【正文】 d by a possible business failure. Customers expect quality in the products they buy, and if a manufacturer expects to establish and maintain a name in the business, quality control and assurance functions must be established and maintained before, throughout, and after the production process. Generally speaking, quality assurance enpasses all activities aimed at maintaining quality, including quality control. Quality assurance can be divided into three major areas. These include the following: ① Source and receiving inspection before manufacturing。 ② The diminishing crosssectional area of the specimen, suffered on account of the plastic deformation. The more plastic deformation the metal undergoes, the harder it bees, due to workhardening. The more the metal gets elongated the more its diameter (and hence, crosssectional area) is decreased. This continues until the point S is reached. After S, the rate at which the reduction in area takes place, exceeds the rate at which the stress increases. Strain bees so high that the reduction in area begins to produce a localized effect at some point. This is called necking. Reduction in crosssectional area takes place very rapidly。 ⑥ Environmental damage。 ④ Corrosion。 ② Deformation。 ④ Applied with continuous direction reversals。 ② Suddenly applied。 Strength is a measure of the ability to resist, without fails, forces which cause stresses and strains. The forces may be。 ⑦ Environment effects。 ⑤ Manufacturing。 ③ Materials selection。Design of machine and machine elements Machine design Machine design is the art of planning or devising new or improved machines to acplish specific purposes. In general, a machine will consist of a bination of several different mechanical elements properly designed and arranged to work together, as a whole. During the initial planning of a machine, fundamental decisions must be made concerning loading, type of kinematic elements to be used, and correct utilization of the properties of engineering materials. Economic considerations are usually of prime importance when the design of new machinery is undertaken. In general, the lowest overall costs are designed. Consideration should be given not only to the cost of design, manufacture the necessary safety features and be of pleasing external appearance. The objective is to produce a machine which is not only sufficiently rugged to function properly for a reasonable life, but is at the same time cheap enough to be economically feasible. The engineer in charge of the design of a machine should not only have adequate technical training, but must be a man of sound judgment and wide experience, qualities which are usually acquired only after considerable time has been spent in actual professional work. Design of machine elements The principles of design are, of course, universal. The same theory or equations may be applied to a very small part, as in an instrument, or, to a larger but similar part used in a piece of heavy equipment. In no ease, however, should mathematical calculations be looked upon as absolute and final. They are all subject to the accuracy of the various assumptions, which must necessarily be made in engineering work. Sometimes only a portion of the total number of parts in a machine are designed on the basis of analytic calculations. The form and size of the remaining parts are designed on the basis of analytic calculations. On the other hand, if the machine is very expensive, or if weight is a factor, as in airplanes, design putations may then be made for almost all the parts. The purpose of the design calculations is, of course, to attempt to predict the stress or deformation in the part in order that it may sagely carry the loads, which will be imposed on it, and that it may last for the expected life of the machine. All calculations are, of course, dependent on the physical properties of the construction materials as determined by laboratory tests. A rational method of design attempts to take the results of relatively simple and fundamental tests such as tension, pression, torsion, and fatigue and apply them to all the plicated and involved situations encountered in presentday machinery. In addition, it has been amply proved that such details as surface condition, fillets, notches, manufacturing tolerances, and heat treatment have a market effect on the strength and useful life of a machine part. The design and drafting departments must specify pletely all such particulars, must specify pletely all such particulars, and thus exercise the necessary close control over the finished product. As mentioned above, machine design is a vast field of engineering technology. As such, it begins with the conception of an idea and follows through the various phases of design analysis, manufacturing, marketing and consumerism. The following is a list of the major areas of consideration in the general field of machine design: ① Initial design conception。 ② Strength analysis。 ④ Appearance。 ⑥ Safety。 ⑨ Reliability and life。 ① Gradually applied。 ③ Applied under impact。 ⑤ Applied at low or elevated temperatures. If a critical part of a machine fails, the whole machine must be shut down until a repair is made. Thus, when designing a new machine, it is extremely important that critical parts be made strong enough to prevent failure. The designer should determ