding operations. Two of these next-generation precision grinding methods are high-speed grinding and continuous path-controlled grinding. Both are already replacing conventional grinding methods and influencing grinding machine designs.

High-speed grinding (HSG) with cubic boron nitride (CBN) wheels removes large amounts of material. In certain applications, HSG's removal rates are several times higher than conventional grinding processes. Cutting and feed speeds, along with feed motions, all increase during HSG. Also, an integral part of HSG technology is super-hard abrasive CBN.

Partnering CBN grinding wheels with specially designed machines steps up production processes like automotive camshaft grinding. During centerless grinding, several sets of grinding wheels can simultaneously cut all the bearing points on these shafts.

However, the downside to using HSG technology to its fullest is that it puts stress on a grinding machine. To combat this, most HSG-equipped grinding machines use oil as both a coolant and lubricant. Therefore, these machines must be totally enclosed and outfitted with safety equipment. For instance, some safety features on HSG machines include fire and explosion protection equipment and a vacuum-powered oil spray removal system.

In addition, HSG machines must reach high rotational and peripheral speeds. Machines must have both static and dynamic rigidity to handle not only these high speeds, but also the amounts of material being removed.

Continuous path-controlled grinding
Continuous path-controlled (CPC) grinding was developed as an alternative to plunge grinding. For CPC grinding, a narrow CBN wheel passes along the contour of a workpiece in one motion. This procedure is also called peel-grinding and is particularly suited for short runs. Grinding machine setup for CPC is quick and easy because wheels are not changed and similar parts are ground one after another.

Combining CPC grinding with HSG and CBN wheels on a machine designed for concave cam grinding can reduce machining time by more than 60% over conventional grinding. One such machine is the Schaudt CF41, which features hydrostatic guides and motors with digital position controls. These controls operate with a workpiece spindle (C axis), grinding wheelhead (X axis), and liquid-cooled spindle drive.

Normally, concave forms limit the size of grinding wheels used. But the CF41 has a second, swiveling wheel that produces any concave form with a minimum wheel diameter of 3.2 in. The ability to swivel this smaller wheel around the axis of a larger one also ensures precisely ground parts.

In one setup, the CF41 rough-grinds a cam contour with a large CBN wheel and then finish grinds using a smaller, swivel-mounted CBN wheel. Roughing takes three sec, while finish grinding is done in four sec.

In another example, a grinding machine's special design along with CPC grinding and CBN wheels eliminates time-consuming and expensive eccentric clamping fixtures for grinding crankshafts. The Schaudt CR41 rotates crankshafts concentrically around the center axis of the main bearing. The lift-bearing is ground in a continuous path-controlled operation by interpolating the X axis (grinding wheel) and the C axis (workpiece). Using this procedure, all lift-bearings are machined quickly and precisely in only one run. This has to be done with an extremely hard CBN grinding wheel.

CPC grinding on the CR41 also eliminates manual loading of crankshafts for crankpin grinding and improves the accuracy of angular positioning. To rapidly set up for any crankshaft, the machine's NC programs lift-height, angular and axial positioning, and diameter of the crankpin. Machining times are cut considerably by doing away with downtime for reclamping. Conventional methods require that operators loosen the chucking clamp and rotate the crankshaft into the relative position of the next crankpin.

Crankshaft machining places high demands on grinding machines in terms of dynamics and precision. The CR41 meets these dynamic requirements on the X axis (grinding wheel) using a hydrostatic guiding device and a hydrostatic threaded spindle with a directly coupled servomotor. Along with freedom from wear, these components reduce noise and offer good dynamic stiffness. The other important axis in the interpolation pair is the C axis (work-piece), which is designed as a motor spindle with a high-resolution optical sensor.

Finally, a modern, high performance vector control in the digital control system optimally governs the axes involved in interpolation to precision grind crankpins to less than a 2-µm roundness error.

Simultaneous operations
Efficiently combining production operations in one machine considerably enhances the profitability and precision of parts machined. Repeated loading or even additional machines are no longer needed, and setup and downtime can be reduced or even eliminated for parts such as pilot valves.

Pilot valves for hydraulic units, like power steering systems, require precise machining operations for the control edges. These edge positions are influenced by the tolerances of the cylindrical grinding process. Grinding all the functional surfaces in a single setup not only delivers the greatest positioning and form accuracies but also reduces downtime.

For single-setup grinding, a Studer S40CNC cylindrical grinder has a grinding spindle turret, which holds the necessary wheels and gaging calipers. The machine also incorporates a feeder robot into the machine casing and an automatic cylinder-error compensation feature.

The first operation on the pilot valves involves the cylindrical grinding of the outer diameter. Afterwards, gaging calipers position and check the rough-milled grooves. A vertical grinding spindle then grinds the groove edges.

To machine chamfers on the grooves, the C axis (workpiece) and the X axis (grinding wheel) are continuous path-controlled. This method makes it possible to completely machine, in one setup, the diameter, chamfers, and pilot valves fast and accurately.

The CBN outlook

Higher accuracies and more difficult-to-grind materials such as chilled and nodular irons, carbon and forged steels, and PM materials will continue to drive the CBN conversion. As recently as 1992, less than 10% of new camshaft grinding machines were designed to use CBN. Today, more than 95% of new machines are CBN capable.

Automotive applications account for more than one quarter of all CBN consumption in Europe and about the same in America. Although automotive camshaft grinding has been one of the most dramatic conversions and largest applications for both Europe and America, there are many other CBN uses within the industry including turbocharger bearings, piston rings, pump components, yokes, valves, and more.

As new workpiece materials are brought to market and process developments such as high-speed, creep-feed, and continuous path-controlled grinding take place, new abrasives must be developed. The latest in CBN crystal technology from GE Superabrasives was introduced in 1996.

Borazon* CBN 400 and 420 are a totally new class of monocrystalline CBN products with unique toughness and fracture characteristics that provide extended life—up to twice that of previous CBN products—while generating lower grinding forces and less grinding heat. These products have provided outstanding performance in applications with large wheel/workpiece contact areas as well as heat and pressure sensitive operations.

As would be expected, Japan has taken the lead in applying these products in its industries, but tool manufacturers in Europe and the Americas are moving quickly to catch up. Europe's greatest developments are in the areas of resin bond applications with CBN 420, while the Americas have seen the greatest growth in vitreous bonds with the uncoated CBN 400.

In the automotive industry, growth will be greatest in existing applications, where CBN could displace many conventional abrasive operations.

Single-layer CBN wheel technology has been paramount in the development of high speed grinding with CBN. Only a few short years ago, wheel speeds above 50 m/s were considered high speed. Today, speeds between 50 and 125 m/s are becoming commonplace, and wheel speeds of 250 m/s and higher are being studied in laboratories and universities in Europe and the U.S. The vitreous bonding system, with its ease of dressing characteristics, is also being developed with high speed applications in mind.

CBN grinding wheels, when manufactured to run at higher speeds on specially designed machines, can provide significantly longer life and form retention, better surface finishes, and higher removal rates without thermal damage to the ground surface.

This information is from a presentation at the 1998 JIDA seminar by Alan Carius, manager of Borazon CBN programs at GE Superabrasives, Worthington, Ohio.

*Trademark of General Electric Co., USA.

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