Predicting high-speed machining dynamics by substructure analysis

Tony L. Schmitz
Automated Production Technology Division
Manufacturing Engineering Laboratory

High-speed machining (HSM) is an emerging area of technology within manufacturing engineering. Recent improvements in high-speed spindle design (reliable 4189 rad/s (40,000 rpm) / 40 kW spindles are commercially available) and new slide system developments (velocities and accelerations approaching 1 m/s and 20 m/s^2, respectively) have made HSM an economically viable alternative to other manufacturing processes such as forming and casting. At this time, the most dramatic applications of HSM have been in the manufacture of aluminum components where volumetric material removal rates can exceed thousands of cubic centimeters per minute. In the aerospace industry, for example, HSM is changing the way aircraft are manufactured by enabling the replacement of sheet-metal assemblies with machined monolithic components resulting in substantial cost savings and improved performance. These monolithic structures can be stronger, lighter, and more accurate than the sheet-metal build-ups and provide a large reduction in inventoried jigs and fixtures.

The practical implementation of HSM requires accurate knowledge of the machine dynamics. Typically, the tool point frequency response function is measured for each tool/holder/spindle combination on a particular computer numerically-controlled machining center and a stability analysis performed for each case to select appropriate spindle speeds for maximum material removal rate (experimental and analytical analyses have shown that the most favorable tooth passing frequencies for process stability occur when operating at a substantial integer fraction of the most flexible natural frequency of the structure). These multiple frequency response measurements typically require a trained technician and considerable machine down time. To reduce measurement time, receptance coupling substructure analysis has been applied to the prediction of the tool point dynamic response, combining frequency response measurements of individual components (i.e., tool, holder, and spindle) through appropriate connections to determine assembly dynamics. A tool tuning example provides verification of this method. Experimental and predicted results show dramatic variations in the tool point response (and process stability) as the tool length is changed and the tool cantilever mode interacts with different holder/spindle modes, similar to the effect seen in dynamic absorbers. Predicted local increases in the critical stability limit match empirical results recorded previously by NIST researchers.