THERMAL-EXPANSION OF THE WORKPIECE IN TURNING

Thermal expansion of the part can be a significant source of dimension al and form errors in precision machining operations. This paper descr ibes a method for calculating the thermal expansion of an axisymmetric workpiece. The analysis is based on a commercially available boundary element code modified to properly represent concentrated moving heat sources such as those produced in machining. The inputs required are t he amount of hear entering the parr from the cutting zone and the ther mal properties of the workpiece material. Calculations are compared wi th direct measurements of expansion from tests on large diameter 2024 aluminum sleeves. The agreement between calculated and measured values is generally reasonable, although calculated expansions are consisten tly smaller than measured expansions. This error is probably due to er rors in estimating the heat input to the part, and particularly the ne glect of flank friction in heat input calculations. Sample calculation s for hard turning of a wheel spindle show that expansions can approac h tolerances on critical surfaces. Based on sample calculations therma l expansion is likely to be significant when hard turning parts with t olerances on the order of 0.01 mm. For these applications, critical su rfaces should be machined first, before cuts on other sections heat th e part, and gaging should be carried our only after the part has coole d.