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****************** Recommended Reading: Manufacturing Engineering and Technology Marks' Standard Handbook for Mechanical Engineers ******************
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Changing the direction of the table feed relative to the cutter rotation has a marked effect on cutting characteristics. Fig 1.4.1.
Click the image for full size picture Fig 1.4.1. shows two milling techniques known as up-cut and down-cut milling. During up-cut milling Fig 1.4.1.a) the table feeds the workpiece in the opposite direction to the cutter rotation, this results in three distinct phases of chip formation, i) the cutting edge slides over the surface, ii) it starts to crush the surface material and iii) the cutting edge gains a purchase and chip formation begins, to a maximum thickness as the tooth leaves the workpiece. The initial sliding and crushing action of the tooth can lead to work-hardening of the material and dulling of the cutting edge, resulting in premature failure of the cutting tool; also as the chip thickness is at a maximum as the tool exits the workpiece there is a sudden release of energy that can lead to bounce, chatter and increased fatigue in the machine bearings and transmission. There is also a tendency to lift the workpiece of the table. There is however, a significant advantage to up-cut milling in that the cutter opposes the feed – therefore eliminating the possibility of snatching the workpiece and taking too large a cut if any backlash is present in the table feed lead screw. During down-cut milling Fig 1.4.1.b) the table feeds the workpiece in the same direction as the cutter rotation. As the cutter contacts the workpiece, chip thickness is maximum and gradually reduces to nil as the cutter exits the material. The gradual decrease in cutting force and absence of sliding and crushing usually results in a much better finish than is normal in down cut milling; also tool life is improved. Power required for the table feed is reduced, and the cutter tends to push the workpiece more firmly to the table, however there must be no backlash in the table feed mechanism or the cutter will snatch the workpiece leaving at best an uneven surface; at worst a broken tool or arbour. A further disadvantage of down cut milling is that any scale or surface hardening of cast or forged blanks can rapidly dull or chip the cutting edge.
Suggested Reading: Manufacturing Engineering and Technology Kalpakjian, Schmid and Schmidt. Marks' Standard Handbook for Mechanical Engineers |
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