Tham khảo tài liệu 'drilling and associated technologies part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Drilling and Associated Technologies 97 NB OIL COOLANT HOLES NOT ILLUSTRATED ARE AUTOMATIC CHIP- BREAKER ie TWIN FLUTE DRILLS OR THOSE WITH DOUBLE MARGINS ASYMMETRICAL DRILLS. Figure 48. Symmetrical twist drill cross-sectional profiles After Spur and Masuha 1981 98 Chapter 3 workpiece see Fig. 49 . The thrust force Fig. 49 is the result of the selected penetration rate . feed in combination with the bulk hardness of the workpiece and its work-hardening ability and the efficiency of the coolant supply - if any - to the cutting edges lips . The resolution of the cutting resistance into their various components when twist drilling is shown in Fig. 49 at a mid-point along the lips. The thrust force is just one of the cutting resistances in a drilling operation contributions to drill resistance are from the Lips - equal lip lengths and angles are important for a balanced cutting action this being considered an efficient cutting process Chisel edge - is highly negatively skewed and as it acts like a blunt wedge-shaped indentor extruding the workpiece material from this vicinity Figure 49. The balanced cutting forces resulting from drilling holes utilising twist drill geometries. After Kaczmarek 1976 Drilling and Associated Technologies 99 Land or margin - via a rubbing or frictional action. NB The latter two are relatively inefficient processes moreover the resistance components of the lips and chisel edge are the product of resistance of the undeformed chip to plastic strain in combination with resistance due to external friction. The land resistance occurs from the friction . rubbing against the side of the drill s hole. When symmetrical twist drilling illustrated in Fig. 50 the undeformed chip can be characterised by its Cutting depth a - where a d 2 with d being the drill s diameter mm Feedrate s - this being the distance the cutting edge moves in the drilling axis direction during one revolution. Normally two rigidly joined cutting edges are cutting at any
