Tools Difference and selection between down milling and up milling X Vf Vf Y B Up milling magnified X Dowm milling magnified Y Climb milling (also called down milling): the feed direction of workpiece is the same as that of the milling rotation at the connecting position. Conventional milling (also called up milling): the feed direction of workpiece is opposite to that of the milling rotation at the connecting position. In down milling, the major force of borne is compressive stress; in up milling, bears the tensile stress. The compressive strength of cemented carbide material is larger than its tensile strength. In down milling, chip becomes thin from thick gradually, and workpiece press each other. The friction between edge and workpiece is small, thus can reduce the abrasion of edge, the hardening of workpiece surface and the surface roughness (Ra). In up milling, chip becomes thick from thin gradually. When insert cutting into the workpiece, it generates strong friction and more heat than down milling, and make workpiece surface harden. In up milling, because horizontal direction of cutting force that milling cutter conducting on workpiece is opposite to the feed direction of workpiece, therefore the lead screw of worktable joints closely with one side of screw nut. In down milling, the direction of cutting force is same as the feed direction. When edge s radial force on workpiece is big enough to some extend, the worktable will bounce left and right, thus make the gap fall behind. The gap will return to front side along with the continuing rotation of lead screw. At this moment the worktable stops motion, however it will bounce left and right again when the radial cutting force is big enough to some extend again. The periodical bounce of worktable will cause poor surface quality of workpiece and tool breakage. When use end mills for down milling, every time the edges begin the cutting at workpiece surface, therefore end mills are not suitable for machining the workpiece with hardened surface. Up milling is recommended for milling the thin-wall components or square milling with the demand of high precision. B 165
Tools Pitch selection Pitch is the distance between one point on one and the same point on the next edge. cutters are mainly classified into coarse, close and extra close pitches, Operational stability L (Low) M (Medium) H (High) Coarse pitch Close pitch Extra close pitch Differential pitch When the milling width equal to diameter of cutter, the machining system is stable and main power of machine is sufficient, selecting coarse pitch can achieve high productive efficiency. General milling function and multiple mixed productions When the milling width is less than diameter of cutter, cutting by maximum edges can achieve high productive efficiency. Selection of approach The approach is composed by insert and tool body, Chip thickness, cutting forces and tool-life are affected especially by the approach. Decreasing the approach reduces chip thickness and spreads the cutting area between and workpiece for a given feed rate. A smaller approach also guarantee that it is stable entrying into or exiting workpiece, to protect the and extend tool life. However this will increase higher axial cutting forces on the workpiece, thus is not suitable for machining thin workpiece such as thin plate. Approach Feed rate per tooth Real maximum cutting depth 90 f z h ex =f z sinkr 75 f z h ex =0.96 f z 60 f z h ex =0.86 f z 45 f z h ex =0.707 f z Round insert f z h ex = ic2 (ic-2ap ) 2 ic fz B 166
Tools General formula Vc :cutting speed(m/min) Dc :nominal diameter of milling tool(mm) n :spindle speed(rev/min) zn :number of teeth Q :metal removal rate(cm 3 /min) Vf :feed rate of worktable ( feed speed)(mm/min) fz :feed rate per tooth(mm/z) π :circumference ratio 3.14 Tc :machining time(min) fz :feed rate per revolution (mm/rev) Cutting speed π Dc n Vc= (m/min) 1000 Spindle speed n = 1000 V c (rev/min) π Dc Feed rate of worktable ( feed speed) Vf = fz n zn (mm/min) B Feed rate per tooth f z= V f n Zn (mm/z) Feed direction Feed rate per revolution f n= V f n (mm/rev) Minor Tooth shape Feed rate per tooth(fz) Machining time Tc= 1000 Vc π Dc (min) Metal removal rate a p ae Vf Q = 1000 (cm 3 /min) B 167
Tools Function of each part in face milling Main s of face mills Main s of face mills Rake R Radial rake r p Axial rake r f Approach Kr λs Inclined of Designation Function Effect Axial rake rf Radial rake rp Approach Kr Rake R Inclined of λs Determining the chip direction Determining whether the cutting is light and fast or not Determining the chip thickness Determining whether the cutting is light and fast or not Determining the chip flow direction negative, excellent capability of chip removal Positive, good cutting performance Kr,chip thickness ;Kr,chip thickness ; Poor cutting high strength of Poor cutting high strength of (-) 0 (+) (-) 0 (+) Good cutting low strength of Good cutting low strength of Characteristics of different rake s combined Negative rake - Double positive Double negative r f (+) r f (-) One positive, one negative r f (+) 0 o 0 rake Positive rake + r p (+) r p (-) r p (-) Applicable material machined Axial rake rf + - + Radial rake rp + - - P M K N S B 168
Tools Cutting performances of different approach s Approach Schematic diagram Instruction 45 Axial force is largest. It will bend when machining thin-wall workpiece, and reduces the precision of workpiece. It is benefit to avoid fringe breakage of workpiece when machining cast iron 75 90 The main purpose is to resolve the radial cutting force, it is often used for general face milling. The axial force is zero in theory, suitable for milling thin plate workpiece. B Wiper insert It has axial and radial run out because fz fn Rz of tools and inserts exist manufacturing tolerance. The axial runout lead to poor surface roughness. Solution Aassembling wiper inserts usage The wiper insert must protrude below the other Wiper insert Common insert inserts by 0.03-0.10 mm at axial direction, only that the wiping function can take into effect. Generally speaking, a cutter can just assemble only one wiper insert. If the diameter of cutter is much bigger or cutter's feed rate per revolution is bigger than the length of wiper edge, 2 to 3 wiper inserts can be assembled. B 169
Tools Selection of cutting width and tool cutting diameter in face milling Generally speaking, the relation between cutting width and tool cutting diameter is Dc=(1.2 1.5) ae. In the machining practice, it need to avoid coincidence of tool center and workpiece center as much as possible. (1.2-1.5)ae Dc=ae Dc:Tool cutting diameter ae:cutting width B 170