Grinding the Hard Stuff
When conventional machining can't cut it
By Robert B. Aronson
Expanding use of ever-harder materials has opened the door to wider use of grinding processes on materials such as titanium, ceramics, and superalloys. These are often materials with properties such as higher strength, wear resistance, and light weight, which are important to the electronics, aerospace, and medical industries. In some cases, high tool wear or time constraints make conventional milling, turning, and drilling economically impractical, so it's grinding to the rescue.
Complete Grinding Solutions, LLC (CGS) (Springboro, OH) does not sell grinding equipment; they are a process design group. Their engineers evaluate a customer's needs, then recommend the best equipment and processes needed to meet those needs. According to Beat Maurer, president of CGS, "We design systems for manufacturing everything from prototypes to high-volume production.
As to trends, currently we are doing a lot of work with carbide and ceramic coatings, which are quite popular, particularly in the aerospace and automotive industry."
These types of hard coatings are needed for greater strength and wear resistance. Normally they are applied in a thickness of around 0.0020" (0.025 mm, ground to specific shapes.
For the harder materials, the company uses diamond wheels. "One complication is that they have to be dressed with a diamond dressing disk, so you are cutting diamonds with diamonds," explains Maurer. It's a matter of matching wear ratios. The dressing system includes an acoustic sensor, which detects the contact of the dressing and working wheels at the beginning of the dressing sequence.
"Currently the two most demanding jobs are aircraft parts and injectors for the auto market," says Maurer. Smaller diesel engines for passenger cars are a market that is really growing, particularly in Europe.
"But you can't buy a diamond wheel and a dressing disk, put it on an old machine, and hit cycle start. You need state-of-the-art equipment that will do the job.
"Look at the entire process. In particular, the grinding machine has to be rigid, equipped with high-frequency drives so that dressing parameters can be manipulated," he concludes.
"We are seeing an increase in exotic materials, including higher heat treating in the RC 60–70 range, and lots of hard coating," says Nelson Beaulieu, Grinding Products Manager, Hardinge North America (Elmira, NY). "Typically this kind of work needs a light cut at high speed with particular attention to coolant. The nozzles not only have to be in the right place, but of the right size and carrying the right volume of coolant. Ideally size, shape, and position of coolant nozzles are critical to quenching the heat derived from the higher speed. In particular, the flow must be tangential to the point of contact. For example, our new Bridgeport FGC2 Flexible Grinding Center, used in aerospace turbine blade and vane grinding, has programmable nozzles so the flow follows the part as grinding progresses."
Many of the harder parts are very heat sensitive. So the fast, shallow cuts are necessary to carry off the heat with the swarf before it influences the workpiece. Surface speed is 400–500 ipm (10,160–127,000 mm/min), with cuts of 0.0004–0.0005" (0.10–0.13 mm). "The old style of creep feeding won't do the job as quickly as with the Viper method," says Beaulieu.
The Viper method is a patented process of using aluminum oxide grinding wheels with the use of programmable coolant nozzles. One of the nozzles is used to direct coolant tangential to the wheel and workpiece as the wheel moves around the workpiece. The second nozzle is a high pressure cleaning nozzle which blasts debris out of the wheel to increase cutting efficiency and reduce the number of dresses or the amount of dressing needed to keep the wheel performing optimally.
For their Bridgeport FGC2 grinding center, they use a machining center base rather than modify a grinder to handle machining speeds.
"We do one piece on-and-off," explains Beaulieu. "If you blow a fixture full of parts, you have a real problem. You are into the second part before you finish the first, so the errors transfer over until the entire fixture has passsed under the wheel. With the Viper method, you make corrections if you are working with only one part at a time."
Most heat-sensitive parts are mounted vertically to expose the part to let the coolant in. Nozzles can be arranged so they reach not only the part, but the spindle and fixture. With the part lying down, a lot of hardware blocks the flow.
With ceramics, Hardinge recommends wheel speeds in the 450–500 fpm (137–152 m/min) speed range with an oil emulsion coolant, not synthetic. Ceramic easily chips, but grinds very well. To minimize stick-slip and vibration that can easily destroy a ceramic part, they use hydrostatic ways.
"We recently were able to do a job where the tolerance was plus 0, minus 40 millionths. It's never impossible to grind. Its just difficult," Beaulieu concludes.
A Hauser S35-400 jig grinder made by Hardinge Kellenberger (Elmire, NY) is being used by Penn United Technologies Inc. (Saxonberg, PA) to grind carbide and steel. According to company President Bill Jones, these operations have produced a positional accuracy of 0.000040" (0.001 mm) for a series of holes. In another job for a razor company, Penn United trimmed materials to 0.0008" (0.02 mm) where punch-to-die clearance had to be held to 0.0001" (0.003 mm) and positional tolerances to 0.000040 or 0.000050" (0.001–0.0013 mm).
More new high-hardness coatings are coming onto the market for two reasons. First, there is a strong push to have jet engines run hotter for both greater power and lower pollution. Second, some alloys of chromium, an old reliable coating material, have been declared carcinogenic.
One of the leading new coatings method used chiefly by the aerospace industry is high velocity oxygen fuel (HVOF). In this technique, the coating is applied in layers up to 0.0040" (0.10-mm) thick, then about 0.008–0.012" (0.2–0.3 mm) is ground away to obtain the proper surface. It offers good protection against wear and corrosion.
HVOF applied materials and other new coatings are so hard that conventional machining is not practical. "Metals have reached the limits of common machining approaches," says Hans Ueltschi of United Grinding Cylindrical Products (Miamisburg, OH).
"We recommend using vitrifieddiamond abrasive. Plated-diamond or resin-bonded diamond wheels will work, but vitrified diamond wheels condition more easily and the dressing process can be more easily automated.
"Reconditioning the wheel is one of the more complex aspects of the process," says Ueltschi. The grinding machine has to be set up for rotary dressing, not a stationary tool. The dressing system must be adjustable in both speed range and direction. This type of dressing requires an acoustic sensor that can precisely find the wheel's edge with an accuracy of 0.000020–0.000040" [0.0005–0.001 mm].
To grind hard materials, such as ceramics or carbide, it's essential to have a stiff machine. This allows the use of "hard" wheels and precise truing and dressing. The results are good surface finish and quality part size and geometry.
"For someone just starting this type of work, the main issue is understanding the entire process. You can't just change a wheel and press go. It takes time to establish a process," Ueltschi concludes.
Morgan Advanced Ceramics, Rugby, UK, got its start as a sparkplug maker and has followed the rising star of ceramics for some time. Currently, the medical market is a strong focus of attention. They have been making ceramic hip joints since the early '80s. "Ceramic joints are more widely accepted in Europe," says bioceramic product manager Steve Hughes. "Traditional joints that rely on polyethylene on metal will usually show wear far sooner than a ceramic joint, which can last a lifetime. The ceramic used is aluminum oxide, which is very pure and has a uniform microstructure. We finish it using diamond tooling. CBN is not hard enough."
The equipment the company uses is similar to that used with metals, but both it and the machines and the tooling have to be modified. "You can't just put on another wheel and be in business," says Hughes. "Understanding the material side is the big issue. You must know how the ceramic will perform under various process conditions."
Hexavalent chrome is becoming a no-no in manufacturing because of its link to cancer. "We are looking for coatings to replace it in cases where corrosion protection and surface finish are critical," says Mike Hitchiner, Technology Manager for St. Gobain (Worcester, MA). "Aerospace is a big market, as is construction equipment. HVOF is emerging as a major answer, and because an HVOF-coated part often needs finishing, this need indirectly influences the need for grinding."
Generally, there are more materials being used that have a hardness of RC 65. For example, there is greater use of ceramics in medical products, while Tribaloy, another intermetallic alloy with good wear properties, is another material that is growing in popularity, but very difficult to grind.
New grinding machines are also on the increase, particularly CNC machines with dressable diamond wheels that don't need conditioning. These machines are also highly automated for greater productivity and the elimination or minimization of an operator.
"Combinations we have never seen before are on the market, such as a single machine using flexible belts for roughing and diamond-bonded wheels for finishing," says Hitchiner. "There is an increase in the usage and regrind of ultrahard tools such as PCD and PCBN, which have to be ground."
Rollomatic Inc. (Mundelein, IL) has solved a number of grinding problems for customers that are working with harder materials. In one case, core pins for an injection molding machine, formerly made of HSS, had to be made of ceramic.
According to Eric Schwarzenbach, company president, "We were able to make these parts with unmodified equipment. The only change was making the feed rates for the diamond wheel substantially lower. This was to avoid the heat problems which can easily shatter ceramic if unchecked. We also added higher coolant flow and more powerful drive motors."
In a problem solved with a kind of reverse logic, they were having trouble grinding nickel titanium to be used in dental tools. This material is unhardened, but in terms of grinding it behaves like hardened material where the hardness is only about RC 45. "First we tried CBN wheels unsuccessfully," says Schwarzenbach. "Then we went to diamond wheels and they did the job."
Another interesting project was fiber-optic connectors; a part made from a hard, easily shattered material that also required a superior finish. To ensure transmissibility, the surfaces had to be very precise. Again, more coolant and slower wheel speeds did the job.
The cutting tool industry is moving to more inserts and less solid carbide. As a result, Rollomatic has more work from customers who want to grind inserts; threading inserts in particular. CBN inserts are one such job. "They are harder than carbide and take more time to grind. Actually, it's more of a polishing," concludes Schwarzenbach.
Because of Rollomatic's experience with precision grinding, they plan to introduce their own CNC dressing machine shortly.
Larry Marchand, manager aerospace products, United Grinding Technologies (Miamisburg, OH) notes that about 90% of his work is with very challenging superalloys, particularly in turbine engine applications.
As noted earlier, there is a strong push to increase the operating temperatures of jet engines to increase fuel economy and reduce pollution. "Land-based turbines are using more superalloys in place of stainless, and aircraft turbines are replacing titanium with similar superalloys," says Marchand.
Many projects require the best of both grinding and milling, and to meet this need UGT offers an "almost all-purpose" grinding/milling machine system. It has an automatic toolchanger that offers a variety of abrasive wheels along with traditional machining tools.
For example, a grinding wheel may be the proper process for most areas of a part, but it may also need a hole or chamfer that requires a drill or milling head. The machine can apply either a plated CBN wheel for grooves or a continuous dress grind to move a heavy volume of stock.
Dressing is done with an overhead system with an 8" (203-mm) wide arbor and an in-and-out axis. The CNC axis positions the dresser over the wheel form that needs dressing.
The best design for cutting hard materials, such as ceramic-coated superalloys, is a stiff machine that can dampen the vibration. Even a small amount of harmonics getting through to a ceramic workpiece can cause it to chip. "We dampen vibration by using hydrostatic guideways that are standard on the Mägerle grinders," says Marchand. "That small film of pressurized oil does a great job of damping."
Another advance for superalloys comes from the company's Blohm division. The new Prokos model utilizes linear motors for high-speed reciprocal grinding. Its speed is about three times that of a conventional grinding machine. It takes very shallow cuts with little pressure on the part, which is essential for work in the harder materials. It comes in three or five-axis versions for better access to the workpiece.
Among the more popular of the newer coatings is High-Velocity Oxygen Fuel (HVOF) spray. One of the suppliers is National Thermospray (Pasadenia, TX), a company that specializes in HVOF services in which a hot combustion-driven, high-speed gas jet creates a strong, wear-and-corrosion-resistant coating. In the process, a gun emits a stream of combustion gas that burns at a temperature between 5000 and 6000°F (2769–3316°C) along with a powdered material.
The fuel and powder are blended to match the substrate. The resulting coating is high density, has a bond strength in excess of 12,000 psi (83 MPa), and has zero porosity. The HVOF process contributes little heat to the workpiece.
Substrate temperature is less than 300°F (149°C), which does not affect the substrate's mechanical properties. No stress relieving is required.
This article was first published in the August 2007 edition of Manufacturing Engineering magazine.