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‘Pain-Free’ Precision with Centerless Grinding

Alan Rooks
By Alan Rooks Editor in Chief, Manufacturing Engineering
TGS-2-centerless-grinder-199x300.jpg
Once a centerless grinder is set up for a specific part,
the operator needs to make only minimal adjustments
to maintain the OD size.

Centerless grinders have been making precision parts for more than 115 years, and many of the machines are so durable that 40- and 50-year-old machines are still grinding away in shops across the US. Older machines, with strong, cast-iron frames and beds, are often rebuilt with new features.

Yet turnkey automated centerless technology is producing higher-precision parts faster and with less operator input. High-precision spindles, granite and composite bases, linear guideways, and automatic wheel balancing systems are reducing vibration and improving the grinding process—key features when part tolerances are within millionths of an inch.

Likewise, offline gaging systems help develop better parts and online gaging systems enable real-time process control. And growing use of load-and-unload conveyors, robots, and gantry loaders has reduced the need for operator intervention, enabling lights-out, 24/7 operation—a key issue when some through-feed applications are producing multiple parts per second.

Tight Tolerances, Mass Quantities

“Centerless grinding produces a cylindrical form to tight tolerances on a wide variety of parts,” said Phil Plainte, senior applications engineer, Norton | Saint-Gobain Abrasives (Worcester, MA). “It can make parts ranging in size from miniature stainless pins for fuel-injection systems to railroad freight-car axles.” It is also versatile, capable of producing mass quantities of automotive shock absorber shafts to short runs of multi-diameter shafts for a wide range of industries and applications, he said.

Through-feed (continuous machining of straight parts) and plunge (where the workpiece is static in the axial direction and parts with complex geometric shapes are producedare the two basic centerless grinding processes. “Unlike cylindrical grinding, centerless grinding requires no centers to hold the part, which enables highly productive rough or finish grinding,” said Plainte.

He noted that Cincinnati Milacron is the “grandfather” of centerless technology, having purchased the patents for the process in 1924. “While Cincinnati has not produced centerless machines in many years, their machines are still prevalent,” he said. “Rebuilders are repurposing these machines with advanced technology. And developments in new abrasive grains and bonds allow for higher metal-removal rates, better surface finishes and improved part quality—at a lower cost per part.”

Machine Improvements

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A centerless grinding operation.

While rebuilding remains popular, new machines offer even more improvements to spindles, bases and guideways that allow machines to grind faster and to tighter tolerances.

“Our older, legacy products were built on weldment frames with cast-iron beds, which were more than adequate for the tolerances at the time,” said Robert Gleason, vice president of engineering for Glebar Co. (Ramsey, NJ). “As our customers pushed the tolerance envelope, we developed a line built on a granite machine bed, which gives you much better thermal stability, flatness and rigidity. Now, for the highest precision parts, we use a 6000-lb composite mineral cast base that is extremely rigid and has amazing thermal properties and excellent vibration-dampening characteristics.”

Ralf Schürl, area sales manager for Schaudt Mikrosa (Leipzig, Germany), a company of United Grinding (Bern, Switzerland and Miamisburg, OH), agreed that composite beds help deliver the highest possible precision. “Our sister company, Studer, developed Granitan, a composite base that we use in our centerless grinders as well,” he said. Granitan consists of natural hard stone of various sizes mixed with a two-part bonding agent and hardened. “It dampens vibration six times faster than cast-iron machines. And when ambient temperatures in the shop change, the Granitan base follows with size changes much slower than a comparable cast-iron machine base,” said Schürl.

At Republic Lagun Machine Tool Co. (Harbor City, CA), the centerless grinder structure is Meehanite cast iron, heat treated prior to machining slideways, which are high-frequency hardened and precision ground for maximum material stability, deformation-free performance and outstanding wear resistance, according to Jim Garvey, Western states regional manager for Lagun.

Another way to control vibration is a CNC program from Schaudt Mikrosa called Heureka. “If, for example, your reg wheel and your grinding wheel are in sync, those harmonics could produce vibration that can cause out-of-round parts.” said Schürl. “Heureka calculates the likelihood of harmonic vibration in specified speed zones and warns the operator to move out of those unstable zones by adjusting wheel speed or workpiece height-above-center.”

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A fully automated GT-610 Zero Defect automotive centerless grinding cell from Glebar.

In another front in the war on vibration, spindles have been redesigned. For example, United Grinding designs spindles using different materials and thicknesses in order to dampen the frequencies the spindle emits. At Lagun, grinding wheel spindles and regulating wheel spindles are precision machined from nickel chromolybedenum alloy steel (SNCM 220). The inside of the spindle is hardened to HRC 25–30; the spindles’ outer surface is hardened to HRC 62.

Another aspect of spindle design—bearings—also needed improvement, according to Joe Giacalone, co-founder of Total Grinding Solutions (TGS; Warren, MI). “Plain, pneumatic bearings riding on a film of oil is still a good design. However, we decided to get rid of all the hydraulics, and now our spindles all use angular-contact, spherical roller bearings. With this precision, we can hold 0.2 µm roundness when grinding fasteners or water pump shafts, for example.”

According to Giacalone, pneumatic bearings can become a maintenance “nightmare,” requiring frequent oil and filter changes and hydraulic tank cleanouts. “If some of the maintenance falls by the wayside, dirt and grinding swarf can end up in the hydraulic tanks and go right into the spindles.”

The Move to Linear

Linear-motion bearings, or linear slides, which provide free motion in one direction, are changing the way centerless grinders move. “The first centerless grinders were designed to hold tolerances of tenths of an inch, then thousandths of an inch,” said Giacalone. “Today we are holding millionths on a steady basis, so design had to evolve.” He noted that TGS uses Cross-Roller linear-motion bearings on all of its slide assembles, which, coupled with new sensor technology, allows machines to compensate to 0.1 µm on a regular basis. “That’s because I no longer have metal-to-metal contact as in old dovetail guideway designs, which produced slip/stick.”

TGS’ dressing wheels are also on linear rails to enhance precision, according to Giacalone. “Our machines are used to grind aerospace fasteners, which have tight tolerances on the radiuses. To create those radiuses, we dress them into the wheel,” he said. “If my dresser is not accurate, the radius may be incorrect since it goes right into the part. As a result, all our dressers are full CNC and have linear rails and precision ball screws with zero backlash on both axes.”

Glebar’s Gleason agreed that linear guideways are a major improvement. “With dovetail ways, when the ram is stationary and you command it to move with a ballscrew, there is always a bit of hesitation [stick/slip] due to the friction between the slides,” said Gleason. “Roller guideways minimize that friction. We also use 0.1 µm glass scales to close the loop on positioning.”

CNC control is also enabling innovation in centerless grinding. For example, to create a straight-line passage for some parts through the grinding zone in a through-feed unit, or to come to a dead stop in plunge applications, an “hourglass” shape needs to be dressed into the regulating wheel, which is tilted during operation.

“Using a manual dresser involves a lot of trial and error,” said Schürl. “Instead, with our CNC control we measure the angle of the reg wheel and the height of the workpiece above center. With these parameters, we calculate the path the dresser must use to create the perfect hourglass shape, which reduces setup time dramatically.”

Automation Nation

To reduce cycle time and need for skilled labor, more manufacturers are automating their centerless grinders with load and unload conveyors, robots and gantry loaders. With the speed at which modern centerless grinders operate, automation is often a necessity, not an option.

For through-feed operations, bowl feeders are used for small to medium-sized parts, and for long, heavy parts, step feeders are typically employed, said Schürl of Schaudt Mikrosa. After grinding, robots such as flex pickers can unload parts from the outgoing conveyor with a cycle time of less than one second and place them in trays or blisters.

For plunge grinding, a typical automation system involves an operator loading a shuttle system. A gantry (often a double gantry) integrated with the grinding machine picks workpieces from the shuttle and loads the machine—often four to six parts at a time. Health and safety requirements on new machines call for closed housings to avoid contaminating the workplace with aerosols. That limits operator access because stopping and starting the wheel when changing a workpiece is not an option.

“For an additional few thousand dollars, it is much more efficient to use a full CNC machine with an internal gantry than a hand-actuated loading system,” said Schürl. “You get all the benefits of the CNC. You can teach it loading positions, recall them when needed as they are stored in the program, and changeover is faster than when you have to turn wrenches to adjust a manual system.”

Gleason agreed, adding that Glebar often recommends double-gantry systems, which position parts in the grinder at the same time they position parts in the gages. “For example, we recently developed a turnkey package to grind a thin-wall tool steel cup. The workpiece is measured prior to grinding, ground, re-measured and inspected after grinding. That automation worked perfectly; the challenge was the grinding wheel. The tool steel cup was very springy, so without the right grinding wheel, we would have deformed the material. We found a wheel to cut the product cleanly and provide the required surface finish. As a result, our customer is making millions of parts per year at a rate of two parts per second.”

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The KRONOS S 125 centerless grinder from Mikrosa (United Grinding) offers high-precision grinding for small workpieces. It has a working range of 0.5–30 mm, plunge length up to 120 mm, a Granitan composite machine bed, and cross-slide systems for grinding and regulating wheels.

TGS sells many of its centerless grinders to auto part producers, and automation is key in that industry. “Our grinders produce balance shafts using a FANUC robotic cell to load and unload multiple machines producing six million parts per year,” said Giacalone. “They are running 24/7 with a cycle time of 21 seconds.

“We also do a lot of camshafts, and for them we lean more toward gantry systems,” he continued. “One camshaft has seven different diameters and lobes. I have a twin-gantry loader picking up out of my load conveyor, dropping one part off the machine and at the same time unloading the part from the machine.”

Lagun offers the choice of multiple controls. “For example, we offer a FANUC CNC with six-axis capabilities, said Garvey. “We also offer auto load /unload for fully automatic through-feed or infeed applications. Through-feeds use a conveyor mounted on a moveable, mounted stand, while infeeds use a robot arm to place parts into the grinder while another arm off loads the finished part. Our machines offer automatic unloading equipment for through-feed grinding.”

More customers are specifying automation when they buy new centerless grinders, according to Giacalone. “Automation on our machines has tripled over the last few years,” he said. “Twenty years ago, people would manually load parts, stick their hands between the grinding wheels and grab them out. You can’t do that today.”

Gaging the Future

Online and offline gaging systems are another key area of improvement. While tactile and air gaging still predominate, laser gaging is making inroads. Nearly all gaging of centerless grinding takes place post-process because it is difficult or impossible to position a gaging system in the work zone. Automated post-process gaging, however, can provide online feedback control to the grinder, making size adjustments when needed.

“Tactile gages are flexible and can measure a wide range of parts,” said Schürl. “And air gages, which surround the part, are also useful. However, air gages can only bridge a maximum gap of ±40 µm, so if part size changes the gage ring must be replaced. Automatic feedback control allows post-process gaging to develop the process over time; if, for example, ambient temperature rises, the gages instruct the CNC to compensate for temperature growth, or for diamond wear on the wheel.”

Glebar uses both laser and contact gaging: in-line, continuous-flow laser measurement for through-feed, and contact gages for plunge. “Contact gages are typically accurate to 0.1 µm, and laser is even better than that,” said Gleason.

He added that Glebar developed a patent-pending process that measures more than part diameter; the process feeds back the entire geometric profile of a part to the machine, automatically correcting the wheel dress profile. “In typical centerless grinding, only the diameters are measured,” Gleason said. “But that doesn’t take into account the actual shape of the part, which could include tapers. The P4K, an offline Glebar metrology system, feeds back part geometry to grinder, then the grinder corrects the wheel dress shape for the actual part profile. It’s faster and more accurate than a more traditional way of doing things — you’re trading setup time for production time.”

One of the issues with laser gaging is that the part must be cleaned of coolant and debris prior to measurement. However, TGS uses an in-process automatic blow-off, such as an air knife or an air tunnel, to remove coolant from the part so it can be laser gaged. “You can also set filters on a laser gage to ignore some of the impurities,” said Giacalone. “I would say 80 percent of our machines have some kinds of in process or post process gages. Lagun’s centerless grinders also use in-line laser measurement systems (with part cleaning systems) to ensure tolerances. Part size determines the correct system to use, according to Garvey.

Wheeling and Dealing

With companies grinding different materials, such as heat-resistant superalloys, the selection of grinding wheels is changing.

“As new and repurposed centerless machines are put into demanding markets and applications — such as automotive, aerospace, medical, and oil and gas — materials provide significant grinding challenges,” said Norton | Saint-Gobain’s Plainte. In these cases, abrasives provide the muscle behind the machine.”

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The RL-CL-1810E centerless grinder from Republic Lagun.

He noted that Norton Abrasives provides a full complement of products to handle these challenging materials. Products include conventional abrasive (aluminum oxide, ceramic) and superabrasive (diamond and CBN) with complementary bond systems. Typically, resin is used for through feed and vitrified products are used for plunge, according to Plainte. (Editor’s Note: for more information from Phil Plainte on selecting and using coolant for centerless grinding and the dressing of grinding and regulating wheels, visit www.advancedmanufacturing.org and search for “Tips for Trouble-Free Centerless Grinding.”)

Giacalone of TGS noted that every grinding wheel is specific to a family of parts, and that is where a lot of wheel selection mistakes can be made. “Some job shops buy one wheel to grind different types of materials,” he said. “They can’t understand why they ground a great steel part to tight size, but when they switched to aluminum they can’t hold size at all. You don’t see that with large companies, such as Timken, or GM, because they are mainly grinding a certain process and a certain material.”

Balancing grinding wheels is also critical to successful centerless grinding. To increase throughput and improve part profiles, automatic balancing systems, once an option, have become standard equipment for dressing and grinding operations on many centerless grinders.

“On a full CNC centerless machine, dressing the wheel is a piece of cake,” said Schürl. “All parameters are fixed; the operator presses a button, the wheel is dressed and compensated, and grinding resumes immediately. Manual dressing takes much longer, and after it is completed the operator has to zero the wheel in again. As a result, many operators put off dressing, leading to excessive wear on the grinding wheel. Also, with manual dressing the operator doesn’t really know how much of the wheel he is removing. With automatic dressing, you can dress after a certain number of parts and know exactly how much of the wheel you are taking away.” In Schaudt Mikrosa’s system, the sounds that wheel dressing makes are made visible on the machine display via acoustic sensors and help the operator determine if the wheel is fully dressed.

Giacalone agreed that automatic balancing systems are a must. “All of our machines get automatic balancers; I can’t remember the last time we built a machine without one. It’s the icing on the cake; if you have a great machine and the process is right, automatic balancing helps you trim off that last 0.25 µm. Also, if you’re having trouble balancing with your automatic system, say, five years down the road, it’s a good indicator that you probably have a spindle issue, not a balancing issue.”

Today, centerless grinding is an interesting mix of old and new, with some shops running decades-old manual machines and successfully making parts, while others use new, largely automated machines. But as tolerance demands grow and part volumes increase, the balance appears to be tipping toward modern centerless grinding.

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