Chand Kare Develops Grindability Test System


	Chand Kare Develops Grindability Test System Chand Kare Technical Ceramics (Worcester, Massachusetts) began working to develop a cost-effective Ceramic Grindability Test System (CGTS) in October 1993. A prototype of the test system has been developed, and extensive tests with various ceramics have been completed. The nearly finished computer-controlled, fully automatic CGTS uses controlled-force grinding with a diamond belt. Extensive measurements (using the CGTS) for grindability of various technical ceramics indicate that grindability is a unique property of ceramics that cannot be characterized by hardness or flexural strength alone. Ceramics of similar mechanical properties can have very different grindabilities, and ceramics of very different mechanical properties can have similar grindabilities. Therefore, in evaluating ceramics for their cost-effective application, grindability should be considered, as well as other mechanical properties such as hardness, flexural strength, and fracture toughness. Today, many ceramic manufacturers around the world supply ceramic materials of similar properties. However, these materials often reveal very different machining characteristics that require different machining procedures. To achieve overall cost-effectiveness, ceramics with superior properties and good machining characteristics are required. Furthermore, a scientific method is needed for quickly and cost-effectively choosing the appropriate process parameters for each ceramic. Grindability studies and testing become necessary to provide a fundamental understanding of the grinding behavior of ceramics. Mathematical Definition of Grindability Grindability can be considered in terms of productivity, cost, and quality. The cost-effective machining of ceramics requires that measures of grindability should be easy to obtain experimentally. Volumetric removal rate under controlled-force grinding is a cost-effective measure of ceramic grindability. Under controlled-force grinding, material-removal rate is a function of normal force F(n), wheel speed v(s), material properties (o)c, and wheel characteristics (o)d, or To find a parameter depending only on material properties, we define Experimental results indicate that proportional relationships exist between material-removal rate and both belt speed and normal force (Fig. 2); therefore, grindability G(c) can be defined as: The Test Machine (CGTS) A test machine employing controlled-force grinding with diamond belts has been developed (Fig. 1). Experiments reported here were conducted using the prototype of this machine. Basic features of Chand Kare's CGTS follow: Full computerization to eliminate operator influence. Diamond-belt grinding to eliminate wheel-dressing influence. Accommodation of cost-effective, standard MOR bars as test specimens. Computer-controlled force and speed adjustment for various ceramics. In-process automatic removal-rate gaging with analog and digital output. Grindability Measurements Extensive tests were conducted on 18 ceramics, as shown in Table 1 and Fig. 3. Grindability results for each material group were significantly different. The twelve Si3N4 materials were further compared. ANOVA (for analysis of variance) evaluations indicate that the twelve materials are significantly different. Statistical tests (T-tests) indicate that materials 6 and 7, 7 and 8, and 2 and 9 showed no significant difference. Actually, materials 6 and 7 are from two batches of the same material, and materials 2 and 3 are from two batches of another material. Mechanical Properties and Grindability No simple correlation was found between grindability and either Vickers hardness or flexural strength. Results in Fig. 4 indicate that some materials with the same properties have very different grindabilities (materials 2, 3, 6, and 7), and other materials of very different properties have very similar grindabilities (materials 7 and 8). This indicates that the development of ceramics with both superior mechanical properties and good grindability should be possible. Therefore--besides hardness, toughness, and flexural strength-grindability should be evaluated in ceramic material development to achieve the overall cost-effectiveness of ceramic components.
	Reprinted from Ceramic Technology Newsletter, Issue No. 47, Summer 1995.
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