Каталог Dormer Pramet фрезерование 2021 - 2022 - страница 769

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9 WORKING CONDITION WHEN MILLING When performing a milling operation, the edge of the milling cutter If engagement conditions are chosen incorrectly, this shock can cause almost always makes interrupted (intermittent) cuts. Each edge en- brittle damage to the edge, in the form of either fracturing or crum- ters and exits the workpiece at least once within a single revolution bling of the edge. of the tool. Position of the milling cutter relative to the workpiece is thus a very In addition, a periodic change in chip thickness takes place dur- important factor. There are essentially three possible milling cutter ing each revolution of the milling cutter. This results in fluctuations positions: side up milling, centre milling and side down milling. For in the size and direction of the tangential component of the cut- indexable tools, we recommend using co-directional engagement ting force. The edge of the milling cutter is thus subjected to cy- (so that the cutter forms thick chips on entry and thin chips on exit). clic stress which results in specific wear. The durability of the mill- However, there are notable exceptions (workpieces with surface skin, ing cutter edge is therefore dependent on the conditions in which machines with worn feed screws...). the edge enters and exits the workpiece. Proper choice of these conditions significantly affects the milling process and its results in terms of cutting power and quality of the machined surface. At the moment the edge enters or exits the workpiece, the edge is subjected to more or less intense mechanical shock which caus- es mechanical stress in the immediate vicinity of the cutting edge. h=0 vf vc h max UP SIDE MILLING h vc CENTRE MILLING vf DOWN SIDE MILLING vc h max vf h=0 During face milling, where the width of the milled surface ae is equal made (see correction tables on page 699). Either way, we should try to the diameter of the milling cutter, follow the values recommended to ensure that the tool does not enter or exit the cut in an area close specifically for the inserts. If the engagement width is less than the to the centre of the milling cutter (so-called dead zone). diameter of the milling cutter, then the key factor is whether we ma- chine with the centre or the side of the milling cutter, as mentioned above. In both cases, corrections in feed and cutting speed should be f ae When the edge exits from the cut, this is accompanied by both stress- of flexible deformations, particularly in the surface layers of the work- ing of the edge due to rapid cooling of the surface layers of the insert piece after a rapid decrease in cutting force. near the cutting edge and by mechanical shock caused by the release 768 DC/2