Grinding Research Pays Big Dividends

Blurring the line between itself and other metalworking processes, grinding is experiencing nothing less than a technical renaissance. Research spanning the last decade has produced spectacular changes in grinding technology. Many of these advances are a direct result of research collaborations between federal research labs, major universities, end users, and manufacturers. Other advances are the culmination of exhaustive field tests conducted within companies to improve existing products and address the trends in workpiece-material development.

However, overall objectives remain the same:

• To grind at higher speeds.
• To enable the use of advanced, hard-to-machine workpiece materials.
• To design and control the manufacturing process.

These efforts have already produced some extremely sophisticated grinding machines, wheel configurations, and fluid compositions, and these innovations have altered the grinding process itself.

Ongoing Research Programs

At Oak Ridge National Laboratory, Oak Ridge, TN, the High Temperature Materials Laboratory (HTML) is conducting research into the development of piston and gas turbine engines using structural ceramics such as silicon nitride (SiN). The two obstacles hindering this process are the difficulty of making low-cost ceramic powders and the high cost of machining these hard materials to accurate dimensions.

A U.S. Department of Energy-supported Cost-Effective Machining of 
Ceramics (CEMOC) program at HTML is developing improved tools to machine ceramics at lower costs. The CEMOC program includes cooperative R&D agreements, subcontracts with industry on ceramic manufacturing, and user projects at HTML. Subcontracts are in place with several companies to demonstrate specific high-accuracy ceramic-component manufacturing technology. Other companies have been contracted to develop better grinding wheels and evaluation instruments, including high-accuracy-instrumented ceramic grinding, dimensional analysis, and surface metrology.

HTML is using a CNC vertical grinding center that Cincinnati Milacron, Cincinnati, supplied as part of an in-kind contribution under the terms of a cooperative R&D agreement. The equipment, originally configured as a vertical machining center (VMC), was modified for high-speed grinding.

"We are grinding SiN, a fairly difficult-to-machine ceramic, with superabrasives-diamond or cubic boron nitride (CBN)-at high speeds," says Sam McSpadden, group leader for the Machining and Inspection Research Group at HTML. "We've come up with a wheel that has a composite material as its core and can run up to 20,000 sfm. We are using the centerless grinder for plunge grinding to make a button-head tensile specimen that is pulled until the material fails to see if the material was damaged by grinding it."

The SiN material is GS-44 from Allied Signal Corp., Morristown, NJ, one of the more useful materials in making valves. Because internal flaws will cause a part to fail during the grinding process or during service after grinding, very high-quality materials are required. Cycle time to make parts is less than one minute, whereas it takes up to 30 minutes to make with a conventional centerless grinder.

According to McSpadden, "There was a concern about the bearings on the spindle being able to handle the higher speed without overheating. So we put temperature-measuring equipment on the machine-thermocouples-went to a thinner viscosity hydraulic fluid for the hydrostatic bearings, and replaced the cooling fan in the spindle motor with a constant-speed cooling fan. A lot of the technology for Cincinnati Milacron's Viking came out of the work here."

In addition to the centerless cylindrical grinder, HTML uses several surface grinders, a creep-feed grinder, and a 3-axis grinder that is similar to a VMC. The CEMOC program's next step is obtaining an active industrial partner to supply a valve design.

Advanced materials and precision grinding also are the focus of a program at the National Institute of Standards and Technology (NIST), Gaithersburg, MD, that specifically addresses the technologies required to produce ceramic and composite parts for the automotive industry. Sponsored by Cummins Engine Co., Columbus, IN, the Motor Vehicle Manufacturing Technology, Submicron Precision Grinding of Advanced Engineering Materials project will develop cost-effective technologies for grinding advanced ceramic and composite materials to submicron tolerances.

The project team will test new methods of truing and dressing a grinding wheel and investigate additional technologies to make ceramic grinding cost effective. Other machining technologies under investigation include grinding wheels, a coolant-filtration system, a laser truing system, and sensors and algorithms for truing.

In addition, the project will evaluate advanced materials for diesel applications and develop the technologies for prototype grinding machines. Ultimately, the project will produce a system for shop-floor testing. Agreements were recently inked and the project got underway in mid-March, according to NIST spokesman Richard Bartholomew.

The Center for Grinding Research & Development at the NSF Industry/University Cooperative Research Center, University of Connecticut, Storrs, CT, also is addressing production-grinding problems. Technical areas include centerless grinding, dressing, damage assessment, truing, and lobing in precision abrasive-material-removal processes.

Projects cover every facet of the grinding operation, including acoustic emission in grinding, grinding-wheel structure improvements, truing and dressing CBN/seeded-gel hybrid wheels, microorganisms in metalworking fluid, optimized ceramic grinding, analysis of surface integrity in ground tool steel, and the thermal limitations of creep-feed grinding.

Companies involved in this program include Norton Co., Worcester, MA; The Timkin Co., Canton, OH; Bryant Grinder Corp., Springfield, VT; Cincinnati Milacron, Cincinnati; and Weldon Machine Tool Inc., York, PA.

Research efforts paid big dividends with the 1632 Gold CNC cylindrical grinder design from Weldon Machine Tool by minimizing or eliminating secondary operations once associated with other metalworking processes. Weldon works with the University of Connecticut, Massachusetts Institute of Technology (MIT), Cambridge, MA, and Penn State University, State College, PA, in various research programs. The company's work with MIT is apparent in its cylindrical-grinder design that uses patented technology from the machine's base to its spindle. Developing this technology has yielded more precise finished parts, smoother surface finishes, and greater productivity in a more aggressive grinding operation.

Precision Products, Springfield, TN, uses cylindrical grinders to produce tooling parts for the can industry.

Better Quality and Productivity

Precision Products, Springfield, TN, produces cold-formed necking dies, carbide punches, and other parts used to manufacture steel and aluminum cans. Its capabilities include grinding carbide and ceramic parts and nonround parts. The company wanted to shift its manufacturing processes away from conventional plunge or traverse grinding, using CNC-interpolation grinding instead to produce better quality parts with shorter lead times.

"No one else was able to do the interpolation grinding with superabrasives to the extent that we've been able to do it," explains Bob Rawls, process engineer. "In 1989, some of the major grinder manufacturers told us that it was impractical; the time and money invested couldn't be justified."

A collaboration between Precision Products and Weldon has not only changed its manufacturing process from plunge or traverse grinding, it has reduced the steps required to make the part.

"We built in operator feel and touch to the machines and our supplier opened our macro routines to allow us to customize routines, so that in a span of four to five years, the program runs on all machines and downloads from a remote site," says Rawls. In addition, the company's 20 operators can now produce twice as much and the 11 original operations have been consolidated into two setups on two parts, greatly reducing the need to handle parts. Operators also are able to tend to more machines simultaneously.

"We are doing linear circular interpolation with superabrasives, and for years people said that it could not be done," adds Rawls. "We do it to whatever tolerance is required using a combination of the machine and a family of superabrasive wheels that were developed for us by a manufacturer and by programming the controller.

"Our competition is getting in close with NC machining and finishing conventionally. But they are doing secondary operations to get what we are doing in one. That's our claim to fame." Precision has since purchased five more grinders, each time incorporating the changes it had made on its existing machines and adding further refinements.

Milling, Machining, or Grinding?

The grinding process, taken to higher speeds using superabrasive grinding wheels and equipped with CNC, resembles metalworking processes once considered secondary to the metal-removal process. Thanks to the advances in technology, grinding-machine builders have been able to catapult the concept of high-speed grinding into the mainstream, unlike the grinding center predecessors who introduced the concept in the late 1980s and early 1990s. For instance, superabrasive machining systems developed by Edgetek Machine Corp., Meriden, CT, use diamond or CBN electroplated wheels to machine at spindle speeds up to 20,000 rpm. CNC programming guides three, four, or five axes of machine movement. The machines are used in a variety of applications, such as slotting, forming, and roughing, and replace some metalworking processes.

It is analogous to high-efficiency deep grinding (HEDG); however, the forms and grooves are not as wide as what a creep-feed grinder would accommodate, and the machines cost much less. Manufacturing steps are eliminated as parts are ground in the hardened state as opposed to milling a soft metal, heat treating, and then regrinding.

Alternately, Junker Machinery Inc., Chicopee, MA, has taken another tack by employing "point technology" in its grinding machines. Again, the features are strikingly similar to those used in lathes or milling machines, because the tool is programmed to contact the workpiece at specific intervals, removing material directly from the part as opposed to creating the part's mirror image on the grinding wheel.

A CBN grinding wheel measuring 4mm to 6mm wide is vertically tilted into the workpiece in a programmed grinding path to perform tapers, shoulders, plunge cuts, or radius grinding in a single chucking. According to company officials, the reduced wheel wear and cutting forces improve surface quality and dimensional stability. Cutting speeds can reach 27,600 sfm to grind a variety of materials. "All grinding machines have a line contact between the wheel and the workpiece, whereas we have a very small contact," claims Markus Lipphardt, sales manager. "When you have a complicated part, you do not dress the wheel, you do not put the form on the wheel-plunge in and out-and thereby generate a profile of the part.

"A CNC controls two axes, and you take this point to grind along whatever shape you want to generate. The wheel is always the same, so in terms of flexibility, parts can be changed without changing the wheel. In conventional grinding, every time a part is changed, a different wheel is necessary."

Dave Johnston, a manufacturing engineer at Briggs & Stratton, Auburn, AL, uses several of Junker's grinding machines to make cast-iron engine components. The company has a total of five Junker machines at its facility. The grinding machines were used to produce a new V twin engine in the 18- to 20-hp market. The cast-iron components include crankshafts, cam gears, the hardened pin on the crank, the main bearings, and the taper on the crankshaft.

"We produced a scrap part when we changed wheels, and we only changed wheels to let people gain experience," says Johnston. "The current projected life of the wheel is one to possibly two years, and we run 400 parts daily. We also are getting 600 cranks between wheel dressings."

About the Author
Melissa Kennedy is a freelance business writer based in Cleveland, who specializes in the metalworking industry.