机械工程英语——Lesson 14 The lathe

The lathe（车床） is one of the most useful and versatile（多才多艺的） machines in the workshop, and is capable of carrying out a wide variety of machining operations. The main components of the lathe are the head stock 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（横向拖板） which enables it to move sideward across the saddle（鞍状物） or carriage（车架） as well as along it depending on the kind of job it is doing. The ordinary center lathe can accommodate（容纳） only one tool at a time on the tool-post, but a turret lathe（六角车床） is capable of holding five or more tools on the revolving turret（回转六角刀架）. The lathe bed must be very solid to prevent the machine from bending or twisting under stress.

The headstock incorporates（合并） the driving and gear mechanism, and a spindle（心轴） which holds the work-piece and causes it to rotate at a speed which depends largely on the diameter of the work-piece. A bar（棒） 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 tail-stock hold the work firmly between them. A feed-shaft from the headstock drives the tool-post along the saddle（鞍状物）, either forwards or backwards, at a fixed and uniform（均匀的） 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 deep or roughing cut the forward movement of the tool should be less than for a finishing cut.

Fig 14-1 Lather

Centers are not suitable for every job on the lathe. The operator can replace them by various types of chucks（卡盘）, 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. A transverse movement of the tool-post across the saddle enables the tool to cut across the face of the work-piece and give it a flat surface. For screw cutting, the operator engages the lead-screw（导螺杆）, a long screwed shaft which runs along in front of the bed and which rotates with the spindle. The lead-screw drives the tool-post forwards along the carriage at correct speed, this ensures that the threads on the screw are of exactly the right pitch. The operator can select different gear speed, and this will alter the ratio of spindle and lead-screw speeds and therefore alter the pitch of threads. A reversing lever（换向杆） on the headstock enables him to reverse the movement of carriage and bring the tool back to its original position.