Overhung-boring bars : the performance of undamped and damped bars under static and dynamic conditions when machining metals
The work of the author was to investigate the static, dynamic and machining behaviour of some new designs of slug damped boring bars with a 10 to 1 overhang ratio. The bars were mounted on a centre lathe. The static behaviour of a boring bar in relation to the geometric form errors that might be produced during boring was studied both analytically and experimentally. Specifically, two types of errors were considered, namely, a) errors that arise on entry of the boring tool into the workpiece) known as the "bell-mouth" errors; and b) reproducibility of eccentricity errors, known as the "copying" errors. The theory for "bell-mouth" errors did not seem to fit the results well; however, the theory did prove that such errors could exist. The theory for “copying” errors agreed remarkably well with the results provided that the initial eccentricity was small compared with the depth of cut. The dynamical behaviour of the slug damped boring bar was modelled by a mathematical analogue. Despite its inability to properly account for the compressibility effect of the gaseous damping fluid, the model revealed the possibility of design improvements. In consequence, the optimally-tuned slug-damped tungsten-bunged bar was conceived, Manufactured and tested along with a solid bar for comparison purposes, a slug-damped recessed bar and a slug-damped steel-bunged bar. The machining behaviour of a boring bar was studied in terms of the maximum depth of cut that it could cope before the occurrence of chatter. At first, a stability model was developed based on the mathematical analogue formulated in the study of the dynamical behaviour. But since this analogue did not fit the results accurately, a second and more precise model was set up using the frequency response obtained from dynamic experiments instead. The concept of negative damping coefficient was used; and a one-to-one correspondence between the asymptotic value of the negative damping coefficient and the limiting depth of cut was found to exist. By virtue of this, it is in principle possible to predict the limiting depth of cut of any machine tool system whose frequency response characteristics are known. Compared with other bars tested, the optimally-tuned tungsten-bunged bar was found to have the best dynamic and machining characteristics as reflected in the limiting depth of cut of 0.10511 (2.67 mm) to 0.110" (2.79 mm) at the feed of 0.0065"/rev (0.165 mm/rev) and the speed of 500 rprn on a 3.5" dia. bore (140 m/min) of EN8 steel. By constrast, the solid bar was hardly able to cut stably even at the light cut of 0.005".