Radial throw in micromachining: Measurement and analysis

This paper presents a comprehensive approach for measurement and analysis of radial throw and the associated tool-tip trajectory in micromachining when using ultra-high-speed (UHS) spindles. The effect of radial throw on micromachining accuracy could be significant due to the strict absolute-tolerance requirements. However, accurately determining radial throw in micromachining poses challenges due to the micron-scale tool dimensions and high rotational speeds. In contrast to run-out, radial throw depends on the tool-rotation angle and dictates the instantaneous position (trajectory) of the cutting edges. This work first presents a mathematical framework to obtain the radial throw at the cutting edges by measuring radial throw at two locations on the tool shank. A laser Doppler vibrometry-based experimental approach is then described to accurately measure the radial throw from the tool shank in two mutually-perpendicular directions. Next, the variations on radial throw measurements are evaluated, the effect of spindle speed on the radial throw is analyzed, and the predictions of tool-tip radial throw are compared with those directly measured from the tip of a micro-tool blank. A study is then performed to assess the effect of tool attachment/detachment cycles on radial throw. Lastly, a set of simulations is conducted to demonstrate how the radial throw magnitude and orientation affect the surface location error, sidewall surface roughness, and flute-to-flute chip-thickness variations. It is concluded that the presented mathematical framework and measurement approach can be used to determine the tool-tip radial throw magnitude with better than 3.5% +/- 2% accuracy. Increased spindle speed considerably increases the radial throw, e.g., from 8 mu m at 60,000 rpm to 16 mu m at 130,000 rpm. The tool attachment/detachment induces less than +/- 6.9% variation to the mean radial throw magnitude at 60,000 rpm. The main contributor to the variation of the radial throw orientation is identified as the inherent inaccuracies of the spindle.