Taking Advantage Of Superabrasives

Using the cutting capabilities of a superabrasive grinding wheel requires a systematic processing approach. It takes more to get good process results than just slapping a superabrasive grinding wheel on a machine tool.

Article From: 6/1/1997 Modern Machine Shop, ,

Metalworking has two major technology incubators—cutting tools and machine tools. Each is integral to the operation of the other yet both are often approached independently. Historically they have leap-frogged each other in terms of technological advancement.

An advance in one industry spurs a development project in the other. Happily it's the cutting tool and machine tool users who benefit from this developmental leapfrogging.

Until recently, superabrasive grinding wheel technology has had the capability lead over most machine tools. In centerless grinding, for example, superabrasive wheels with a cutting capability of 10,000 sfm and more have been largely underutilized because most of the older machine tools on which the wheels are applied cannot take full advantage of superabrasive cutting capability.

It would appear that is changing. Some machine tool builders have upped the ante for superabrasive grinding by developing machines whose capabilities not only meet performance specifications of current wheel design, but exceed them. Superabrasive grinding, specifically diamond, is currently being swept into the stream of "match this if you can."

To find out about how this contest is shaping up, we talked Al Maietta, of Universal-Fantastic Company (Livingston, New Jersey), about superabrasive diamond centerless wheels. And we talked to Kevin Bevan at Cincinnati Machine (Cincinnati, Ohio), about their new centerless grinding machine.

The Wheel

Superabrasive grinding wheels are built like any grinding wheel. The major components are grit and binder. How these two ingredients are selected and blended determines the performance characteristics of the grinding wheel.

One customer of Universal-Fantastic uses an 8-inch-wide wheel on their new CNC grinder to through-feed some small tungsten-carbide parts. "It has a modified phenolic bond with lubricants included," says Mr. Maietta. "We devised this formula especially for high speed grinding [up to 10,000 sfm]."

Universal-Fantastic is working to develop some new wheel and grit combinations to help this customer take better advantage of their new machine. Currently the wheel they've built uses 120-grit, synthesized diamond from GE Superabrasives, Worthington, Ohio. "We treated the diamond to increase its friability," says Mr. Maietta.

At higher grinding speeds, there is a tendency for the grinding wheel to become harder acting. Harder acting means that the rate at which the wheel exposes new superabrasive grits is too slow. The binder is not releasing the diamond soon enough and it is being allowed to dull. This creates excess heat. "If you run the same wheel at 8500 sfm, change nothing, and bump the speed to 9500 sfm, that same wheel will act harder at 9500," says Mr. Maietta.

Wheel manufacturers must modify the binder-to-grit ratio to accommodate these changes. Ideally, the diamond grit should break away from the binder just as it becomes dull, exposing a fresh cutter to the work. If the grit stays too long, the workpiece can be burned by too much heat. If the grits break away prematurely, wheel life suffers.

Freer cutting action is the goal for the specially formulated wheels that Universal-Fantastic has developed. To avoid losing size, the grinding wheel modifications have produced a softer acting wheel for the higher speed machines. "With only occasional dressing, the diamond wheel should last up to a year in normal production," says Mr. Maietta.

Cutting Carbide

Tungsten carbide material is an ideal application for superabrasive diamond. Similar to the binder and grit that make up a grinding wheel, carbide is a mixture of cobalt (which acts as binder) and carbide grains.

The carbide's hardness is created by manipulating the cobalt-to-carbide ratio along with other processing variables in the pressing and sintering process.

"Pure carbide is tailor-made to be cut with diamond," says Mr. Maietta. "It cuts freely with little or no loading of the wheel. Cobalt is necessary to keep the carbide grains together, but it increases the degree of grinding difficulty. A higher cobalt content produces a softer workpiece, which is harder to grind. More cobalt makes the part grind more like steel than carbide."

Raising The Bar

Many shops believe they can take their good old grinder, slap on a superabrasive wheel, either CBN (cubic boron nitride) or diamond and reap immediate double-figure production increases. Adding superabrasives is only one part of the equation. A machine tool that has been designed to deliver sufficient stiffness to take advantage of superabrasive cutting is the other critical factor.

Until recently, superabrasive wheel makers like Universal-Fantastic and others had a product that was generally more capable than the machine tools on which it was fitted. The new Cincinnati Machine Viking centerless grinding machines are rated at 18,000 sfm—effectively raising the bar on superabrasive grinding wheel performance.

The machine was designed in a cooperative venture between Cincinnati Machine, the National Center for Manufacturing Sciences (NCMS), and with input from several of the company's customers. The result is a centerless grinding machine with a static stiffness of more than 3 million pounds. "This very high stiffness makes the machine capable of the impressive wheel speeds and in many cases enables shops to make fewer grinding passes to finish the part," says Mr. Bevan.

It's a full CNC machine with an open control system for DNC or networking. Workpiece range for the machine is 0.05 to 2.5 inches in diameter. The rigidity of the machine's design makes it capable of cutting beyond the range of superabrasive wheels currently available.

"The question becomes, `What is the optimum superabrasive cutting speed?'," says Mr. Maietta. "At some point, spinning the wheel faster doesn't improve its ability to cut. We just don't know what that optimum is. It might be 13,000 sfm or 23,000 sfm."

Now, however, the challenge is made. "It's the superabrasive wheel makers' turn to jump over the bar that we [Cincinnati Machine] and other high speed grinding machine tool makers have set," says Mr. Bevan. Ultimately, the grinding shop is the beneficiary of this technological leap-frogging.

It's A Movement

Successful shops in any industry have a single mantra that drives their business decisions—keep the customer happy. Even in a captive shop, continuous efforts to reduce costs and improve quality and deliveries are expected.

There are many things a shop can do to incrementally improve the response to customer demands. Eventually, when many of the easier solutions are in place, a shop has to bite the technology bullet.

In superabrasive centerless grinding, this means investing in new grinder technology. Time is relative for many manufacturing operations. It doesn't matter at what rate a given technology changes. What's key is to make a decision when it's right for the business and to choose technology with a future that corresponds with the plans for the business.

A Case In Point

Shops are looking at ways to improve efficiency. Centerless grinder technology has moved beyond the 6500 sfm cutting capability of the machines currently used by many carbide grinding shops.

In one application, a shop grinding carbide slugs (89 Rc hardness) required an average of just over five passes for every piece produced. The older machines the shop had just didn't have the stiffness to cut any more aggressively.

"Superabrasives are all you want to consider for centerless grinding of these parts," says Mr. Maietta. "It could be done with conventional abrasives but the job would become so inefficient, from a dollars and cents point of view, that anything short of superabrasive is just impractical."

Last year, this shop purchased its first new centerless grinder machines in many years. It's a 40-hp, 8-inch Viking machine built by Cincinnati Milacron.

The impact on their process was immediate. The starting point for process development was 8500 sfm at 2,000 rpm, about a 30 percent increase.

Part production also saw a big improvement. Stiffness of the new machine allowed the diamond wheel to perform closer to its 10,000 sfm recommended maximum. The number of passes per part is being reduced from an average of more than five to an average close to three passes. That's a 40 percent reduction. With the grinder's 18,000 sfm potential, a total of two passes—one rough and one finish—may be attainable, which will increase production even more.

To get the slugs to size, two rough passes and one finish pass are programmed into the grinder. One machine is used for both rough and finish—an advantage of CNC.

For example, a half-inch diameter slug can run oversize from 0.519 to 0.522 inch. In spite of these variances, the first rough pass is set to take 0.012 inch. It may actually take 0.014 inch on some parts and only 0.010 inch on others.

The second pass then takes the work down to 0.001 inch above size. That leaves a finish cut of 0.001 inch. Surface finish is 8 to 12 Ra for the final grind.

Through-feed angle on the old machines was 3 degrees with the regulating wheel set at 40 rpm. Because the new machine and superabrasive wheel could cut more aggressively, the through-feed angle was changed to 2 degrees and the regulating wheel bumped up to between 55 and 60 rpm.

However, the lower feed angles put more pressure on the grinding wheel. Because they were grinding faster, the wheel acted harder. To prevent the parts from burning, the shop had to back off to 3 degrees and order a softer wheel. When it comes in, they expect to be successful with the 2-degree feed angle.

The grinding process is a system. Each variable is interdependent with the others. Taking one variable at a time allows a shop to slowly eke out a little more production. It's a delicate balance and should be approached incrementally. Getting a new process as stable as the process it replaced takes time but the important gains (for example, a 40 percent increase in throughput) are definitely worth the effort.

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