1、Dimensional Tolerances and Surface Roughness The manufacture of machine parts is founded on the engineering drawing. Everyone engaged in manufacturing has a direct or indirect interest in understanding the meaning of the drawings on which the entire production process is established. The engineer in
2、 industry is constantly fated with the fact that no two machine parts can ever be made exactly the same. He learns that the small variations that occur in repetitive production must be considered in the design so that the tolerances placed on the dimensions will restrict the variations to acceptable
3、 limits. The tolerances provide zones in which the outline of finished part must lie. Proper tolerancing practice ensures that the finished product functions in its intended manner and operates for its expected life. A designer is well aware that the cost of a finished product can increase rapidly a
4、s the tolerances on the components are made smaller. Designers are constantly admonished to use the widest tolerances possible. Situations may arise, however, in which the relationship between the various tolerances required for proper functioning has not been fully explored. Under such conditions t
5、he designer is tempted to specify part tolerances that are unduly tight in the hope that no difficulty will arise at the time of assembly. This is obviously an expensive substitute for a more thorough analysis of the tolerancing situation. The allocation of proper production tolerances is therefore
6、a most important task if the finished product is to achieve its intended purpose and yet be economical to produce. The size of the tolerances, as specified by the designer, depends on the many conditions pertaining to the design as well as on past experience with,similar products if such experience
7、is available.A knowledge of shop processes and machine capabilities is of great assistance in helping to determine the tolerances in the most effective manner. A revision of the design may be called for if the tolerances are too small to be maintained by the equipment available for producing the dim
8、ension. Ambiguities in engineering drawing can be cause of much confusion and expense. When specifying the tolerances, the designer must keep in mind that the drawing must contain all requisite information if the designers intent is to be fully realized. The drawing must therefore give complete info
9、rmation and at the same time be as simple as possible. The detail of drawing must be capable of being universally understood. The drawing must have one and only one meaning to everyone who will use it - the design, purchasing, tool design, production, inspection, assembly, and servicing departments.
10、 Tolerances may be placed on the drawing in a number of different ways. In the unilateral system one tolerance is zero and all the variation of the dimension is given by the other tolerance. In bilateral dimensioning a mean dimension is used with plus and minus variations extending each way from the
11、 mean dimension. Unilateral tolerancing has the advantage that a tolerance revision can be made with the least disturbance to the remaining dimensions. In the bilateral system a change in the tolerances also involves a change in at least one of the mean dimensions. Tolerances can be easily changed b
12、ack and forth between unilateral and bilateral for the purpose of making calculations. A part is said to be at the maximum material condition (MMC) when the dimensions are all at the limits that will give a part containing the maximum amount of material. For a shaft or external dimension, the fundam
13、ental dimension is the largest value permitted, and all the variation, as permitted by the tolerance, serves to reduce the dimension. For a hole or internal dimension, the fundamental dimension is the smallest value permitted, and the variation as given by the tolerance serves to make the dimension
14、larger. A part is said to be at the least material condition (LMC) when the dimensions are all at the limits that give a part with the smallest amount of material. For LMC the fundamental value is the smallest for an external dimension and the largest for an internal dimension. The tolerances thus p
15、rovide parts containing larger amounts of material. Maximum material tolerances have a production advantage. For art external dimension, should the worker aim at the fundamental or largest value but form something small, the parts may be rework to bring them within acceptable limits. A worker keepin
16、g the mean dimension in mind would have smaller margins for any errors. These terms do, however, provide convenient expressions for denoting the different methods for specifying the tolerances on drawings. Dimensional variations in manufacturing are unavoidable despite all efforts to keep production
17、 conditions as constant as possible. The reasons for the variation in a chosen dimension on parts all made by the same process are of interest. The reasons can usually be grouped into two general classes: assignable cause and chance causes. Assignable causes. A small modification in the process can
18、cause variations in a dimension. A slight change in the properties of the raw material can cause a dimension to vary. Tools will wear and must be reset. Changes may occur in the speed, the lubricant, the temperature, the operator, and other conditions. A systematic search will generally bring such m
19、uses to light and steps can then be taken to have them eliminated. Chance causes. Chance causes, on the other hand, occur at random and are due to vague and unknown forces which can neither be traced nor rectified. They are inherent in the process and occur even though all conditions have been held
20、as constant as possible. When the variations due to assignable causes have been located and removed one by one, the desired state of stability or control is attained. If the variations due to chance causes are too great, it is usually necessary to move the operation to more accurate equipment rather
21、 than spend more effort in trying to improve the process. Todays technology requires that parts be specified with increasingly exact dimensions. Many parts made by different companies at widely separated locations must be interchangeable, which requires precise size specifications and production. Th
22、e technique of dimensioning parts within a required range of variation to ensure interchangeability is called tolerancing. Each dimension is allowed a certain degree of variation within a specified zone, or tolerance. For example, a parts dimension might be expressed as 20 0.50, which allows a toler
23、ance (variation in size) of 1.00 mm. A tolerance should be as large as possible without interfering with the function of the part to minimize production costs. Manufacturing costs increase as tolerances become smaller. There are three methods of specifying tolerances on dimensions: Unilateral, bilat
24、eral, and limit forms. When plus-or-minus tolerancing is used, it is applied to a theoretical dimension called the basic dimension. When dimensions can vary in only one direction from the basic dimension (either larger or smaller) tolerancing is unilateral. Tolerancing that permits variation in limi
25、t directions from the basic dimension (larger and smaller) is bilateral. Tolerances may also be given in limit form, with dimensions representing the largest and smallest sizes for a feature. Some tolerancing terminology and definitions are given below. Tolerance: the difference between the limits p
26、rescribed for a single feature. Basic size: the theoretical size, form which limits or deviations are calculated. Deviation: the difference between the hole or shaft size and the basic size. Upper deviation: the difference between the maximum permissible size of a part and its basic size. Lower devi
27、ation: the difference between the minimum permissible size of a part and its basic size. Actual size: the measured size of the finished part. Fit: the tightness between two assembled parts. The three types of fit are: clearance, interference and transition. Clearance fit: the clearance between two a
28、ssembled mating parts. Interference fit: results in an interference between the two assembled parts-the shaft is larger than the hole, requiring a force or press fit, an effect similar to welding the two parts. Transition fits: may result in either an interference or a clearance between the assemble
29、d parts-the shaft may be either smaller or larger than the hole and still be within the prescribed tolerances. Selective assembly: a method of selecting and assembling parts by trial and error and by hand, allowing parts to be made with greater tolerances at less cost as a compromise between a high
30、manufacturing accuracy and ease of assembly. The basic hole system utilizes the smallest hole size as the basic diameter for calculating tolerances and allowances. The basic hole system is efficient when standard drills, reamers, and machine tools are available to give precise hole sizes. The smalle
31、st hole size is the basic diameter bemuse a hole can be enlarged by machining but not reduced in size. The basic shaft system is applicable .when shafts are available in highly precise standard sizes. The largest diameter of the shaft is the basic diameter for applying tolerances and allowances. The largest shaft size is used as the basic diameter because shafts can be machined to smaller size but not enlarged. International tolerance (IT) grade: a series of tolerances that vary with basic size to provide a uniform level of accuracy within a given grade. There are I8 IT
