外文原文：Five Basic Machining Techniques 1 The five basic techniques of machining metal include drilling and boring, turning, planing, milling, and grinding. Variations of the five basic techniques are employed to meet special situations.2 Drilling consists of cutting a round hole by means of rotating drill, on the other hand , involves the finishing of a hole already drilled or cored by means of a rotating, offset, single-point tool. On some boring machines, the tool is stationary and the work revolves; on others, the reverse is true.3 The lathe, as the turning machine is commonly called, is the father of all machine tools. The piece of metal to be machined is rotated and the cutting tool is advanced against it. We will discuss the structure and functions of lathe in later paragraphs of this article.4 Planing metal with a machine tool is a process similar to planing wood with a hand plane. The essential difference lies in the fact that the cutting tool remains in a fixed position while the work is moved back and forth beneath it. Planers are usually large pieces of equipment; sometimes large enough to handle the machining of surfaces 15 to 20 feet wide and twice as long. A shaper differs from a planer in that the workpiece is held stationary and the cutting tool travels back and forth.5 Milling consists of machining a piece of metal by bringing it into contact with a rotating cutting tool which has multiple cutting-edges. There are many types of milling machines designed for various kind of work. Some of the shapes produced by milling machines are extremely simple, like the slots and flat surfaces produced by circular saws. Other shapes are more complex and may consist of a variety of combinations of flat and curved surfaces, depending on the shape given to the cutting-edges of the tool and on the travel path of the tool. 6 Grinding consists of shaping a piece of work by bringing it into contact with a rotating abrasive wheel. The process is often used for the final finishing to close dimensions of a part that has been heat-treated to make it very hard. This is because grinding can correct distortions that may have resulted from heat treatment. In recent years, grinding has also found increased application in heavy-duty metal removal operations.7 The Lathe：The lathe is one of the most useful and versatile machines in the workshop, and is capable of carrying out a wide variety of machining operations. The main components of the lathe are the headstock and tailstock at opposite ends of a bed, and a tool-post between them which holds the cutting tool. The tool-post stands on a cross-slide which enables it to move sidewards across the saddle or carriage as well as along it, depending on the kind of job it is doing. The ordinary centre lathe can accommodate only one tool at a time on the tool-post, but a turret lathe is capable of holding five or more tools on the revolving turret. The lathe bed must be very solid to prevent the machine from bending or twisting under stress. 8 The headstock incorporates the driving and gear mechanism, and a spindle which holds the workpiece and causes it to rotate at a speed which depends largely on the diameter of the workpiece. A bar of large diameter should naturally rotate more slowly than a very thin bar; the cutting speed of the tool is what matters. Tapered centers in the hollow nose of the spindle and of the tailstock hold the work firmly between them. 9 A feed-shaft from the headstock drives the tool-post alone the saddle, either forwards or backwards, at a fixed and uniform speed. This enables the operator to make accurate cuts and to give the work a good finish. Gears between the spindle and the feed-shaft control the speed of rotation of the shaft, and therefore the forward or backward movement of the tool-post. The gear which the operator will select depends on the type of metal which he is cutting and the amount of metal he has to cut off. For a deep or roughing cut the forwards movement of the tool should be less than for a finishing cut. 10 Centers are not suitable for every job on the lathe. The operator can replace them by various types of chucks, which hold the work between jaws, or by a front-plate, depending on the shape of the work and the particular cutting operation. He will use a chuck, for example, to hold a short piece of work, or work for drilling, boring or screw-cutting. 11 A transverse movement of the tool-post across the saddle enables the tool to cut across the face of the workpiece and give it a flat surface. For screw-cutting, the operator engages the lead-screw, a long screwed shaft which runs along in front of bed and which rotates with the spindle. The lead-screw drives the tool-post forwards along the carriage at the correct speed, and this ensures that the threads on the screw are of exactly the right pitch. The operator can select different gear speeds, an