Tham khảo tài liệu 'cutting tool technology industrial handbook by graham t smith_4', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 60 Chapter 2 is the strain-hardening tendency of the workpiece material. Therefore the greater the strain-hardening exponent the higher will be its BUE formation. From experiments conducted on BUE formation it would seem that the higher the cutting speed the less is the tendency for BUE to form. Whether this lack of BUE formation at higher cutting data is the consequence of increased strain rate or the result of higher interface temperatures is somewhat open to debate. However it would seem that a paradox exists because as the speed increases the temperature will also increase but the BUE decreases. The propensity for BUE formation can be lessened by I. Changing the geometry of the cutting edge -by either increasing the tool s rake angle or decreasing the DOC or both II. Utilising a smaller cutting tip radius III. Using an effective cutting fluid or IV. Any combination of these factors. Discontinuous chips - consist of adjacent workpiece chip segments that are usually either loosely attached to each other or totally fragment as they are cut Fig. 29 . The formation of discontinuous chips usually occur under the following machining conditions I. Brittle workpiece materials - these materials do not have the machining capability to undergo the high shear strains II. Hard particles and impurities - materials with these in their matrix will act as stress-raisers and actively encourage chip breakage III. Very high or low cutting speeds - chip velocity at both ends of the cutting spectrum will result in lack of adherence fragmentation of the chip segments IV. Low rake angles large DOC s - either small top rakes and heavy DOC s will decrease the adherence of the adjacent chip segments V. Ineffective cutting fluid - poor lubricity combined with a meagre wetting ability will encourage discontinuity of chip segments VI. Inadequate machine tool stiffness - creating vibrational tendencies and cutting instability leading to disruption of the machining dynamics and loosening of .
