英文：
Engineering T olerance
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 asmbled 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 process. 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 performa
nce.
Component Tolerances
Control of dimensions is necessary in order to ensure asmbly 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 .冰雪奇缘的故事
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.
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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
民间补肾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 asmbly 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.
洁银牙膏
甘蔗种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 ud by convention to designate all external features of a part.
Hole：the term ud by convention to designate all internal features of a part.
Heat Treatment of Metal禁毒宣传图片
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朝三暮四文言文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 purpo of hot working(as in forging operations) is excluded from this definition．Likewi，the types of heat
treatment that are sometimes ud 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．Becau 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 diffu 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．The points are then plotted on a graph with time an d tem perature as th e ax es．
Classification of Heat Treating Process
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 becau the same technique frequently may be ud to obtain different objectives ．For example, stress relieving and tempering are often accomplished with the same equipment and by u of identical time and temperature cycles．The objectives，however，are different for th e two process ．
The following descriptions of the principal heat treating process 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 tho achieved by full annealing ；for most ferrous alloys, normalizing and annealing are n ot syn onym ou s.
Normalizing usually is ud 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 ud primarily to soften metallic materials，but also to simultaneously produce desired changes in other properties or in microstructure．The purpo 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 increa dimensional stability．When applied solely to relieve stress, it commonly is called stress-relief annealing, synonymous with stress relieving．
When the term “anneali ng 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 gaous medium．Quenching media commonly ud 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
