Intermittent single point machining of brittle materials

Abstract

A series of tests were undertaken to explore diamond tool wear in the intermittent cutting of brittle materials, specifically silicon. The tests were carried out on a plain way No. 3 Moore machine base equipped as a flycutter with a motorized Professional Instruments 4R air bearing spindle. The diamond tools were made by Edge Technologies with known crystal orientation and composition and sharpened with either an abrasive or chemical process, depending on the individual test. The flycutting machine configuration allowed precise control over the angle at which the tool engages the anisotropic silicon workpiece. In contrast, the crystallographic orientation of the silicon workpiece changes continuously during on-axis turning. As a result, it is possible to flycut a workpiece in cutting directions that are known to be easy or hard. All cuts were run in the 100 plane of the silicon, with a slight angle deliberately introduced to ensure that the 100 plane is engaged in ''up-cutting'' which lengthens the tool life. A Kistler 9256 dynamometer was used to measure the cutting forces in order to gain insight into the material removal process and tool wear during testing. The dynamometer provides high bandwidth force measurement with milli-Newton resolution and good thermal more » stability. After many successive passes over the workpiece, it was observed that the cutting forces grow at a rate that is roughly proportional to the degradation of the workpiece surface finish. The exact relationship between cutting force growth and surface finish degradation was not quantified because of the problems associated with measuring surface finish in situ. However, a series of witness marks were made during testing in an aluminum sample that clearly show the development of wear flats on the tool nose profile as the forces grow and the surface finish worsens. The test results show that workpieces requiring on the order of two miles of track length can be made with low tool wear and excellent surface finish. With longer track lengths, the tool forces (and presumably tool wear) begin a roughly linear increase as surface finish steadily worsens. No catastrophic tool failures were observed, only slow changes as the track length increases. Interestingly, the specific cutting energy did not remain constant with depth of cut, suggesting that there are significant friction forces in the cutting of silicon. This finding supports published results emphasizing the importance of a large clearance angle on the tool and hints that fairly aggressive cuts may be the most efficient way to remove material. That is, tool life may turn out to scale with track length, not volume indicating that machining parameters for silicon should be chosen to minimize track length by taking heavier cuts. « less