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Overcoming ceramic-machining phobia.

Fear of failure. That's what seems to be holding people back from experimenting with ceramic machining. It's common knowledge that ceramics are a tough grind, a process that inevitably takes a long time, weakens the part, and creates a lot of scrap. To even attempt it, you must be driven by a clear and present benefit that ceramic parts could bring to your product line.

The Norton Ceramic Machining Technology Center, Worcester, MA, is a small pioneering group of practical engineers dedicated to dispelling the myth that ceramics are too difficult to machine, or that in so doing, you weaken the part. To demonstrate their progress, they have created ceramic springs and other odd artifacts to demonstrate the outer limits of ceramic machining. They plan to grind a simple ceramic nut and a bolt to show how strong that simple fastener can be made that its strength is undiminished by their grinding process.

They have been doing this kind of thing in ceramics for four years now (even longer in electronics applications), led by Dr K (Subbu) Subramanian. "We had to learn an awful lot about operational factors," he says "grinding cycle, part design, and process parameters, and how they influence each other. We had to go even further into the scientific fundamentals on how the grinding of ceramics differs from our hundred-year experience in grinding of metals."

Too often, the bottom line is the need for a high-tech grinder, a quarter-million dollar machine. "In situations where the production process is already in place, and the commitment defined, we can show a clear economic justification for such a machine. But, not for those who have yet to select a ceramic material to use or evolve a process around it. If they have yet to machine even a few good quality ceramic parts, let alone production quantities, and I say to them, 'If you buy a quarter-million-dollar machine, your problems will be solved,' they throw me out, pronto ! "So we had to overcome this barrier. We feel ceramic applications are going to grow. For example, if you're making punches and dies, and you want to improve their quality and the productivity of the punching process, you will need to evolve to ceramics."

Not just aerospace

It's not just the far-out ceramic projects you've read about-turbine rotors or engine blocks-his group is excited about. "Basic engineering components of relatively complex geometry can, by radical substitution of ceramic for metal, achieve enormous differences in productivity. We are trying to change the perception that ceramics are only for automotive parts or bearings. Think of carbide. People turn to it as an alternative to steel. All of those applications are potential candidates for ceramics." Thus, the opportunity: a broad industrial base in need of this technology. And the problem: most do not have the experience or machine tools to be successful in ceramic production. "We felt a new level of systems integration was critical if ceramics is ever going to take off. So our objective in creating this center last year was to develop a partnership between ceramics users and machine-tool builders. We are bringing machine tools here on a floating basis so that we will always have the latest machine technology to combine with our experience in ceramic grinding-the manufacturing process, the machining variables, and the knowledge of real-life manufacturing in the wide variety of industries that Norton serves. We help our customers study prototype grinding, move into pre-production grinding, and when they are ready, go into full production." They are also helping the donor grinder builders build better machines.

Build partnerships, build parts They have no intention of seeking any Federal assistance. "Although there is an interest on the part of the government to help," he explains, "we wanted to keep this on a business level-build business partnerships-rather than an R&D level where you publish papers, give seminars, but nothing concrete ever comes of it-nobody makes a single ceramic part ! "All our projects are on a contract basis. We feel anyone who wants to do something like this should have a buy-in into the process. For example, small job shops. They can't afford to buy the equipment, but have a chance to bid on machining a few prototype components. So, they can come to us, we will grind the initial parts for them, and once they get enough volume, they can then go out, buy their own grinding machine, and go into full production. That is our key objective-helping as many people as possible get into ceramics machining."

Getting full part strength

Because of ceramic hardness, every grinding wheel cutting ceramics must be a diamond wheel. "Our problem," says Subbu, "was not deciding which wheel to use-that's clear-but how to grind ceramics without causing damage. The question people always ask is

'Can you get parts of good strength?' If ceramic grinding is always going to be associated with damage, then you might as well kiss it good-bye and go on to something else. Because of this fear, the feeling is that you must always minimize grinding.

"We have spent a lot of our research in trying to fully understand how ceramics can be ground to high strength and high surface finish, without compromising part quality. We looked at grit-size effects, and found that bending strength is independent of the direction of grinding (i.e., equal to the virgin strength of the material) when grain size approaches zero. The rougher grind creates surface cracks that precipitate breakage, and thus, the finer the finish, the less likely the part will break. Because of plastic deformation in metals, they are much less crack susceptible.

"Once we found out that, we said you don't have to be afraid of grinding ceramics any longer. We could demonstrate we could make the most complex parts possible even springs-with strengths comparable to virgin materials."

Cost factors

Ceramics are expensive materials to work with because, like powder-metal parts, they have two things going against them: Raw-material purity is critical to part properties, as is grain size and distribution. Both are expensive to control. Beyond that is the need to form the ceramic closely to its final size to reduce material waste and machining requirements. Unlike metals, there are no forging or rolling processes, or welding technologies.

With any near-net shape process, tooling is expensive, particularly when production volume is small as it usually is with ceramics. And here, the net-shape philosophy does not achieve its goal of reducing machining costs, Subbu feels. "We have demonstrated many times the ability to take simpler shapes and grind them to final shape at relatively lower cost and higher structural reliability. With near-net shape, your losses due to differential shrinkage and breakage exceed the material-savings gain. Out of ten parts, you lose two, whereas with ten parts of a simple shape, you can machine ten good parts."

Concludes Subbu, "Don't bother with near-net shape and all its complexity. Use a simple shape with reliable surfaces, and use the machining process to take off most of the stock."

For those experiencing ceramicgrinding costs as high as 40% of total part cost, Subbu has some good news. "The fascinating thing here is that when you ask why it is that high, you usually find it's because yield is only 20%. If that yield were instead 100%-all good parts-then grinding cost would be only 8%.

"Next, they explain that it requires eight setups, or 20 hours per part. If we can cut that down to 2 hrs for them, then machining costs come down to 1 %. Then, even with a machining cost of $150/hr, if you are adding value to that ceramic part in the grinding process, you can easily justify going into full production."

Plastic deformation

It's hard to picture plastic deformation in a ceramic. "The extent of plastic deformation is much smaller than for metals, but it's the degree of plastic deformation on the surface that we are looking at. If the preponderance of plastic deformation is in the production of the chip, this minimizes crack generation in the remaining material. We have identified the conditions to achieve this deformation, and once you know that, you can cut parts as complex as you want."

But there is a price to be paid: this requires a state-of-the-art grinding machine, one that is very stiff and has multiaxis CNC to make up in cycle time for the higher setup times required to rigidly support the part. It still takes at least three times longer to grind ceramic parts than metal parts.

Sometimes, it's necessary to calculate grinding stresses in the part to know where it must be supported, but Subbu's group has ground cantilevered surfaces without worry about breakage, or calculations.

But what about those without this confidence? "It's very hard to translate all of this experience and information into technical papers and seminars," Subbu admits. "That's why we say just give us your part, and we will grind it, give you the exact fixturing and process parameters, and then, you can go out and duplicate it."

By Eugene Sprow Special Projects Editor
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Author:Sprow, Eugene
Publication:Tooling & Production
Date:Dec 1, 1991
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