学习啦【英语散文】 编辑：韦彦 发布时间：2016-09-14 16:38:56
Milling and Milling Cutter
Milling is a machining process that is carried out by means of a multiedge rotating tool known asa milling cutter.
In this process, metal removal is achieved through combining the rotary motion of the millingcutter and linear motions of the workpiece simultaneously. Milling operations are employed inproducing flat, contoured and helical surfaces as well as for thread- and gear-cuttingoperation.
Each of the cutting edges of a milling cutter acts as an individual single-point cutter when itengages with the workpiece metal. Therefore, each of those cutting edges has appropriaterake and relief angles.
Since only a few of the cutting edges are engaged with the workpiece at a time, heavy cuts canbe taken without adversely affecting the tool life. In fact, the permissible cutting speeds andfeeds for milling are three to four times higher than those for turning or drilling.
Moreover, the quality of the surfaces machined by milling is generally superior to the qualityof surfaces machined by turning, shaping, or drilling.
A wide variety of milling cutters is available in industry. This, together with the fact that amilling machine is a very versatile machine tool, makes the milling machine the backbone of amachining workshop.
As far as the direction of cutter rotation and workpiece feed are concerned, milling isperformed by either of the following two methods.
Up milling (conventional milling). In up milling the workpiece is fed against the direction ofcutter rotation, as shown in Fig.12.3a. As we can see in that figure, the depth of cut (andconsequently the load) gradually increases on the successively engaged cutting edges.
Therefore, the machining process involves no impact loading, thus ensuring smootheroperation of the machine tool and longer tool life. The quality of the machined surface obtainedby up milling is not very high. Nevertheless, up milling is commonly used in industry, especiallyfor rough cuts.
Down milling (climb milling). As can be seen in Fig.12.3b, in down milling the cutter rotationcoincides with the direction of feed at the contact point between the tool and the workpiece. Itcan also be seen that the maximum depth of cut is achieved directly as the cutter engages withthe workpiece.
This results in a kind of impact, or sudden loading. Therefore, this method cannot be usedunless the milling machine is equipped with a backlash eliminator on the feed screw. Theadvantages of this method include higher quality of the machined surface and easier clampingof workpieces, since the cutting forces act downward.
Types of Milling Cutters
There is a wide variety of milling cutter shapes. Each of them is designed to perform effectively a specific milling operation.
Generally, a milling cutter can be described as a multiedge cutting tool having the shape of a solid of revolution, with the cutting teeth arranged either on the periphery or on an end face or on both. Following is a quick survey of the commonly used types of milling cutters.
Plain milling cutter. A plain milling cutter is a disk-shaped cutting tool that may have either straight or helical teeth, as shown in Fig.12.4a. This type is always mounted on horizontal milling machines and is used for machining flat surfaces.
Face milling cutter. A face milling cutter is also used for machining flat surfaces. It is bolted at the end of a short arbor, which is in turn mounted on a vertical milling machine. Fig.12.4b indicates a milling cutter of this type.
Plain metal slitting saw cutter. Fig.12.4c indicates a plain metal slitting saw cutter. we can see that it actually involves a very thin plain milling cutter.
Side milling cutter. A side milling cutter is used for cutting slots, grooves, and splines. As shown in Fig.12.4d, it is quite similar to the plain milling cutter, the difference being that this type has teeth on the sides. As is the case with the plain cutter, the cutting teeth can be straight or helical.