机械工程英语Unit 7

Unit 7 Metal Cutting
➢ importance of metal cutting ➢ Introduction of a typical cutting tool ➢ Chip Formation ➢ Shear zone ➢ Cutting Tool Materials ➢ Surface Finish ➢ Cutting Fluids
Fig 7.3 Three main types of chips a）Discontinuous chip b) Continuous chip c) Continuous chip with BUE
❖ Discontinuous Chip
The segmented chip separates into short pieces, which may or may not adhere to each other. Severe distortion of the metal occurs adjacent to the tool face, resulting in a crack that runs ahead of the tool. Eventually, the shear stress across the chip becomes equal to the shear strength of the material, resulting in fracture and separation.[1] With this type of chip, there is little relative movement of the chip along the tool face, Fig 7.3 (a).
The conditions which have an important influence on metal cutting ：
❖ work material（工件材料） ❖ cutting tool material（切削刀具材料） ❖ cutting tool geometry（切削刀具几何参数） ❖ cutting speed（切削速度） ❖ feed rate（进给量） ❖ depth of cut（切削深度） ❖ cutting fluid（切削液） ❖ Three cutting factors：
importance of metal cutting
❖ every product we use in our daily life has undergone this process either directly or indirectly.
❖ In USA, more than $100 billions are spent annually on machining and related operations.
❖ Continuous with a Built-up Edge
This chip shows the existence of a localized, highly deformed zone of material attached or “welded” on the tool face. Actually ,analysis of photomicrographs shows that this built-up edge is held in place by the static friction force until it becomes so large that the external forces acting on it cause it to dislodge , with some of it remaining on the machined surface and the rest passing off on the back side of the chip, Fig 7.3(c).
cutting speed（切削速度）——v feed rate（进给量） —————f depth of cut（切削深度） —— d
Chip Formation
Metal cutting process is a very complex process. Fig 7.2 shows the basic material removal operation schematically. The metal in front of the tool rake face gets immediately compressed, first elastically and then plastically.
Shear zone
❖ two schools of thought in the analysis of the metal removal process：
① deformation zone is very thin and planar as shown in Fig 7.4 (a). ②actual deformation zone is a thick one with a fan shape as shown
Introduction of a typical cutting tool
Fig 7.1 The general characteristics of a metal cutting tool
A typical cutting tool in a simplified form is shown in Fig 7.1. The important features to be observed are follows：
❖ Continuous Chip
The continuous chip is characterized by a general flow of the separated metal along the tool face. [2] There may be some cracking of the chip, but in this case it usually does not extend far enough to cause fracture. This chip is formed at the higher cutting speeds when machining ductile materials. There is little tendency for the material to adhere to the tool. The continuous chip usually shows a good cutting ratio and tends to produce the optimum surface finish, but it may become an operating hazard, Fig 7.3 (b).
②Clearance angle （后角）
This is the angle between the machined surface and the underside of the tool called the flank face. The clearance angle is provided such that the tool will not rub the machined surface thus spoiling the surface and increasing the cutting forces. A very large clearance angle reduces the strength of the tool tip, and hence normally an angle of the order of 5-6°is used.
❖ The maximum limit to the rake angle is generally of the order of l5°for high speed steel tools cutting mild steel. It is possible to have rake angles at zero or negative.
❖ Then the deformed metal (called chip ) flows over the tool (rake) face. If the friction between the tool rake face and the underside of the chip (deformed material) is considerable, then the chip gets further deformed, which is termed as secondary deformation. The chip after sliding over the tool rake face is lifted away from the tool, and the resultant curvature （曲率）of the chip is termed as chip curl.
❖ A large majority (above 80%) of all the machine tools used in the manufacturing industry have undergone metal cutting.
❖ An estimate showed that about 10 to 15% of all the metal produced in USA was converted into chips.
① Rake angle（前角）
❖ It is the angle between the face of the tool called the rake face and the normal to the machining direction.