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Ceramic grinding: the next generation.

If you're not grinding ceramic components today, you are likely to be in the not-too-distant future. From high-performance ceramics for heat engines, aerospace, electronics, and medical applications to "everyday" uses in scissors, baseball bats, and screwdrivers, the use of a variety of ceramics is increasing rapidly.

Ceramics are greatly affected by raw material and processing conditions, such as variations in raw material particle size. Pressing, firing, sintering, and infiltration control the density, hardness, and strength of the resulting ceramic component.

Another critical determinant of component performance is surface integrity. Machining of the component to final shape must be accomplished without introducing flaws that can cause failure of the component in service. Much of industry perceives the task of ceramic grinding as difficult, but the difficulties are often attributable to use of the wrong tools and/or wrong methods.

The present dilemma

Generally, it is the machine technology that is not suited to machining ceramics. Machines designed to machine metal parts cannot cope with the dynamics and radically different grinding forces of ceramics. The rigidity requirement for grinding machines to properly machine ceramics is extraordinary.

Grinding forces are generally measured in two modes: the normal force and the tangential force. The normal force is required to penetrate the surface of the material with the abrasive, and the tangential force is needed to remove the material. When machining metals, the ratio of the two forces can be between 2:1 and 5:1, with the normal force always the higher of the two. In grinding ceramics, the normal force can be as much as 40 to 50 times the tangential.

It was never the intent to remove a great deal of stock with a grinder, so they are generally light machines with stiffness ratings in the area of 75,000 to 150,000 lb/in. This means, for example, that the spindle of a grinding machine with a stiffness of 100,000 lb/in will deflect 0.0001" when subjected to a normal force of 10 lb on the grinding wheel. Surface grinding a ceramic with a depth of cut of 0.001" could easily exert 10 lb of force on the spindle. The deflection is therefore as much as 10% of the wheel depth of cut.

With such "flimsy" machines, the abrasive particles on the wheel periphery tend to attack the ceramic surface like a series of hammer blows, leaving it shattered and pitted. The spindle not only deflects, but may also chatter and bounce up and down as it grinds, seriously decreasing wheel life and probably cracking and edge chipping the workpiece. To minimize these problems, the depth of cut is often reduced along with the feedrate, resulting in inordinately long processing times.

Current technology

One technology available now is creep-feed grinding, which has been around since the late 1950s. The process is starting to find applications across industry, and particularly in machining of ceramics. Creep-feed grinding requires a very stiff machine, which generally has CNC control of at least four axes.

Creep-feed grinders take a large depth of cut using a grinding wheel with a slow feed, much like a milling process. The large depth of cut and the large area of contact between the wheel and workpiece tend to dampen process vibration. The stock removal capability of creep-feed grinding is phenomenal, and the resulting surface is superior to that obtained with any other grinding process.

The principles of creep-feed grinding--using a highly stiff machine with slow feedrates--have shown great benefit for machining of ceramics. Even the best creep-feed grinders, however, have stretched the ancient C-frame design to the limit. Industry is ready for the "next-generation" grinding machine.

The next generation

The next-generation grinding machine under development at Advanced Manufacturing Science and Technology, Cincinnati, OH, is a significant departure from the current norm. It can be configured as a coupled or de-coupled system, and provides for in-cycle loading and unloading of the workpiece without compromising wheel stability and rigidity.

The next-generation machine is being developed from a list of requirements based on research that has taken place over the last 25 years and the direction in which materials science is taking industry. These requirements include:

* Very high stiffness.

* Freedom from vibration.

* A mechanically and electronically stable movement system.

* Higher and more stable wheel speeds and proper wheel dressing/preparation systems.

* A fast, accurate method of wheel changing.

* A mechanism for coping with the large volumes of swarf and cutting fluid generated by high stock removal rates.

* Ability to load and unload workpieces out of cycle to maximize machine efficiency and productivity.

* Ability to manipulate the workpiece only, leaving the grinding wheel stationary.

Once identified, these requirements make a clean paper approach to designing a next-generation grinding machine quite simple.

Stiffness, stability, and ease of changing the grinding wheel quickly have been addressed in a bearing system registered under a US patent in 1987. Using a system of hydrostatic bearings and "squeeze-films," a grinding wheel is held in a doubly supported yoke which does not have to be dismantled to change the wheel. The result is an extremely stiff wheelhead, which sacrifices none of its inherent rigidity to effect a wheel change. This bearing system has a calculated stiffness in excess of 6,000,000 lb/in, an order of magnitude more rigid than the best creep-feed grinding machines today.

Bearing in mind the volume of swarf and cutting fluid generated by rapid stock removal, it would be advantageous to surface grind in a vertical direction so that the swarf and fluid are blown down and away from the work fixture, keeping the area free from contamination. This is unlike the traditional surface grinding machine, where the swarf and grinding grit drop down onto the machine table and heavily contaminate workpiece, fixture, locators, and clamping mechanisms.

Rather than attempt to control the movement of the rotating body of the grinding wheel towards the workpiece, the wheelhead will remain stationary. It will be the function of a part manipulator to position the workpiece with respect to the grinding wheel. Depending on the type of fixture and control system, the part manipulator can perform surface, plunge form, contour, cylindrical, or cam grinding, all on the same machine. The next-generation grinding machine will truly be an answer to the problems inherent in machining of ceramics.

Dr Stuart C Salmon President Advanced Mfg Science & Technology Cincinnati, OH
COPYRIGHT 1993 Nelson Publishing
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Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Author:Salmon, Stuart C.
Publication:Tooling & Production
Date:Mar 1, 1993
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