Abstract

Micromilling can fabricate complex features in a wide range of engineering materials with excellent finish but the limited flexural stiffness of the micro-end mill can result in catastrophic tool failure. This issue can be overcome by using high rotational speeds. Note that the combination of high rotational speeds and low flexural stiffness can induce process instability which is aggravated by the accelerated wear of the microtools at high speeds, specifically, for Ti-alloys. The effect of progressive tool wear on the stability has been investigated in micromilling of Ti6Al4V. For incorporating tool wear, the cutting force coefficients are modeled as a function of instantaneous cutting edge radius (CER). These coefficients also depend on feed/flute and the initial CER of the micro-tool (due to inherent variability in grinding process). A significant increase (85%-114%) in the instantaneous CER is observed with an increase in the length of cut. At high rotational speeds, a zero-order approximation frequency domain model may not be accurate. Therefore, a 2-DOF time domain model based on semidiscretization method has been used to characterize the evolution of stability limits with an increase in the length of cut. The frequency spectra of velocity and surface topography have been analyzed for the model validation. The progressive tool wear affects the stability limits along with the initial CER and the feed/flute. At higher speeds (90,000-110000 rpm), the effect of progressive tool wear is pronounced and the stability limits reduce by ~30% in that range.

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