From Odd Shapes to Superhard Materials: Advancing Technology
Precision grinding processes match tooling, parts requirements
By Jim Lorincz
If it uses grit and takes off material, we’re probably in it," says Harlen Gibbs, operations manager, Minnesota Grinding (Crystal, MN), and he means that quite literally. If you add the word "true" to Gibbs’ characterization, you would be describing the precision machining possible with the latest advances in grinding technology. Minnesota Grinding’s evolution from a small tool-grinding shop in 1959 to becoming a 42,000 ft² (3902-m²) precision-grinding job shop with high bays and overhead cranes for handling large workpieces, employing 35 across three shifts, isn’t an uncommon tale.
Minnesota Grinding’s customers cover nearly every industry from food processing to roofing manufacturers to medical, automotive, and aerospace. Typical jobs in the shop at any time could be centerless grinding bar stock, grinding pins for hard drives on the same machine, a few hours later on the same machine, repairing a large roller, while thread-grinding titanium bone screws in another.
"We started as a tool-grinding shop and gradually expanded into centerless, cylindrical, and thread grinding. Eventually, we added machines for flat grinding, honing and lapping. Currently, we provide centerless, cylindrical, thread grinding, lapping, honing, double-disk grinding, Blanchard and surface grinding, as well as some CNC lathe and Swiss work," David Schranck, HR/IT manager explains.
Harnessing Precision Grinding
Workhorses in Minnesota Grinding’s stable are four Studers—an S20, two S40s, and a new S33—from United Grinding Technologies (UGT; Miamisburg, OH). "We found the rigidity of the Studers key to our precision and repeatability requirements, and the service we’ve received from UGT, the technical support, whether it’s helping with a programming issue or just fixing or adjusting the machine—has been remarkable," says Gibbs. "Those are the best things about Studer grinders. They are very rigid and run forever. The S20, for example, has been running for 24 years. It still runs today, every day. We run the grinders five days a week, 24 hours a day," Gibbs says.
The parts they’ve been grinding have tolerances of ±0.0001 to 0.0004" (0.003–0.010 mm), with an 8–16 RA. "They tackle just about any material, from aluminum, titanium, Hastelloy, stainless, Inconel, carbide, plastic—pretty much the whole gamut. We just had a job that was made out of Rhenium, an exotic, very expensive material for an aerospace coupler requiring ID, OD, and thread grinding," Gibbs says.
"There are some parts that we can put on the S33 and grind the OD, the face, and an ID in one chucking, whereas we used to have to put this same part on three different grinders to do the one job," Gibbs says. "Plus, the S33 has a shoulder probe, and a B axis that automatically rotates the wheelhead around to do different kinds of features—ID grinding, for example. The S33 grinder is equipped with the latest generation of high-frequency internal grinding spindles. The machine can grind with speeds up to 120,000 rpm, thus allowing the grinding of very small bores."
In the past, Minnesota Grinding probably would have quoted jobs based on running a part on multiple machines. The new Studer S33 with the Fanuc 0i-TD control can do multiple diameters, multiple faces, and shoulders in one setup on a single machine. It has changed the company’s philosophy and opened up other opportunities because of its precision, speed, and flexibility.
Joel Schoening, grinding technician and lead man, describes how Cpk on one job was improved on the S33: "The part with nine diameters is a 16" [406-mm] long shaft. We grind the entire shaft in two operations. I’ll chuck on one end and grind six diameters from the shoulder down to the end. Then I flip the shaft around and grind the other shoulder and the three remaining diameters. The diameters are from 1.5" [38 mm] at the largest, and the far ends are about 0.750" [19 mm] with a keyway on one end. The diameters gradually step down. There are bearing journals on each side that are about 1" [25.4 mm] in diameter. There is a keyway on one end.
"These parts have a print tolerance of 0.0005" [0.013-mm] runout. We’ve run these same shafts on other machines and achieved 0.0003" [0.008 mm] from diameter to diameter, giving us a Cpk of 1.394. On our new S33 we are holding 50 millionths from diameter to diameter giving us a Cpk of 3.0134. In fact, when I initially ran this part on the S33 and checked it with my tenths indicator, the needle didn’t even move. I had another operator check the part and he said my indicator must be broken. Then we checked with a 20-millionths indicator, turning the part slowly, and found it to be 50-millionths runout."
Smart Toolmaking Processes
Rollomatic Inc. (Mundelein, IL) has made five- and six-axis GrindSmart tool and cutter grinders since 1995 for production-grinding end mills, drills, reamers, specialty cutting tools, form tools, inserts, and medical and dental devices including form drills, orthopedic drills, and bone drills. "Recent developments include a wheel changer for production flexibility and continuing software development to meet the changing requirements for grinding high-performance end mills," says Eric Schwarzenbach, president.
"Our software development is important for specialty tooling and inserts and for grinding end mills with variable indexing, variable leads, and variable rake angles to overcome machine harmonics that cause chatter. Reamers had unequal indexing for years to defeat chatter. It has only been in the last half-dozen years the unequal index or variable-lead helix was recognized as the solution for end mill chatter," Schwarzenbach explains.
Rollomatic’s wheel changer introduced for its five and six-axis tool and cutter grinders allows the user to dress and preset six wheel packs in the machine. "It’s very handy for short-run work or for long runs where wheels wear and have to be replaced. They can be replaced without shutting the machine down to pull the wheel pack," says Schwarzenbach.
Two tool-and-cutter grinder models are available. The GrindSmart 528XS is a five-axis machine for manufacturing and regrinding end mills, drills, and deburring tools from 1 to 25-mm diam. The GrindSmart 628XS is a six-axis grinder for manufacturing and regrinding round and nonround cutting tools from 0.1 to 16-mm diam. It features a V-block and steadyrest support system for handling metal, composite, and wood cutting tools, inserts, and medical and dental instruments. 3-D tool and machine simulation and Virtual Grind software enable precision manufacturing.
Another grinding technology available from Rollomatic employs precision pinch/peel cylindrical grinding for tooling-blank preparation, for pins and punches in the mold and die industry, and for assembly pins and fiber-optic applications in the electronics industry.
"In pinch/peel grinding the roughing wheel is slightly advanced of the finishing wheel, both in the axial and the radial direction," Schwarzenbach points out. "The roughing wheel peels the material, it plows in there, and takes the material out. The finishing wheel travels a little behind to put the final finish on the part. The pinch comes from the fact that both wheels engage the workpiece simultaneously providing opposing forces on either side to eliminate deflection or bending." A rolling wheel pushes the spinning workpiece into a V block which provides support when the blank exits machining.
Rollomatic has introduced a steadyrest attachment to catch long and thin parts that come out of the V block to prevent long and thin workpieces from whipping, which causes out-of-roundness and finish problems. Long thin parts up to 12" (305-mm) benefit from the steadyrest tailstock. Range of diameters of these parts is 1/8–3/8" (3.2–9.5 mm). Anything larger doesn’t whip, including ½" (12.7-mm) carbide.
Global Automotive Sets Standard
Cinetic Landis Corp. (Hagerstown, MD) prides itself on its expertise in the grinding of automotive components, in particular camshafts and crankshafts. Between it and a sister company in the UK, automotive and, increasingly, applications from industrial compressors to diesel-powered off-road equipment are served with the latest orbital grinding designs. Products are designed and manufactured in-house based on proven machine platforms that have developed to cope with the rigorous demands of high-volume production, yet still be flexible enough to be used in batch production. Cinetic Landis products feature no-wear hydrostatic spindles and slide ways, with linear motor technology on all feed axes.
The automotive industry where large V8 engines are largely a thing of the past is trending toward universal adoption of 4- and 6-cylinder engines, with an emerging demand for 3-cylinder power plants. New generation engines are smaller, requiring smaller components such as camshafts and crankshafts. This, coupled with a growing interest in more flexible equipment, has led Cinetic Landis to re-engineer its popular LT2 orbital grinder for global automotive manufacturers. The LT2E has the same capacity of the earlier design orbital grinder it replaces—shafts up to 750-mm long with a swing diameter to 180 mm—yet it uses less than half the floor space, has a lower more ergonomic centerline and incorporates smaller, more economical grinding wheels. The grinder’s twin wheelheads provide simultaneous grinding with wheels spaced as close as 20 mm.
A continuing emphasis on process flexibility, coupled with a rapidly growing interest in energy efficiency in manufacturing processes, has created a promising market for Cinetic Landis’ new EcoFlex traverse/plunge/contour grinder. Late in 2011, the company shipped the first EcoFlex grinder for use on a family of parts at a facility in Asia.
"The biggest challenges we face are meeting our customers’ demands for machine tools that are flexible enough to accommodate families of parts, yet be able to maintain and improve quality while optimizing process performance," explains Tim Hykes, chief engineer. "We’re always looking at optimizing the process to be able to handle families of parts with different dimensions while making cycle times faster."
"What Tim is getting at is the fact that there’s a tradeoff in cycle time and reducing floor time in creating a flexible process," says Dwight Myers, director of operations. "As the industry moves toward smaller batches of families of parts, customers we’re setting these production lines up for aren’t 100% sure what parts are going to be made on that line, sometimes even four to six months after they’ve ordered it.
The new EcoFlex grinder is a machine platform that is designed for handling larger parts for mining and heavy-industry applications like compressors for refrigeration where the challenge again is to consolidate manufacturing and be flexible. Cinetic Landis addressed two key issues—flexibility and economy of ownership—in designing the Ecoflex line of traverse/plunge/contour CNC grinders. The modular platform can accommodate either conventional aluminum oxide or superabrasive CBN grinding wheels. It incorporates a B axis that allows using multiple spindles and programmable angles to handle combined operations in a single fixture for better geometric accuracy. Linear motor drives are incorporated for the grinding wheel in-feed as well as the traverse axis to eliminate stick-slip positioning limitations. Target parts include large rolls, drive shafts, and similar large components. The base machine is targeted for parts up to 1.5-m long and 400-mm diam with a 750-mm part swing available as an option. ME
This article was first published in the May 2012 edition of Manufacturing Engineering magazine. Click here for PDF.