While cylindrical grinding has been used for some time, its capabilities may not be fully appreciated. New grinding wheel technology, controls, materials and dressing processes, coolant and pump options—as well as smarter software—make today’s machines worth getting acquainted with.
With multiple-wheel machines that allow a single setup for a complex process to probes that measure parts on the machine to maintain critical tolerances, cylindrical grinders have grown more flexible and easier to operate.
One process gaining traction is peel grinding, which removes a large amount of steel or carbide in a single pass using high feed rates and spindle speed in a manner reminiscent of a lathe.
Meanwhile, oil-based coolants are finding favor over water among some users. Improved chillers and heaters are maintaining stable processing temperatures vital for meeting the tighter specs required of aerospace, automotive and other components made with super-hard carbide, ceramic or coated materials. Programmable variable frequency pumps provide the exact amount of coolant when it’s needed most.
And, more sophisticated hinged or stationery probes can measure complex part features—length, shoulders, tapers, outer diameter (OD) and even internal bores—while compensating for temperature and wheel wear, all without unclamping the workpiece.
It all adds up to a contemporary lineup of cylindrical grinders that deftly handles parts as large as shafts and gears or small as ballscrews, threads and cutting tools.
Feeds, Speeds and Flexibility
The quest to accelerate grinding machine setup and part production and expand usefulness has led to notable modernizations.
For instance, the Studer machines from United Grinding North America (Miamisburg, OH) provide a new level of flexibility, said Hans Ueltschi, vice president of cylindrical sales. A contemporary part might be partly coated with carbide or another material via High Velocity Oxygen Fuel (HVOF), requiring a diamond wheel for that section. The alloy or steel substrate requires a different wheel. With a multiple-wheel setup, the grinder can process the entire part, avoiding multiple setups and runout issues.
Studer machines are also being asked to grind more bearings, especially for the aerospace industry. With one setup, the machine can grind the OD of the ring and interior of the race with the same clamping in the shoe system. Traditionally, Ueltschi said, bearing grinders would perform different functions on up to three or four machines.
He also noted that the Studer line features more accurate linear scales graduated in nanometers instead of microns.
Meanwhile, recognizing that some cutting tools initially require a round of cylindrical grinding, ANCA Inc. (Wixom, MI), has refined its machines to allow them to spin the headstock at up to 3000 rpm or house a 10″ (254-mm) diameter wheel. ANCA machines also feature in-process gaging.
On tool and cutter grinders, the option for a cylindrical grinding first step “is a nice feature to have,” said ANCA President Russell Riddiford. It minimizes the number of setups or “hand-holding” of the tool from one machine to another.
ANCA’s portfolio includes the MX7 Linear and larger TX7 Linear, which can be used to grind a range of parts, including pinion gears for power steering shafts, journal bearings, helical gears and carbide blanks for cutting tools like end mills and drills. The MX7 features up to six wheel pack stations for configuring multiple operations; the TX7 cell features two to 21 wheel packs.
“You have a library of wheels you can call upon,” Riddiford said. “You can tell the machine to get wheel pack number 15 and do this particular operation, and that wheel change time is about 12 seconds.” Mounting and truing the wheels prior to operation can take about 15 minutes.
He noted that peel grinding with cylindrical machines is gaining traction for its ability to remove several millimeters of material in a single pass. “We’re doing a lot of development with grinding wheel suppliers,” Riddiford noted. “There’s a lot of focus being put on peel grinding and the grinding wheel’s ability to hold the edge for long periods of time to allow this mass removal of material.”
The feed rate for peel grinding “is quite aggressive,” he continued. The machine calculates optimal spindle rpm when the operator inputs the job’s surface footage on the controller.
Parts and Processes
Among recent additions in the cylindrical grinding repertoire is the replacement of chrome plating with HVOF spray coatings, said Rob Titus, grinder product specialist for Okuma America Corp. (Charlotte, NC). The aerospace industry in particular is utilizing the process to great effect.
In the past, end users would chrome-plate parts, grinding off some chrome to maintain durability in a final assembly. HVOF involves spraying a chromium carbide mixture on a part—mild steel, for example—that allows more even and consistent coating and greater wear resistance, he explained.
Titus noted that a potential hindrance to optimal use of cylindrical grinding is often the practice of using decades-old process specifications limiting adoption of current grinding speeds. Today’s superabrasive wheels can be fed at Q-prime rates in excess of 5 mm3/min, he explained, whereas aerospace specs crafted in the 1980s might be based on 2.5 Q-prime.
“Grinding technology is a little bit different than the rest of the machining world,” Titus explained, noting that there are features and practices that smaller customers or those who don’t use grinding as a primary process might not be using and benefiting from.
For instance acoustic emissions sensors have been in Okuma and other machine makers’ machines for 25-plus years. They listen for when the grinding wheel touches the workpiece to slow the wheel to grinding speed, as well as for when the dresser contacts the wheel during dressing. But Titus still encounters customers who have not heard of them.
In the automotive industry, grinders are adjusting to tighter tolerance features and lighter-weight components, said Shane Farrant, national product manager for grinders at Toyoda Machinery USA (Arlington Heights, IL). Toyoda specializes in camshaft and crankshaft production.
“Various industries are introducing new materials for us to work with, which is good to see and challenges us and the customers with application solutions. In many cases we look to our grinding wheel manufacturers for recommendations for handling materials outside of our typical mild and hardened steels and cast iron.”
Dressing for Success
There are numerous techniques for renewing the cutting action of or truing the wheel on a cylindrical grinder—including single-point, roll, crush—in-process while the wheel is in contact with a part, and postprocess, said ANCA’s Riddiford. Grinding wheel makers are developing units that minimize the number of dressings required and work longer and harder.
Aluminum-oxide wheels are the more conventional and relatively less-expensive option, generally used for steel and flexible in their ability to be dressed with various profiles and shapes.
“The makeup of those wheels in terms of their compounds and other chemical agents vary based on application,” Riddiford said. “You might use a larger grit wheel for a roughing application, then a finer grit wheel for finishing.”
Optimizing wheel life and output depends on a variety of factors, he noted: part run; listening to the wheel; examining parts coming off the machine; monitoring CNC controller data that tracks a wheel’s amperage draw, which increases as the wheel dulls; and using software that adjusts rpm to keep the wheel working within a given parameter range.
Grinding wheel suppliers like 3M and Saint-Gobain “will give you good information about a starting point in terms of speeds and feeds,” he said, guiding users on where their wheels work the best. “But then you put that grinding wheel on two different machines and it is likely to perform differently on both. It can depend on whether one machine is more rigid than another, for example.”
To facilitate wheel R&D, two of ANCA’s larger machines are in the US labs of key grinding wheel manufacturers, who develop and test their products with them.
To keep grinding wheels working with optimum effect, United Grinding is unveiling the 2.0 version of its WireDress technology for metal-bonded diamond or cubic boron nitride (CBN) units.
WireDress is an EDM method for removing bonding material from superabrasive wheels, explained Ueltschi, allowing sharper wheels with easier-to-produce profiles. The smaller, less-intrusive version being introduced fits right on Studer machines. The technology allows better control over grinding wheel shapes, profiles and accuracies as opposed to traditional methods.
“On a conventional wheel, we mainly use a single-point dresser,” Ueltschi explained. “If it is a plated wheel, you cannot dress it—you cannot influence the shape or the geometry. If it is a vitrified-bond wheel, you can use a rotary disk-type dresser and dress the shape, but you are limited in what type of profiles you have—typically a straight shape or basic profile. If it’s a resin bond diamond wheel, you are reduced to truing the wheel with a silicon carbide grinding wheel.”
However, with the WireDress electrical discharge method, wire-generated sparks remove bonding between the diamond particles of a metal-bonded wheel to expose the diamond or CBN grit and hone the wheel’s grinding ability. “It clears the bond without pressure or deflection,” all with simple CNC programming.
It’s particularly useful for generating a very fine-pitch thread form on the wheel to produce a thread on a carbide workpiece, he added.
Keep Things Cool
Historically, said ANCA’s Riddiford, water-based coolants have been favored for OD grinding, but oil is coming to the fore for some users, being better for the wheels, surface finish and the machine overall.
“Oil now has a very high flash rate for fires,” he explained. “It’s much safer to use oil as your main part coolant, especially for peel grinding, which requires good lubricity.”
The importance of coolant goes far beyond just keeping the part cool.
“The position of the coolant is critical in all grinding operations, especially OD,” he said. “The nozzles that project coolant onto the wheel need to be in the right place” to allow cooling at the point of contact between the wheel and part. Proper coolant flow is required to remove the resultant particles, or swarf.
Different materials produce different swarf, he added. For instance, OD grinding stainless steels produces stringy swarf, whereas a heat-treated blank will produce particles like fine sand.
While thicker synthetic or general cutting oils are available, the ideal oil is thin and low viscosity with high flash points, he advised. “A lot of the grinding wheels have been developed around oil-based coolant.” And, grinding oil makers like Oelheld and Blaser SwissLube are “constantly working on the chemical makeup of their coolants.”
Coolants are also application driven, he continued; carbide might require something different than stainless steel. Whereas a job shop might seek a coolant with average features for a range of grinding jobs, a production run dedicated to carbide alone would allow use of a higher-end coolant.
Oil is “absolutely” better for the grinding process, said Okuma’s Titus. “If you talk to a grinding wheel manufacturer about a superabrasive grinding wheel, he’s going to say, ‘I would love for this to be run in oil because it will perform much better and last a lot longer.’” That said, about 90% of the company’s customers prefer water as a coolant in light of the environmental impact of oil disposal. Also, the requisite fire-suppression equipment in the machines is an added cost.
Improved grinder design is helping reduce thermal displacement issues. Toyoda has changed its casting design to control heat fluctuations in the manufacturing atmosphere by isolating the casting from the coolant with a steel plate with an air gap, explained Farrant.
To evacuate coolant, Toyoda added a channel beneath the wheelhead to eliminate pooling around the wheelhead. “Being thermally stable helps reduce the number of offsets needed when operators return from a break or during the machine warm-up period,” Farrant said.
Software and Sensors
Calculating feeds and speeds, spark-out times and other process parameters to ensure desired accuracies and surface finishes requires grinding software to be more technology driven, advised United Grinding’s Ueltschi. A lack of skilled operators demands that software allow cylindrical grinders to calculate how to grind and how fast.
In addition to libraries of wheels and wheel profiles, United Grinding software offers a quick-set option that uses the machine’s probe system to measure parts without touching off each wheel to the workpiece and recalibrates the machine immediately. Setup time can be reduced up to 10% with one wheel and 90% with four wheels. And, the software can set feed and speed rates and grinding methods, resulting in a good part quickly without an operator taking half a day to dial-in the proper setup. A user could generate a part in 10 minutes instead of about 30 minutes, Ueltschi noted.
Another critical function software provides is wheel balancing, said ANCA’s Riddiford; his company offers a software suite that includes in-machine balancing to prevent vibration and harmonics. With its machines grinding parts anywhere from 20 thousandths up to 6″ in diameter, “we are constantly working on improving the software in terms of getting machines to grind quicker,” particularly by speeding setup.
Meanwhile, Industry 4.0 features appearing in all sectors of manufacturing are making their way to grinders, with sensors monitoring processes and component wear and predicting when machine parts must be repaired or replaced.
“We see more customers requiring some sort of automation system,” Ueltschi said. “In cylindrical grinding, with its short cycle times, you don’t typically talk about lights-out processing.” However, United Grinding’s flexLoad Automation Cell allows an operator to stock enough parts for the grinder to run 30 minutes to an hour untended.
Chevalier Machinery Inc. (Santa Fe Springs, CA) offers two proprietary grinding software solutions. The company’s conversational graphic program offers a “Q & A” setup style that asks the operator what the job requires, then creates the program. “Anyone without experience in engineering, programming or CNC operating can use it,” said Johnson Lan, vice president of sales. Chevalier also offers the iMachine Communication System, which uses its MT-LINKi to monitor performance data, prevent downtime and forecast production issues. “Remote monitoring of the factory will not only increase productivity but also produce systematically controlled outcomes,” Lan noted.
Chevalier also offers two spindle sensors, one for automatic balancing and another for the spindle loading meter to determine the wheel dressing cycle. “Customers can leverage it as an indicator to maintain the machine,” Lan said.
According to Titus, Okuma’s grinders have joined its turning and milling machines in incorporating a Windows-based open architecture OSP-P300G control that is “very suitable for Industry 4.0.” With that comes a conversational CNC programming package, “reducing the need for a dedicated programmer for the machine. The operator goes through a series of menus and fills in the blanks, and that will develop a CNC program based on those inputs,” he said.
And at Toyoda, monitoring factories as a whole as well as machines individually has been a focus for the past year or so, Farrant said. Many grinding-related issues can be identified before they occur by monitoring vibration, noise and temperatures of castings, coolant and lubricants.
Such monitoring is standard on Toyoda grinders in the form of the Toyopuc AAA module, which allows factorywide connectivity. The recently introduced Toyopuc-Touch control “helps facilitate the conversational aspects of the control with the operator,” Farrant said. From a maintenance perspective, the control allows faults to be easily traced in the ladder for quick diagnosis, for example down to a faulty wire, and the grinder’s manuals are available at the touch of a button on the control.
Ultimately, “we want to be able to hit cycle start and let that machine run as long as possible without any intervention.”
About the author: Geoff Giordano has been a contributing editor for SME since 2016 and a manufacturing and tech journalist since 2005. Contact him at firstname.lastname@example.org