Creeping Up

Author Dennis Esford
Published
March 01,2000 - 11:00am

Imagine for a moment that a company asks you to machine a part from a very soft, gummy material—say 50 percent iron and 50 percent nickel. You’ve been trying to land this company’s business for years, and its purchasing agent is desperate. Oh, by the way, the customer needs 2,000 of the parts by next week.

How would you approach it? Could you approach it?

You could if your shop operated creep-feed grinding equipment. A CF grinder can hold extremely tight tolerances in difficult-to-machine materials. And it can do many jobs for pennies, or fractions of a penny, per piece.

Few people outside of the aerospace industry are familiar with the capabilities of CF grinding. The process, which has been around since the late 1950s, is one in which the grinding wheel makes a single pass at a heavy depth of cut—up to 1" or deeper, depending on the material—and at a slow feed rate.

CF grinding is superior to traditional machining in many ways. But the process has never really caught on, making it the Betamax of the machining industry. This could change, though, as demand grows for tighter-tolerance parts manufactured from hard-to-machine materials.

That, anyway, is what one job shop that specializes in CF grinding thinks.

Misunderstood Process
Abrasive Form Inc., Roselle, Ill., is one of the largest creep-feed grinding houses in the United States. It is described as a “leading light” in the industry by Dr. Stuart Salmon, widely regarded as the “father of continuous-dress CF grinding.”

When asked about the relative lack of interest in CF grinding, Abrasive Form Sales Engineer Leo Przybylowski said that people don’t fully understand the process or its capabilities.

“I would say 80 percent of prospective customers have no concept of what [CF grinding] is,” he said.

Przybylowski added that his job is more about teaching potential customers what the process can do than convincing them that Abrasive Form is the company that should do it. Even among its customers, there’s a lack of understanding as to what tasks CF grinding can accomplish. Most people consider grinding to be strictly a finishing operation and don’t realize that CF grinding has awesome roughing capabilities as well.

 


In one pass, a CF grinding wheel can cut a 1"-deep profile.

Abrasive Form President Ken Kummer said, “We often go back to a customer and tell them that not only can we grind the feature they [requested], but we can grind the slot, grind the chamfer, grind the contour and various other surfaces—all in one pass.”

Kummer said that because a CF grinding wheel is like a multifunctional tool, it dramatically reduces tooling costs. “It is pretty common for our grinding wheel costs (per piece) to be about a penny or two,” Kummer said. He added that one production job’s wheel cost was in the range of $0.0003 per part.

The reusability of CF wheels also helps keep costs down. Abrasive Form bought a 20" wheel for $200 and used it for a 500-part job. Afterwards, the shop dressed the wheel’s diameter down to 16" and used it for another job that has since run on and off for four years.

The per-piece cost isn’t always low, though. Kummer said that CF grinding parts made of high-nickel alloys and other exotic materials can run as high as $10 per piece. But, he said, those are materials that conventional CNC lathes and mills can’t machine without incurring high tooling costs per part.

“Some of our wheels might cost $1,000, but they’re worth it on a piece-part basis,” Kummer said.

Continuous wheel dressing lowers costs, too. Continuously sharpening the wheel as it grinds the part dramatically improves productivity and surface integrity and allows a CF grinder to hold extremely tight tolerances.

“We hold tenths in a routine production run,” said Kummer. “Broaching or milling machines cannot consistently operate in that tight a tolerance range.”

Materials Issue
The customers of machine shops will continue to demand tighter tolerances, according to Kummer, who also expects parts-engineering habits to change as the demand for stronger and lighter materials increases. Kummer said that oftentimes a material is chosen for a part not because it is the best material, but because of its machinability “when using the old mill-then-grind approach. I don’t see CNC mills machining carbide or ceramics yet.”

Kummer believes that the ability of a CF grinder to machine almost any material will result in cheaper parts being made from better materials.

“Manufacturers will have no choice but to turn to creep-feed grinding, because we can machine with diamonds and CBN, the two hardest materials known to man,” Kummer said.

He acknowledged that other processes utilize these materials in the form of inserts and coatings, but he said that CF grinding’s low per-piece cost gives it an advantage.

“People don’t realize that tool costs for CNC mills are very high compared with the per-piece cost of a grinding wheel, even in low volumes,” said Kummer.

Stuart Salmon agreed. Salmon, president of the consulting firm Advanced Manufacturing Science & Technology, Rossford, Ohio, added that the aerospace industry isn’t alone in wanting to make better products for less money. So do manufacturers of things like high-speed actuators. In order for these actuators to move rapidly, they need to be as light as possible. This has led to their being manufactured from ceramic instead of steel.

“[Engineers] are now saying, ‘Hey, ceramic doesn’t really cost us much more overall than a piece of steel, and it’s significantly better,” said Salmon.

An assortment of parts that Abrasive Form CF-grinds. The company says that per-piece costs for production runs are often pennies per part.

It seems likely that the growing use of ceramics, high-nickel alloys and other advanced materials will boost the use of CF grinding. But, let’s face it, these materials aren’t that widely used for commercial products.

So why consider using CF grinding for common materials?

One reason cited by Salmon is that CF grinding, despite its high material-removal rate, introduces very little stress into the part. He referred to CF grinding as a “gentle” process, despite the fact that it permits extreme DOCs. He said that the DOCs utilized do generate higher forces between the wheel and the workpiece, making a rigid machine design essential. Although the overall forces are higher than in conventional grinding, the force on each individual grain, in the arc of the cut, is reduced significantly. This, he said, provides excellent form holding and long wheel life.

Less stress on a part—any part—means fewer instances of scrap or failure in the field.

It Ain’t Cheap
Given some of the obvious benefits of creep-feed grinding, why, you might ask, isn’t everybody rushing out and buying CF grinders? One reason is cost.

According to Kummer, CF grinding machines cost anywhere from $300,000 to $1 million and can require an additional $200,000 worth of support equipment, including wheels, spare parts and software, and an experienced operator. He doesn’t advise that anyone try to start up a company with one or two machines and no experience.

Abrasive Form used the creep-feed grinding process to produce electronic clips that are part of a military circuit board (bottom right). The process begins with a 6'-long piece of flat stock—an alloy consisting of 50 percent nickel and 50 percent iron. This is CF-ground parallel and chopped into 1"-long blanks. Then parallel grooves are ground (top left). Next, the side opposite the grooves is ground and parted, creating the ‘goal post’ shape of the final part with tabs to hold the delicate pieces together (top right). Finally, a wire-EDM separates the tabs to complete the process. The radii and parallelism tolerances are so tight that just dropping a clip on the floor will require it to be scrapped.

Salmon attributed the lukewarm reception CF grinding receives to the fact that people in the manufacturing community don’t readily embrace change. He said that’s particularly true for those who have been successful with more traditional processes.

“Go into a company with a milling machine and tell them to forget everything they know about milling and adopt a grinding process and they’ll tell you, ‘No way! We know everything there is to know about milling. Why should we learn about grinding?’” said Salmon.

The fear of change may explain why U.S. manufacturers’ interest in CF grinding fell off quickly after the process was introduced here in the 1970s. At that time, European companies were machining aerospace parts with extremely tight tolerances and very low tool costs. Salmon said that some U.S. manufacturers experimented with the process on similar aerospace parts, but became intimidated by the amount of knowledge required to effectively CF-grind and the catastrophic results of any miscalculation.

“They burned some blades, so [creep-feed grinding] was seen as too risky,” said Salmon.

Another obstacle to acceptance is the long learning curve required to effectively and efficiently CF-grind parts. Kummer estimated that as many as 60 different variables have to be considered before starting a job, and that any miscalculation can drastically reduce the efficiency of the process.

The efficiency “doesn’t just drop from 90 percent to 80 or even 70,” he said. “It drops to 10 percent—almost failure—if a major variable is not accounted for correctly.”

Then there’s the matter of materials. Kummer said that every material is different, and that just because you’re successful machining one doesn’t mean you’ll be successful with another.

Kummer said that experience is the only way to gather the information that’s critical to successful CF grinding.

Even changing a wheel requires a great deal of technical knowledge. “It requires the ability to analyze and make adjustments to dressing parameters, feed rates, rpm of the dresser, rpm of the wheel, in-feed rate, dwell time, etc.,” he said. Kummer said that all the factors have to work in concert or the wheel’s productivity will be reduced.

Kummer said, too, that while there is a myriad of information available about conventional machining processes—technical manuals, classes, Web sites—there is virtually nothing available about CF grinding.

“Nobody teaches it. You go to [industry meetings] and they’ll teach you milling, they’ll teach you how to run a lathe, but they don’t have anything about creep-feed grinding,” Kummer said.

Hands-on experience is even harder to come by. Kummer estimated that there are only a dozen independent CF grinding houses in the U.S. (There are CF grinding departments in automobile plants and the facilities of large equipment manufacturers, but they often only work with a limited number of materials.) This can make it difficult to find trained employees.

Getting Into CF
Creep-feed grinding is a complex process, and one that leaves little room for error. But if someone is determined to get in on it, Kummer said it’s possible.

He recommended that a newcomer partner with a large CF grinding house (see accompanying article, page 40) and start out producing small parts that don’t require high horsepower or a large table.

He also suggested acquiring a high-speed grinding machine, because it can utilize wheels made of cubic boron nitride. Kummer considers CBN the CF wheel of the future.

The president of Abrasive Form said he’d consider setting up a satellite shop for one of his high-volume customers and training the staff to run it.

The thought of launching a possible competitor doesn’t worry him, Kummer said, because most customers like the sense of security a large shop like his offers.

Apparently, there’s room in the market for a couple more CF grinding houses. Considering that there are only a handful of these businesses in the country, the work piles up quickly.

Kummer said he expects to put about 110,000 machining hours on Abrasive Form’s 29 CF grinders this year. “We run 20 hours per day, six days per week and we’re completely booked.”

Maybe more people know about CF grinding than we thought. 

 

Lose an Employee, Gain a PartnerA few years ago, Abrasive Form had a problem. A good problem: too much work.

Ken Kummer, president of the creep-feed grinding shop, was receiving requests from new and existing customers for short-run jobs that cut into his high-volume production schedule. He knew that a good relationship with long-term customers was important, and he also knew that many of his new customers could eventually contract him to run high volumes of parts on a steady basis.

Kummer solved the problem by developing a partnership with an ex-employee who had launched his own job shop specializing in short-run CF grinding.

Former Abrasive Form employee Darrin Knuth owns Contour Tool Works Inc., Wheeling, Ill. He specializes in producing parts in lot sizes of 100 to 500 and handles many of Abrasive Form’s small-lot jobs. Since Knuth runs a small shop of just seven people, he can’t tie up his equipment for extended runs and still provide his customer base with timely deliveries. So when the overflow work from his former employer becomes too great, Knuth shifts the work back to Abrasive Form.

“This arrangement is a ‘win-win’ proposition for both companies,” said Knuth.

In the six years since he started Contour, Knuth has expanded from being solely dependent on work from Abrasive Form to developing a broad base of customers that represent the aerospace, automotive and electronics industries.

Knuth decided to stay in the relatively small world of CF grinding in order to avoid the crowded field—and shrinking profit margins—of traditional CNC job shops. “I knew I needed some sort of an edge, doing something my competitors couldn’t do,” said Knuth. “CF grinding was the edge I needed.”

Specializing in CF grinding means that Knuth is often the vendor of last resort—the guy who can machine what other shops can’t. While this usually means lower pricing pressures, it also means that critical customer deadlines must be consistently met in order to get future work. Knuth’s philosophy for meeting these deadlines is to hire and train operators who will grow to acquire a toolmaker’s level of skill. Knuth then helps these operators marry that skill to the unique processing experience required for CF grinding.

If yours is a traditional CNC job shop, how should you go about getting into CF grinding? Knuth sees only two ways. First, if you are lucky enough to have a good customer that comes to you with a part of sufficient volume to justify a single machine, then purchase a new CF grinder as a turnkey package for that specific part.

“The biggest hurdle is the complexity of the CF process,” said Knuth.

A turnkey package can cost from $300,000 to $750,000, but you’re not only buying the machine, you’re buying the builder’s applications expertise. The latter guarantees that the system produces good parts—or you don’t pay for it. A turnkey package is like an insurance policy against costly mistakes that could threaten the survival of your business.

But remember, warned Knuth, “being able to do one type of part is no guarantee that you will be able to successfully CF-grind other parts. Doing that will require that you fully understand the creep-feed process.”

The other approach, according to Knuth, is to hire a person with CF grinding experience who can help you buy a high-quality, used CF grinder for under $200,000. This person can generate your initial revenue by operating the grinder profitably while becoming a mentor to the other skilled CNC machinists in your shop.

Knuth mentored his first hire, Jeff Domain, a 10-year job shop veteran. Domain was sent to school for additional training to enhance his skills while learning the complex nuances of CF grinding from Knuth. Domain now mentors two other employees who were sifted from a group of 11 young people.

While this group included machinists with CNC experience, the two that went on to run the CF grinders had worked at an oil-changing franchise and a bottled water delivery company. But their solid mechanical aptitude and desire, coupled with mentoring and formal training, turned them into successful CF grinder operators.

With respect to operating a CF grinder, Knuth estimated that it takes about three years to go from button-pusher to setup person.

 

Dennis Esford

Related Glossary Terms

  • abrasive

    abrasive

    Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.

  • alloys

    alloys

    Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.

  • ceramics

    ceramics

    Cutting tool materials based on aluminum oxide and silicon nitride. Ceramic tools can withstand higher cutting speeds than cemented carbide tools when machining hardened steels, cast irons and high-temperature alloys.

  • computer numerical control ( CNC)

    computer numerical control ( CNC)

    Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.

  • creep-feed grinding

    creep-feed grinding

    Grinding operation in which the grinding wheel is slowly fed into the workpiece at sufficient depth of cut to accomplish in one pass what otherwise would require repeated passes. See grinding.

  • cubic boron nitride ( CBN)

    cubic boron nitride ( CBN)

    Crystal manufactured from boron nitride under high pressure and temperature. Used to cut hard-to-machine ferrous and nickel-base materials up to 70 HRC. Second hardest material after diamond. See superabrasive tools.

  • cubic boron nitride ( CBN)2

    cubic boron nitride ( CBN)

    Crystal manufactured from boron nitride under high pressure and temperature. Used to cut hard-to-machine ferrous and nickel-base materials up to 70 HRC. Second hardest material after diamond. See superabrasive tools.

  • dressing

    dressing

    Removal of undesirable materials from “loaded” grinding wheels using a single- or multi-point diamond or other tool. The process also exposes unused, sharp abrasive points. See loading; truing.

  • feed

    feed

    Rate of change of position of the tool as a whole, relative to the workpiece while cutting.

  • flat ( screw flat)

    flat ( screw flat)

    Flat surface machined into the shank of a cutting tool for enhanced holding of the tool.

  • gang cutting ( milling)

    gang cutting ( milling)

    Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.

  • grinding

    grinding

    Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.

  • grinding machine

    grinding machine

    Powers a grinding wheel or other abrasive tool for the purpose of removing metal and finishing workpieces to close tolerances. Provides smooth, square, parallel and accurate workpiece surfaces. When ultrasmooth surfaces and finishes on the order of microns are required, lapping and honing machines (precision grinders that run abrasives with extremely fine, uniform grits) are used. In its “finishing” role, the grinder is perhaps the most widely used machine tool. Various styles are available: bench and pedestal grinders for sharpening lathe bits and drills; surface grinders for producing square, parallel, smooth and accurate parts; cylindrical and centerless grinders; center-hole grinders; form grinders; facemill and endmill grinders; gear-cutting grinders; jig grinders; abrasive belt (backstand, swing-frame, belt-roll) grinders; tool and cutter grinders for sharpening and resharpening cutting tools; carbide grinders; hand-held die grinders; and abrasive cutoff saws.

  • grinding wheel

    grinding wheel

    Wheel formed from abrasive material mixed in a suitable matrix. Takes a variety of shapes but falls into two basic categories: one that cuts on its periphery, as in reciprocating grinding, and one that cuts on its side or face, as in tool and cutter grinding.

  • land

    land

    Part of the tool body that remains after the flutes are cut.

  • lathe

    lathe

    Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.

  • machinability

    machinability

    The relative ease of machining metals and alloys.

  • milling

    milling

    Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.

  • milling machine ( mill)

    milling machine ( mill)

    Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.

  • parallel

    parallel

    Strip or block of precision-ground stock used to elevate a workpiece, while keeping it parallel to the worktable, to prevent cutter/table contact.

  • tolerance

    tolerance

    Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.

  • web

    web

    On a rotating tool, the portion of the tool body that joins the lands. Web is thicker at the shank end, relative to the point end, providing maximum torsional strength.

Author

Dennis Esford is senior editor of Cutting Tool Engineering.