Material Considerations in Optimization of Machine Tool Structure

The three most popular choices currently used for the main structural components of machine tools are steel weldments, metal (cast iron) castings and polymer composites. Among the three, polymer composite offers the highest vibration damping and the lowest thermal conductivity. All three approaches have been employed in the design of machine tools to meet the criteria for required rigidity, impact resistance and vibration damping. The final choice is also affected by additional factors including cost footprint (space) requirements and lead times. For most production applications of machine tool structures, (gray cast iron) metal castings remain the primary choice because of cost, ease of sourcing, good damping with relatively high strength, good machinability and well-established and consistently achievable manufacturing and processing requirements. However, fabrications are normally the preferred choice for low volume production of large structures, due mainly to the high up-front molding costs and the difficulties in process control inherent in very large castings. On the other hand, with increasing, emphasis on high speed machining, hard turning, and better and consistent machining accuracies, structural rigidity, thermal stability and vibration damping are becoming major design considerations making polymer composites a leading choice. For this reason, Hardinge Inc., a super precision machine tool builder has traditionally used its proprietary polymer composite (Harcrete®) in its lathe, grinder and machining center bases. Depending on the performance and cost requirements, the base can be all composite or a combination of conventional casting strategically reinforced with composite. With the current market forces and ever increasing competition in the industry, for most machines, value engineering has become a prominent factor. A major consideration is to identify the materials and designs that would provide the best performance of the machine while minimizing the cost. Therefore, new sets of evaluation criteria are necessary to arrive at designs with optimum cumulative impact on various technical, commercial and strategic requirements. This paper proposes such new criteria and examines their suitability based on testing and analyses of structural components in today’s demanding real-world machine tool applications.