中英文翻译(Engineering Tolerance).doc

英文：
Engineering Tolerance
Introduction

A solid is defined by its surface boundaries. Designers typically specify a component’s nominal dimensions such that it fulfils its requirements. In reality, components cannot be made repeatedly to nominal dimensions, due to surface irregularities and the intrinsic surface roughness. Some variability in dimensions must be allowed to ensure manufacture is possible. However, the variability permitted must not be so great that the performance of the assembled parts is impaired. The allowed variability on the individual component dimensions is called the tolerance.
The term tolerance applies not only to the acceptable range of component dimensions produced by manufacturing techniques, but also to the output of machines or processes. For example , the power produced by a given type of internal combustion engine varies from one engine to another. In practice, the variability is usually found to be modeled by a frequency distribution curve, for example the normal distribution (also called the Gaussian distribution).One of the tasks of the designer is to specify a dimension on a component and the allowable variability on this value that will give acceptable performance.

Component Tolerances
Control of dimensions is necessary in order to ensure assembly and interchangeability of components. Tolerances are specified on critical dimensions that affect clearances and interferences fits. One method of specifying tolerances is to state the nominal dimension followed by the permissible variation, so a dimension could be stated as 40.000mm ± 0.003mm.This means that the dimension should be machined so that it is between 39.997mm and 40.003mm.Where the variation can vary either side of the nominal dimension, the tolerance is called a bilateral tolerance. For a unilateral tolerance, one tolerance is zero, e.g. 40+0.006 .
0.000
Most organizations have general tolerances that apply to dimensions when an explicit dimension is not specified on a drawing. For machined dimensions a general tolerance may be ±0.5mm. So a dimension specified as 15.0mm may range between 14.5mm and 15.5mm. Other general tolerances can be applied to features such as angles, drilled and punched holes, castings，forgings, weld beads and fillets.
When specifying a tolerance for a component, reference can be made to previous drawings or general engineering practice. Tolerances are typically specified in bands as defined in British or ISO standards.

Standard Fits for Holes and Shafts
A standard engineering ask is to determine tolerances for a cylindrical component, e.g. a shaft, fitting or rotating inside a corresponding cylindrical component or hole. The tightness of fit will depend on the application. For example, a gear located onto a shaft would require a “tight” interference fit, where the diameter of the shaft is actually slightly greater than the inside diameter of the gear hub in order to be able to transmit the desired torque. Alternatively, the diameter of a journal bearing must be greater than the diameter of the shaft to allow rotation. Given that it is not economically possible to manufacture components to exact dimensions, some variability in sizes of both the shaft and hole dimension must be specified. However, the range of variability should not be so large that the operation of the assembly is impaired. Rather than having an infinite variety of tolerance dimensions that could be specified, national and international standards have been produced defining bands of tolerances. To turn this information into actual dimensions corresponding tables exist，defining the tolerance levels for the size of dimension under consideration.
Size：a number expressing in a particular unit the numerical value of a dimension.
Actual size：the size of a part as obtained by measurement.
Limits of size：the maximum and minimum sizes permitted for a feature.
Maximum limit of size the greater of the two limits of size.
Minimum limit of size：the smaller of the two limits of size.
Basic size：the size by reference to which the limits of size are fixed.
Deviation：the algebraic difference between a size and the corresponding basic size.
Actual deviation：the algebraic difference between the actual size and the corresponding basic size.
Upper deviation：the algebraic difference between the maximum limit of size and the corresponding basic size.
Lower deviation：the algebraic difference between the minimum limit of size and the corresponding basic size.
Tolerance：the difference between the maximum limit of size and the minimum limit of size.
Shaft：the term used by convention to designate all external features of a part.
Hole：the term used by convention to designate all internal features of a part.
Heat Treatment of Metal
The generally accepted definition for heat treating metals and metal alloys is “heating and cooling a solid metal or alloy in a way so as to obtain specific conditions and I or properties.” Heating for the sole purpose of hot working(as in forging operations) is excluded from this definition． Likewise，the types of heat treatment that are sometimes used for products such as glass or plastics are also excluded from coverage by this definition．

Transformation Curves
The basis for heat treatment is the time-temperature-transformation curves or TTT curves where，in a single diagram all the three parameters are plotted．Because of the shape of the curves，they are also sometimes called C-curves or S-curves．
To plot TTT curves，the particular steel is held at a given temperature and the structure is examined at predetermined intervals to record the amount of transformation taken place．It is known that the eutectoid steel (T80) under equilibrium conditions contains， all austenite above 723℃， whereas below，it is pearlite．To form pearlite，the carbon atoms should diffuse to form cementite．The diffusion being a rate process，would require sufficient time for complete transformation of austenite to pearlite ．From different samples，it is possible to note the amount of the transformation taking place at any temperature．These points are then plotted on a graph with time and temperature as the axes．

Classification of Heat Treating Processes
In some instances，heat treatment procedures are clear cut in terms of technique and application．whereas in other instances，descriptions or simple explanations are insufficient because the same technique frequently may be used to obtain different objectives ．For example, stress relieving and tempering are often accomplished with the same equipment and by use of identical time and temperature cycles．The objectives，however，are different for the two processes ．
The following descriptions of the principal heat treating processes are generally arranged according to their interrelationships．
Normalizing consists of heating a ferrous alloy to a suitable temperature (usually 50°F to 100 °F or 28 ℃ to 56℃) above its specific upper transformation temperature. This is followed by cooling in still air to at least some temperature well below its transformation temperature range．For low-carbon steels．the resulting structure and properties are the same as those achieved by full annealing ；for most ferrous alloys, normalizing and annealing are not synonymous.
Normalizing usually is used as a conditioning treatment, notably for refining the grain of steels that have been subjected to high temperatures for forging or other hot working operations．The normalizing process usually is succeeded by another heat treating operation such as austenitizing for hardening, annealing，or tempering.
Annealing is a generic term denoting a heat treatment that consists of heating to and holding at a suitable temperature followed by cooling at a suitable rate．It is used primarily to soften metallic materials，but also to simultaneously produce desired changes in other properties or in microstructure．The purpose of such changes may be，but is not confined to, improvement of machinability, facilitation of cold work ( known as in-process annealing)，improvement of mechanical or electrical properties, or to increase dimensional stability．When applied solely to relieve stresses, it commonly is called stress-relief annealing, synonymous with stress relieving．
When the term “annealing is applied to ferrous alloys without qualification, full annealing is implied．This is achieved by heating above the alloy’s transformation temperature，then applying a cooling cycle which provides maximum softness．This cycle may vary widely, depending on composition and characteristics of the specific alloy．
Quenching is the rapid cooling of a steel or alloy from the austenitizing temperature by immersing the workpiece in a liquid or gaseous medium．Quenching media commonly used include water，5% brine，5% caustic in an aqueous solution，oil，polymer solutions，or gas(usually air or nitrogen)．
Selection of a quenching medium depends largely on the hardenability of the material and the mass of the material being treated(principally section thickness)．
The cooling capabilities ofthe above-listed quenching media vary greatly．In selecting a quenching medium, it is best to avoid a solution that has more cooling power than is needed to achieve the results，thus minimizing the possibility of cracking and warp of the parts being treated．Modifications of the term quenching include direct quenching，fog quenching，hot quenching，interrupted quenching selective quenching，spray quenching, and time quenching．

Tempering ．In heat treating of ferrous alloys ，tempering consists of reheating the austenitized and quench-hardened steel or iron to some preselected temperature that is below the lower transformation temperature (generally below 1300°F or 705℃ ) ．Tempering offers a means of obtaining various combinations of mechanical properties．Tempering temperatures used for hardened steels are often no higher than 300°F (150℃)．The term “tempering” should not be confused with either process annealing or stress relieving．Even though time and temperature cycles for the three processes may be the same，the conditions of the materials being processed and the objectives may be different．
Stress Relieving．Like tempering, stress relieving is always done by heating to some temperature below the lower transformation temperature for steels and irons ．For nonferrous metals，the temperature may vary from slightly above room temperature to several hundred degrees，depending on the alloy and the amount of stress relief that is desired．
The primary purpose of stress relieving is to relieve stresses that have been imparted to the workpiece from such processes as forming, rolling，machining or welding．The usual procedure is to heat workpieces to the pre-established temperature long enough to reduce the residual stresses (this is a time-and temperature-dependent operation) to an acceptable level；this is followed by cooling at a relatively slow rate to avoid creation of new stresses．

Introduction to CAD/CAM
Throughout the history of our industrial society, many inventions have been patented and whole new technologies have evolved. Perhaps the single development that has impacted manufacturing more quickly and significantly than any previous technology is the digital computer. Computers are being used increasingly for both design and detailing of engineering components in the drawing office.
Computer-aided design (CAD) is defined as the application of computers and graphics software to aid or enhance the product design from conceptualization to documentation. CAD is most commonly associated with the use of an interactive computer graphics system, referred to as a CAD system. Computer-aided design systems are powerful tools and are used in the mechanical design and geometric modeling of products and components.
There are several good reasons for using a CAD system to support the engineering design function：
⑴ To increase the productivity
⑵ To improve the quality of the design
⑶ To uniform design standards
⑷ To create a manufacturing data base
⑸ To eliminate inaccuracies caused by hand-copying of drawings and inconsistency between drawings
Computer-aided manufacturing (CAM) is defined as the effective use of computer technology in manufacturing planning and control. CAM is most closely associated with functions in manufacturing engineering, such as process and production planning, machining, scheduling, management, quality control, and numerical control (NC) part programming. Computer-aided design and computer-aided manufacturing are often combined into CAD/CAM systems.

This combination allows the transfer of information from the design stage into the stage of planning for the manufacturing of a product, without the need to reenter the data on part geometry manually. The database developed during CAD is stored; then it is processed further, by CAM, into the necessary data and instructions for operating an controlling production machinery, material-handling equipment, and automated testing and inspection for product quality.

Rationale for CAD/CAM
The rationale for CAD/CAM is similar to that used to justify any technology-based improvement in manufacturing. It grows out of a need to continually improve productivity, quality and competitiveness. There are also other reasons why a company might make a conversion from manual processes to CAD/CAM：

⑴ Increased productivity
⑵ Better quality
⑶ Better communication
⑷ Common database with manufacturing
⑸ Reduced prototype construction costs
⑹ Faster response to customers

CAD/CAM Hardware
The hardware part of a CAD/CAM system consists of the following components：(1) one or more design workstations,(2) digital computer, (3) plotters, printers and other output devices, and (4) storage devices. In addition, the CAD/CAM system would have a communication interface to permit transmission of data to and from other computer systems, thus enabling some of the benefits of computer integration.
The workstation is the interface between computer and user in the CAD system. The design of the CAD workstation and its available features have an important influence on the convenience, productivity, and quality of the user’s output. The workstation must include a graphics display terminal and a set of user input devices. CAD/CAM applica