Cylindrical Machining Workpiece Temperature and Bore Cylindricity

Abstract

Cylindrical machining processes are widely used in industry to achieve better dimensional and geometrical tolerances and finer surface finish on cylindrical workpieces. Hard turning is utilized to machine hardened steels for large bearing rings and finish boring is used to machine cylinder bores during automotive engine block production. Workpiece temperature is critical for cylindrical machining processes. In hard turning, high machined surface temperature leads to the formation of white layer, reducing the workpiece fatigue life. In finish boring, thermal expansion due to workpiece temperature rise causes bore cylindricity errors, leading to engine performance issues. Besides thermal expansion, other factors like cutting force, spindle, and fixture/clamping also affect the bore cylindricity in finish boring. This dissertation studied the cylindrical machining workpiece temperature through both experiment and modelling and identified bore cylindricity error sources in finish boring. Firstly, two experimental methods were developed to measure machined surface temperatures in hard turning. The first method, based on a tool-foil thermocouple, estimated the machined surface temperature using a metal foil embedded in the workpiece to measure the tool tip temperature. The second method used a thermocouple embedded in the tool with its tip continuously sliding on the machined surface behind the cutting edge. The inverse heat transfer method was applied on a three-dimensional thermal model to find the machined surface temperature near the cutting edge. These two methods, although based on distinct approaches, gave correlated predictions in hard turning tests, indicating both to be feasible for the measurement of hard turning machined surface temperatures. Secondly, four finite element method (FEM) models, namely the advection model, surface heat model, heat carrier model and ring heat model, were studied to predict the workpiece temperature in finish boring. Cylinder boring experiments were conducted to measure the workpiece temperature and evaluate the capability of four models in terms of accuracy and efficiency. Results showed good correlations between model-predicted and experimentally- measured temperatures. Advantages and disadvantages of each model were discussed. For studying detailed cylinder boring workpiece temperature, it was suggested to use the ring heat model to estimate the moving heat flux and the heat carrier model for local workpiece temperature calculation. Thirdly, experimental and FEM analysis was combined to identify the bore cylindricity error sources in finish boring. Experiments were conducted to measure the workpiece temperature, cutting and clamping forces, spindle error, and bore shape. FEM analysis of the workpiece temperature, thermal expansion, and deformation due to cutting and clamping forces was performed. The coordinate measurement machine (CMM) measurements of the bore after finish boring showed the 5.6 micrometer cylindricity and a broad spectrum from 2nd to 10th harmonics. The FEM revealed effects of workpiece thermal expansion (1.7 micrometer cylindricity), deformation due to cutting force (0.8 micrometer cylindricity), and clamping force (1.9 micrometer cylindricity) on the finished bore and the dominance by the 1st to 3rd harmonics using the three-jaw fixture. The spindle synchronous radial error motion (3.2 micrometer cylindricity) was dominated by 4th and higher order harmonics and matched well with the high (above 4th) harmonics in CMM measurements (2.9 micrometer cylindricity). The spindle error was found to be the dominant error source for bore cylindricity in finish boring. The experimental methods, FEM models and approaches developed in this dissertation provide better understanding of cylindrical machining processes and are useful for optimization of the process parameters.