Machining Technical Ceramics: The Cost Factor
Applications for technical ceramics are found in all walks of life; however, the semiconductor industry appears to lead in the emergence of new applications. Many other industries, such as the automotive industry and wear parts applications, continue to suffer from the higher cost of a ceramic component when priced against a similar component made from more conventional materials.
Cost of MachiningJust like non-ceramic materials, technical ceramics require machining into a specific component shape and size for application. The common belief, however, is that the components made from technical ceramics cost more than those made from non-ceramic materials. It is further believed that the high cost of ceramic components is directly associated with the cost of machining. To examine the cost difference of machining a component from a technical ceramic versus a hardened steel, we compared the machining time required to machine two simple shapes: a bushing and a 2 in. long rod made from hardened steel and technical ceramic bar stock, respectively.
Table 1 shows that the machining times for hardened steel components are similar to the machining times for ceramic components. In fact, the ceramic bushing can be machined in a shorter time. Further examination of the material removal rates in the grinding of hardened steel, cemented carbides and technical ceramics also showed a close similarity. In prototype quantity, machining costs for components machined from technical ceramics are competitive with those machined from non-ceramic materials.
In going from prototype to production quantity, technical ceramics demand greater innovation. This is due to the fact that at the current technology level, diamond wheel grinding is the only method for machining technical ceramics. One must be very cautious when projecting prototype machining costs to production runs, as direct application of prototype machining methods to production runs is not cost-effective. To ensure cost-effectiveness, innovative methods of applying diamond wheel grinding for machining production quantities are required.
Other FactorsCeramic components, whether in near-net shape blanks or bulk plate, bar, or tube stocks, have greater amounts of material to be machined than non-ceramic materials, which can be pre-machined to closer tolerances. The amount of material to be machined per component has a direct relationship to the cost of machining.
Another factor that influences the cost comes from the unavailability of any data on the machinability of technical ceramics. A machining engineer can often obtain property data sheets from individual manufacturers, but few, if any, address machinability of technical ceramics.
GrindabilityUnder the auspices of the U.S. Department of Energy (DOE), a Grindability Test System was developed to measure the machinability of technical ceramics.
A standardized grinding test on the Grindability Test machine was performed on 98 technical ceramic materials of various compositions from different manufacturers. Results clearly indicate that a technical ceramic material has a distinct machining characteristic, referred to as its “grindability.” Correlation studies have shown that each ceramic material, depending on how it is compounded and processed, establishes its own unique machinability related only to its microstructure.
Even in a family of similar chemical composition, such as silicon nitride, the difference in grindability is significant. The differences in composition and processing conditions are significant enough that they cause the materials to machine differently. Additionally, materials of the same composition from two different suppliers machine differently. Design engineers and fabricators are therefore often limited to a specific supplier. This generally leads to a non-competitive cost structure.
Machining Cost ControlsSeveral methods can be used to control costs when machining technical ceramics:
1. Allow the least amount of material for machining on near-net shape preforms.
2. Reduce the number of setups required by machining different batches at the same time in a production mode.
3. Use grindability data to optimize and standardize the grinding parameters.
Additionally, formal instruction in machining of technical ceramics is needed at the college and university level. Scientific understanding of the diamond wheel grinding process, along with a more detailed specification of the wheel correlated to the material’s grindability, is necessary for cost-effective machining.