文献翻译-车床1

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英文原文

Lathes are widely used in industry to produce all kinds of machined parts. Some are general purpose machines, and others are used to perform highly specialized operations.

Engine Lathes

Engine lathes, of course, are general-purpose machine used in production and maintenance shop all over the world. Sizes range from small bench models to huge heavy duty pieces of equipment. Many of the larger lathes come equipped with attachments not commonly found in the ordinary shop, such as automatic stops for the carriage.

Tracer or Duplicating Lathes

The tracer or duplicating lathe is designed to produce irregularly shaped parts automatically. The basic operation of this lathe is as fallows. A template of either a flat or three-dimensional shape is placed in a holder. A guide or pointer then moves along this shape and its movement controls that of the cutting tool. The duplication may include a square or tapered shoulder, grooves, tapers, and contours. Work such as motor shafts, spindles, pistons, rods, car axles, turbine shafts, and a variety of other objects can be turned using this type of lathe.

Turret Lathes

When machining a complex workpiece on a general-purpose lathe, a great deal of time is spent changing and adjusting the several tools that are needed to complete the work. One of the first adaptations of the engine lathe which made it more suitable to mass production was the addition of multi-tool turret in place of the tailstock. Although most turrets have six stations, some have as many as eight.

High-production turret lathes are very complicated machines with a wide variety of power accessories. The principal feature of all turret lathes, however, is that the tools can perform a consecutive serials of operations in proper sequence. Once the tools have been set and adjusted, little skill is required to run out duplicate parts.

Automatic Screw Machines

Screw machines are similar in construction to turret lathes, except that their heads are

designed to hold and feed long bars of stock. Otherwise, there is little different between them. Both are designed for multiple tooling, and both have adaptations for identical work. Originally, the turret lathe was designed as a chucking lathe for machining small castings, forgings, and irregularly shaped workpieces.

The first screw machines were designed to feed bar stock and wire used in making small screw parts. Today, however, the turret lathe is frequently used with a collet attachment, and the automatic screw machine can be equipped with a chuck to hold castings.

The single-spindle automatic screw machine, as its name implies, machines work on only one bar of stock at a time. A bar 16 to 20 feet long is fed through the headstock spindle and is held firmly by a collect. The machining operations are done by cutting tools mounted on the turret and on the cross slide. When the machine is in operation, the spindle and the stock are rotated at selected speeds for different operations. If required, rapid reversal of spindle direction is also possible.

In the single-spindle automatic screw machine, a specific length of stock is automatically fed through the spindle to a machining area. At this point, the turret and cross slide move into position and automatically perform whatever operations are required. After the machined piece is cut off, stock is again fed into the machining area and the entire cycle is repeated.

Multiple-spindle automatic screw machines have from four to eight spindles located around a spindle carrier. Long bars of stock, supported at the rear of the machine, pass through these hollow spindles and are gripped by collets. With the single spindle machine, the turret indexes around the spindle. When one tool on the turret is working, the others are not. With a multiple spindle machine, however, the spindle itself indexes. Thus the bars of stock are carried to the various end working and side working tools. Each tool operates in only one position, but all tools operate simultaneously. Therefore, four to eight workpieces can be machined at the same time.

Vertical Turret Lathes

A vertical turret lathe is basically a turret lathe that has been stood on its headstock end. It is designed to perform a variety of turning operations. It consists of a turret, a revolving table, and a side head with a square turret for holding additional tools. Operations performed by any of the tools mounted on the turret or side head can be controlled through the use of stops.

Rolling Contact Bearings

The concern of a machine designer with ball and roller bearings is fivefold as follows:（a）life in relation to load; （b）stiffness, i. e. deflections under load; （c）friction; （d）wear; （e）noise. For moderate loads and speeds the correct selection of a standard bearing on the basis of load rating will become important where loads are high, although this is usually of less magnitude than that of the shafts or other components associated with the bearing. Where speeds are high special cooling arrangements become necessary which may increase frictional drag. Wear is primarily associated with the introduction of contaminants, and sealing arrangements must be chosen with regard to the hostility of the environment.

Because the high quality and low price of ball and roller bearings depends on quantity production, the task of the machine designer becomes one of selection rather than design. Rolling-contact bearings are generally made with steel which is through-hardened to about 900 HV, although in many mechanisms special races are not provided and the interacting surfaces are hardened to about 600 HV. It is not surprising that, owing to the high stresses involved, a predominant form of failure should be metal fatigue, and a good deal of work is based on accepted values of life and it is general practice in the bearing industry to define the load capacity of the bearing as that value below which 90 per cent of a batch will exceed a life of one million revolutions.

Notwithstanding the fact that responsibility for the basic design of ball and roller bearings rests with the bearing manufacturer, the machine designer must form a correct appreciation of the duty to be performed by the bearing and be concerned not only with bearing selection but with the conditions for correct installation.

The fit of the bearing races onto the shaft or onto the housings is of critical importance because of their combined effect on the internal clearance of the bearing as well as preserving the desired degree of interference fit. Inadequate interference can induce serious trouble from fretting corrosion. The inner race is frequently located axially by abutting against a shoulder.

A radius at this point is essential for the avoidance of stress concentration and ball races are provided with a radius or chamfer to allow space for this.

Where life is not the determining factor in design, it is usual to determine maximum loading by the amount to which a bearing will deflect under load. Thus the concept of “static load-carrying capacity” is understood to mean the load that can be applied to a bearing, which is either stationary or subject to slight swiveling motions, without impairing its running qualities for subsequent rotational motion. This has been determined by practical experience as the load which when applied to a bearing results in a total deformation of the

rolling-element diameter. This would correspond to a permanent deformation of 0.0025 mm for a ball 25 mm in diameter.

The successful functioning of many bearings depends upon providing them with adequate protection against their environment, and in some circumstances the environment must be protected from lubricants or products of deterioration of the bearing design. Moreover, seals which are applied to moving parts for any purpose are of interest to tribologists because they are components of bearing systems and can only be designed satisfactorily on the basis of the appropriate bearing theory.

Notwithstanding their importance, the amount of research effort that has been devoted to the understanding of the behavior of seals has been small when compared with that devoted to other aspects of bearing technology.

Machining Centers

Many of today’s more sophisticated lathes are called machining centers since they are capable of performing, in addition to the normal turning operations, certain milling and drilling operations. Basically, a machining center can be thought of as being a combination turret lathe and milling machine. Additional features are sometimes included by manufacturers to increase the versatility of their machines.

Numerical Control

One of the most fundamental concepts in the area of advanced manufacturing technologies is numerical control （NC）. Prior to the advent of NC, all machine tools were manually operated and controlled .Among the many limitations associated with manual control machine tools, perhaps none is more prominent than the limitation of operator skills. With manual control, the quality of the product is directly related to and limited to the skills of the operator. Numerical control represents the first major step away from human control of machine tools.

Numerical control means the control of machine tools and other manufacturing systems through the use of prerecorded, written symbolic instructions. Rather than operating a machine tool, an NC technician writes a program that issues operational instructions to the machine tool. For a machine tool to be numerically controlled, it must be interfaced with a device for accepting and decoding the programmed instructions, known as a reader.

Numerical control was developed to overcome the limitation of human operators, and it

has done so. Numerical control machines are more accurate than manually operated machines, they can produce parts more uniformly, they are faster, and the long-run tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology:

1.Electrical discharge machining.

https://www.360docs.net/doc/5616735157.html,ser cutting.

3.Electron beam welding.

Numerical control has also made machine tools more versatile than their manually operated predecessors. An NC machine tool can automatically produce a wide variety of parts, each involving an assortment of widely varied and complex machining processes. Numerical control has allowed manufacturers to undertake the production of products that would not have been feasible from an economic perspective using manually controlled machine tools and processes.

Like so many advanced technologies, NC was born in the laboratories of the Massachusetts Institute of Technology. The concept of NC was developed in the early 1950s with funding provided by the U. S. Air force. In its earliest stages, NC machines were able to make straight cuts efficiently and effectively.

However, curved paths were a problem because the machine tool had to be programmed to undertake a series of horizontal and vertical steps to produce a curve. The shorter is the straight lines making up the steps, the smoother is the curve. Each line segment in the steps had to be calculated.

This problem led to the development in 1959 of the Automatically Programmed Tools （APT）language. This is a special programming language for NC that uses statements similar to English language to define the part geometry, describe the cutting tool configuration, and specify the necessary motions. The development of the APT language was a major step forward in the further development of NC technology. The original NC systems were vastly different from those used today. The machines had hardwired logic circuits. The instructional programs were written on punched paper, which was later to be replaced by magnetic plastic tape. A tape reader was used to interpret the instructions written on the tape for the machine. Together, all of this represented a giant step forward in the control of machine tools. However, there were a number of problems with NC at this point in its development.

A major problem was the fragility of the punched paper tape medium. It was common for the paper tape containing the programmed instructions to break or tear during a machining