Facing. Facing is the producing of a flat surface as the result of a tool?s being fed across the end of the rotating workpiece. Unless the work is held on a mandrel, if both ends of the work are to be faced, it must be turned around after the first end is completed and then the facing operation repeated. The cutting speed should be determined from the largest diameter of the surface to be faced.
Facing may be done either from the outside inward or from the center outward. In either case, the point of the tool must be set exactly at the height of center of rotation. 幻灯片13
5.2 Turning and Lathe
Because the cutting force tends to push the tool away from the work, it is usually desirable to clamp the carriage to the lathe bed during each facing cut to prevent it from moving slightly and thus producing a surface that is not flat. In the facing of casting or other materials that have a hard surface, the depth of the first cut should be sufficient to penetrate the hard material to avoid excessive tool wear. Words & expressions
mandrel ['m?ndr?l] n.车床心轴 clamp [kl?mp] v. 夹紧，夹住 carriage [?k?rid?] n.大刀架，拖板 penetrate ['penitreit] v. 穿透，渗透
tool wear 刀具磨损 幻灯片14
5.2 Turning and Lathe
译文:
车端面可以加工出一个平面，它是当车刀沿回转工件的端面横向进给的结果。除非工件固定在心轴上，如果工件两端都要车端面，必须将一端加工完成后，将工件调头，重复进行车端面加工。车削速度必须由所加工表面的最大直径来决定。车端面可以由外向内也可以由中心向外进行。无论哪种情况，刀尖必须准确地与回转中心对正。由于切削力有将车刀推离工件的趋势，通常最可行的就是在车端面时将拖板卡紧在床身上以避免车刀轻微移动造成加工表面不平整。在对具有硬质表面的铸件或其他材料进行车端面加工时，首次切深要足以穿透硬质材料从而避免过度的刀具磨损。 幻灯片15
5.2 Turning and Lathe
Parting. Parting is the operation by which one section of a workpiece is severed from the remainder by means of cutoff tool. Because cutting tools are quite thin and must have considerable overhang, this process is less accurate and more difficult. The tool should be set exactly at the height of axis of rotation, be kept sharp, have proper clearance angles, and be fed into the workpiece at a proper and uniform feed rate. Words & expressions
remainder [ri'meind?]剩余物 ，余数
overhang ['?uv?'h??] vt.悬于...之上 n.伸出;突出 uniform ['ju:nif?:m] a.相同的,一致的 n.制服;军服 clearance angles 后角 幻灯片16
5.2 Turning and Lathe
译文:
切断是一种利用切断刀（割刀）将工件的一部分与其剩余部分脱离的加工方法。由于切断刀非常薄而且处于悬臂状态，因此这种加工方法不太精确而且加工起来常常比较困难。切断刀必须准确地对正回转轴，保持锋利，具有合适的后角，以合适、不变的进给率沿工件进给。 幻灯片17
5.2 Turning and Lathe
Threading. Threading can be considered as turning since the path to be travelled by the cutting tool is helical. However, there are some major differences between turning and threading. While in turning, the interest is in generating a smooth cylindrical surface, in threading the interest is in cutting a helical thread of a given form and depth which can be calculated from the formulae. helical [?helik?l] a.螺旋状的 译文:
车螺纹可以看作是车削，只是车刀所形成的加工轨迹是螺旋线。但二者也存在一些不同之处。车削主要考虑能否生成光滑的圆柱形表面，而车螺纹注重的是能否切出具有按公式计算出来的规定形状和深度的螺旋线。 幻灯片18
5.2 Turning and Lathe
? There are two basic requirements for thread cutting.
? An accurately shaped and properly mounted tool is needed because thread cutting is a form-cutting operation. The resulting thread profile is determined by the shape of the tool and its position relative to the workpiece.
The second by requirement is that the tool must move longitudinally in a specific relationship to the rotation of workpiece, because this determines the lead of the thread. This requirement is met through the use of the lead screw and the split unit, which provide positive motion of carriage relative to the rotation of spindle. form-cutting operation 成形加工
longitudinally [l?:?d?i'tju:din?li] adv.纵向 lead screw 丝杠 split unit 开合机构 幻灯片19
5.2 Turning and Lathe
译文:
车螺纹有两个基本要求：首先，由于车螺纹是一种成形加工，因此车刀必须具有准确的形状而且要安装合适。所切出的螺纹外形是由车刀形状和它与工件的相对位置决定的。其次，车刀的纵向进给与工件的回转运动具有特定的关系，因此这决定了螺纹的导程。这个条件可以通过采用丝杠和开合机构得到满足，它们可以实现拖板相对于主轴回转运动的精确可靠的运动。 幻灯片20
5.2 Turning and Lathe
Lathe. Many types of lathes are used for production turning. According to purposes and construction, lathe-type machine tools can be classified as follow:
1. Engine lathes 普通车床 2. Vertical lathes 立式车床 3. Turret lathes 转塔车床
4. Single-or multiple-spindle automatic or semi-automatic lathes 5. Contouring lathes 仿形车床 6. Universal lathes 万能车床
7. Special-purpose lathes such as crankshaft lathes, camshaft lathes, car wheel lathes and backing-off lathes, etc. 幻灯片21
Figure 5.1 The engine lathe