Grinding Gets Flexible
Accuracy and process flexibility drive innovations in today's abrasive machining equipment
By Bruce Morey
Customers are demanding more flexibility, higher precision, increased throughput, and longer life for grinding machines. Suppliers are meeting these demands with better machines that feature linear motors, improved abrasives, and multiple wheels and spindles.
A continuing positive trend for the abrasive-machining industry is the transfer of operations from aerospace and automotive OEMs to outside suppliers. Those suppliers need new equipment to meet demand from their customers in Asia, Europe, and South America, as well as the US.
Softer materials that were formerly the domain of grinding are now being machined using EDM or hard turning. "Frankly, the continuing trend towards harder materials, such as Inconel, is what is keeping the grinding industry alive," says Vic Truelsen of Okamoto Machine Tool Works Ltd. (Buffalo Grove, IL).
"In the automotive powertrain market, tolerances are 30–50% tighter than they were five years ago," says Russell Kaiser, vice president engineering, Cinetic Landis Grinding Corp. (Waynesboro, PA). According to Truelsen, tolerance ranges demanded by industry have gotten tighter as well. In both automotive and aerospace, tolerances on parts are being held to a higher Cpk tolerance than in the past. "Typical industry Cpk standard tolerances of 2.0, 1.66, and 1.33 are reducing print tolerances to 40–60% of the total tolerance allowed," observes Truelsen.
For Drake Manufacturing Services Inc. (Warren, OH), tighter tolerances are driven by the market need to reduce noise in geared systems, whether it be automotive power steering, industrial gearboxes, or speed reducers. "Gearbox components that were at AGMA 8 requirements [American Gear Manufacturing Association] in the past are now being specified at AGMA 10–11," says Jim Vosmik, president and CEO of Drake.
Grinding is changing in real time, according to Nelson Beaulieu, grinding product manager-North America, Hardinge Inc. (Elmira, NY). The high-volume market and the job-shop market were once quite different. Today, Beaulieu believes those markets are drifting toward one another. "The typical automotive supplier, for example, is behaving more like a job shop, because of the short run of products now prevalent in that industry. The typical job shop environment, for a variety of reasons, is becoming more like a production house," Beaulieu says. "Flexibility is the key factor on the part of decision makers today."
There are different ways to offer flexibility, including multiple spindles, multiple grinding wheels, or single spindles with automatic wheel changers.
Russell Kaiser agrees that "our customers need to react to market changes quickly and cheaply." Michael DiVentura, a Cinetic Landis vice president, explains that "customers are now demanding a part-changeover in minutes, if at all possible." Some of the company's customers are running multiple parts on the same line, calling for on-demand setup.
Changes to the design of their machines and accompanying processes have allowed Cinetic Landis to meet this challenge. Old grinding machine designs were dedicated; new designs are built with rapid changeout in mind. In one example, multiple spindles on one machine enable a customer to do multiple crankshafts on the same production line.
Drake Manufacturing also offers multiple operations in a single machine as an answer to the need for both flexibility and minimal capital expenditure, Vosmik says. An example is the Drake Universal External/Internal Thread grinder. It has multiple spindles and multiple wheels for shops with both internal and external thread-grinding requirements. Drake is also introducing a flexible grinder that can do flutes and spiral points in one operation.
One of Hardinge's answers to flexibility is a single-spindle offering from Bridgeport called the Flexible Grinding Center 2 (FGC 2). It's a combined-operations center that can do grinding and other machining operations. A vertical creep-feed machine, it employs a 50-hp (37.3-kW) motor on the spindle with 250-mm diam (to 50-mm wide) aluminum oxide grinding wheels, automatic wheel changeout, and automatic dressing.
Erwin Junker Machinery Inc. (Elgin, IL) also strives for flexibility with multifunction systems, according to Denis Fritz, the company's vice president and CEO. Erwin Junker also offers combined grinding-only operations, such as centerless and plunge-grinding operations in a single machine.
Multiple machining operations are also available on machines from United Grinding Technologies Inc. (UGT; Miamisburg, OH). "We have equipment that combines grinding, milling, and drilling," says Chris Stine, vice president of UGT, "and others that can do grinding, turning, and boring."
Flexibility can also be had from a single-spindle machine equipped with an automatic wheel changer. The new Blohm Prokos machine from UGT offers an optional ATC system for its single spindle, allowing complex part grinding with a single setup by combining grinding, milling, and drilling operations.
A continuing trend from past years is the use of linear motors to replace ballscrew elements for positioning. They offer reduced wear, faster cycle times, and greater positional accuracy when compared to ballscrews, and eliminate backlash.
"The use of permanent-magnet linear motors on modern grinding machine wheel-feed systems has enabled a 50–100% increase in acceleration and reversal of the wheelhead compared to conventional ballscrew-type drive systems," says David Myers of Cinetic Landis. All new machines produced by Cinetic Landis are equipped with linear motors instead of ballscrew technology. "There are 800–1000 existing Cinetic Landis grinding machines that use some or all linear motor technology," says Kaiser.
"Linear motors provide a three-times speed improvement over traditional ballscrews in our applications," says Chris Stine. "Linear motors allow us to increase movement speed from 40 m/min using ballscrews to 120 m/min." Linear motors offer not only speed but acceleration as well. Stine reports that linear-motor-driven UGT equipment achieves five times the acceleration of traditional ballscrew drives, an important factor for the aircraft turbine manufacturers that use them.
Drake Manufacturing concurs with the trend toward linear motors. They first used linear motors in their grinding equipment in 2004. Currently, almost half of the company's thread grinders use linear motors.
Although other companies also use linear motors, not all do. Erwin Junker is following a different technical route. Junker's concerns include the magnetic fields from the permanent magnets, introduction of heat into the grinding machine, and energy consumption. Instead, Erwin Junker has improved the standard ballscrew positioning technology by using hydrostatic wear surfaces.
Hardinge also does not use linear motors, for some of the same reasons. Additionally they report concerns about contamination of the motor units.
"High-efficiency deep grinding [HEDG] has come out of the laboratory and is ready for commercial implementation," Drake's Vosmik asserts. HEDG takes a deeper cut than conventional grinding but at a much faster speed than creep-feed grinding. "We have been performing prototype grinds for customers, and we expect to introduce it commercially in 2007." The enablers for HEDG grinding are improved dynamic stiffness of the machine and higher power spindle motors. The productivity gains are significant—Drake Manufacturing is grinding a 4.5-mm deep gear tooth from solid in two cuts at 40 mm/sec.
Another trend in new grinding operations is making shallower cuts at much faster speeds.
Hardinge's Bridgeport FGC uses a fast, shallower cut compared to its typical predecessors. "The FGC runs about 800 ipm [20.3 m/min] with a 1-mm cut," says Beaulieu. He claims this performance results in about eight times more stock removal than that attained by a typical creep-feed grinder. The FGC includes a patented coolant-positioning system and an associated cleaning-nozzle design.
United Grinding's "speed stroke" approach also uses shallower cuts with faster speed when grinding nickel-based alloys. "One can think of the Prokos as the contra-approach to HEDG," remarks Stine, although he says UGT offers HEDG where that approach is appropriate.
"For the Prokos, think of a standard reciprocating grinder on steroids," says Stine. He contrasts the "speed stroke" approach with traditional creep-feed grinding. "For a difficult creep-feed application like grinding the nickel-based alloys used in a jet's hot zone, creep feed might take cuts at 0.040 or 0.060" [1–1.5 mm] depth of cut at 6 ipm [0.15 m/min] infeed rate, whereas the speed stroke approach grinds off a few thousandths at 120 m/min. This produces a faster material removal rate with less heat introduced into the part." He says using linear motors made the "speed stroke" technique possible.
Hydrostatic ways eliminate metal-to-metal contact and satisfy the need for reduced wear and increased positional accuracy, even during high-speed movement. Junker's Fritz believes they both reduce the stick-slip effect of conventional ways and ensure an even thickness of the oil between the surfaces.
Truelsen says that Okamoto's precision variable static way system not only uses a thin film of pressurized oil, but also senses and adjusts the film as necessary. Oil film thickness is sensed every 6" (152 mm). If the thickness is too low or high, the film is pressure-adjusted by zones.
Tru Tech Systems Inc. (Mt. Clemens, MI) has developed carbide work spindles to provide better precision. "Carbide is both heavier and stiffer than the typical steel used for spindles, providing higher dynamic stiffness," says Steve Smarsh, president and CEO. According to his figures, the price of carbide has come down, making it economically feasible to use it in grinding machine spindles. The carbide spindles are used by Tru Tech in centerless grinding operations. Carbide is more than twice as heavy as steel and ten times more rigid.
"Abrasives in general have evolved much more rapidly than the grinding machines," says UGT's Stine. "In today's world, we see changes in abrasives on a daily basis compared to machine improvements, which evolve more slowly."
Use of cubic boron nitride (CBN) as a grinding medium continues to increase, a trend observed for several years. Of course, machines need to be set up for CBN wheels. Not every machine can accept a CBN wheel. Given that fact, CBN is still worth exploring as an option. Although CBN costs more initially, it can save money over the long run by reducing dressing time and improving accuracy, according to Okamoto's Truelsen. It also extends tool life. "I've seen production increases of 20–30% by switching to CBN," he says.
Cinetic Landis agrees that there is a trend towards CBN. According to Russell Kaiser, 90–95% of shipped equipment goes to the user with vitrified CBN wheels. One of the reasons for the specification of CBN is that there are times when the tolerances of the processes are more limited by the tool than by the machine. Use of CBN is necessary to meet very tight tolerances. And in some instances CBN usage is dictated by the grinding methods employed. "Peel grinding, for instance, is enabled more by technical evolutions in the wheel, wheel speed, and control software than in the development of the machine," says Michael DiVentura.
"CBN wheels appear to have come down in price," Smarsh observes, noting that they may not scare customers like they once did. "CBN wheels are much more durable than they were just a few years ago. You can even drop some of them, and they won't break."
Stine of United Grinding sees big advancements in ceramic abrasives, which are more exotic grains that can offer some very productive output. "Ceramic and ceramic-blend grinding wheels are the biggest advancement in the last twelve months, even more so than CBN," he states.
Tru Tech Systems is doing a lot of research and development in wheel technology. "That's one area I'm not afraid of spending a lot of money on," says Tru Tech's Smarsh. The company has developed diamond wheels that can grind steel and carbide, saving money for the customer who needs to grind both.
This article was first published in the February 2007 edition of Manufacturing Engineering magazine.