时间：<2020-07-06 06:04:24 作者：WY请你恰恰广场舞Om8 浏览量：9777
(1.) Name some of the principal points to be kept in view in preparing designs?—(2.) Why should attempts at designing be confined to one class of machinery?—(3.) What objection exists to examining references when preparing designs?A milling tool with twenty edges should represent as much wearing capacity as a like number of separate tools, and may be said to equal twenty duplicate tools; hence, in cutting grooves, notches, or similar work, a milling tool is equivalent to a large number of duplicate single tools, which cannot be made or set with the same truth; so that milling secures accuracy and duplication, objects which are in many cases more important than speed.
3. The saving from condensation, to which steam is exposed, avoidance of heat, and the consequent contraction and expansion of long conducting pipes.As remarked in the case of other machines treated of, there is no use of describing the mechanism of trip-hammers; it is presumed that every engineer apprentice has seen trip-hammers, or can do so; and the plan here is to deal especially with what he cannot see, and would not be likely to learn by casual observation.
Having thus far treated of such general principles and facts connected with practical mechanics as might properly precede, and be of use in, the study of actual manipulation in a workshop, we come next to casting, forging, and finishing, with other details that involve manual as well as mental skill, and to which the term "processes" will apply.To learn to chip and file is indispensable, if for no other purpose, to be able to judge of the proficiency of others or to instruct them. Chipping and filing are purely matters of hand skill, tedious to learn, but when once acquired, are never forgotten. The use of a file is an interesting problem to study, and one of no little intricacy; in filing across a surface one inch wide, with a file twelve inches long, the pressure required at each end to guide it level may change at each stroke from nothing to twenty pounds or more; the nice sense of feeling which determines this is a matter of habit acquired by long practice. It is a wonder indeed that true surfaces can be made with a file, or even that a file can be used at all, except for rough work.
A cubic inch of water, by taking up a given amount of heat, is expanded to more than five hundred cubic inches of steam, at a pressure of forty-five pounds to the square inch. This extraordinary expansion, if performed in a close vessel, would exert a power five hundred times as great as would be required to force the same quantity of water into the vessel against this expansive pressure; in other words, the volume of the water when put into the vessel would be but one five-hundredth part of its volume when it is allowed to escape, and this expansion, when confined in a steam-boiler, exerts the force that is called steam-power. This force or power is, through the means of the engine and its details, communicated and applied to different kinds of work where force and movement are required. The water  employed to generate steam, like the engine and the boiler, is merely an agent through which the energy of heat is applied.The adaptation, endurance, and cost of machines in designing become resolved into problems of movements, the arrangement of parts, and proportions.The subject of steam-power, aside from its mechanical consideration, is one that may afford many useful lessons, by tracing its history and influence, not only upon mechanical industry, but upon human interests generally. This subject is often treated of, and both its interest and importance conceded; but no one has, so far as I know, from statistical and other sources, ventured to estimate in a methodical way the changes that can be traced directly and indirectly to steam-power.
The prevailing custom of casting machine frames in one piece, or in as few pieces as possible, leads to a great deal of bar-boring, most of which can be performed accurately enough by boring bars supported in and fed through bearings. By setting up temporary bearings to support boring-bars, and improvising means of driving and feeding, most of the boring on machine frames can be performed on floors or sole plates and independent of boring machines and lathes. There are but few cases in which the importance of studying the principles of tool action is more clearly demonstrated than in this matter of boring; even long practical experience seldom leads to a thorough understanding of the  various problems which it involves.
5. This rebounding action becomes less as the columns of water projected upon the wheel are increased in number and diminished in size.Impact wheels, or those driven by the percussive force of water, including the class termed turbine water-wheels, are at this time generally employed for heads of all heights.
It is evident that when the parts of a machine are joined together, each piece which has contact on more than one side must have specific dimensions; it is farther evident that as many of the joints in a machine as are to accommodate the exigencies of construction must be without space, that is, they represent continued sections of what should be solid material, if it were possible to construct the parts in that manner. This also demands specific dimensions.This object is attained in planing machines by the friction of the belts, which not only cushions the platen like a spring, but in being shifted opposes a gradually increasing resistance until the momentum is overcome and the motion reversed. By multiplying the movement of the platen with levers or other mechanism, and by reason of the movement that is attained by momentum after the driving power ceases to act, it is found practicable to have a platen 'shift its own belts,' a result that would never have been reached by theoretical deductions, and was no doubt discovered by experiment, like the automatic movement of engine valves is said to have been.
Ordinary machinery, on the reverse, is stationary, generally consumes a regular amount of power, is not subjected to such uncertain strains, and as a rule acts without its operation being controlled by the will of attendants.
In respect to repairing alone, the consideration of having the parts of machinery fitted to standard sizes is often equal to its whole value.