时间：<2020-07-06 07:24:19 作者：RO相术小说SfP 浏览量：9777
Or, more briefly stated—
In order to render what is said of shop processes more easily understood, it will be necessary to change the order in which they have been named. Designing, and many matters connected with the operation of machines, will be more easily learned and understood after having gone through with what may be called the constructive operations, such as involve manual skill.It is no uncommon thing for a skilled latheman to lock the slide rest, and resort to hand tools on many kinds of work when he is in a hurry.The advantages gained by milling, as stated, are speed, duplication, and accuracy; the disadvantages are the expense of preparing tools and their perishability.
(1.) What is principal among the details of steam machinery?—(2.) What has been the most important improvement recently made in steam machinery?—(3.) What has been the result of expansive engines generally stated?—(4.) Why has water proved the most successful among various expansive substances employed to develop power?—(5.) Why does a condensing engine develop more power than a non-condensing one?—(6.) How far back from its development into power can heat be traced as an element in nature?—(7.) Has the property of combustion a common source in all substances?Machine-drawing may in some respects be said to bear the same relation to mechanics that writing does to literature; persons may copy manuscript, or write from dictation, of what they do not understand; or a mechanical draughtsman may make drawings of a machine he does not understand; but neither such writing or drawing can have any value beyond that of ordinary labour. It is both necessary and expected that a draughtsman shall understand all the various processes of machine construction, and be familiar with the best examples that are furnished by modern practice.
6. The loss of power in a steam-engine arises from the heat carried off in the exhaust steam, loss by radiation, and the friction of the moving parts.In handling a weight with the hands it is carefully raised, and laid down with care, but moved as rapidly as possible throughout the intervening distance; this lesson of nature has not been disregarded. We find that the attention of engineers has been directed to this principle of variable speed to be controlled at will. The hydraulic cranes of Sir William Armstrong, for example, employ this principle in the most effective manner, not only securing rapid transit of loads when lifted, but depositing or adjusting them with a care and precision unknown to mechanism positively geared or even operated by friction brakes.To this proposition another may be added, that shop processes may be systematised or not, as they consist in duplication, or the performance of certain operations repeatedly in the same manner.  It has been shown in the case of patterns that there could be no fixed rules as to their quality or the mode of constructing them, and that how to construct patterns is a matter of special knowledge and skill.
CHAPTER XXXVI. SCREW-CUTTING.
This article has been introduced, not only to give a true understanding of the effect and value of machine combination, but to caution against a common error of confounding machine combination with invention.
The life of George Stephenson proves that notwithstanding the novelty and great importance of his improvements in steam transit, he did not "discover" these improvements. He did not discover that a floating embankment would carry a railway across Chat Moss, neither did he discover that the friction between the wheels of a locomotive and the rails would enable a train to be drawn by tractive power alone. Everything connected with his novel history shows that all of his improvements were founded upon a method of reasoning from principles and generally inductively. To say that he "discovered" our railway system, according to the ordinary construction of the term, would be to detract from his hard and well-earned reputation, and place him among a class of fortunate schemers, who can claim no place in the history of legitimate engineering.
One of the problems connected with the handling of material is to determine where hand-power should stop and motive-power begin—what conditions will justify the erection of cranes, hoists, or tramways, and what conditions will not. Frequent mistakes are made in the application of power when it is not required, especially for handling material; the too common tendency of the present day being to apply power to every purpose where it is possible, without estimating the actual saving that, may be effected. A common impression is that motive power, wherever applied to supplant hand labour in handling material, produces a gain; but in many cases the  fallacy of this will be apparent, when all the conditions are taken into account.There is nothing more interesting, or at the same time more useful, in the study of mechanics, than to analyse the action of cutting machines or other machinery of application, and to ascertain in examples that come under notice whether the main object of a machine is increased force, more accurate guidance, or greater speed than is attainable by hand operations. Cutting machines as explained may be directed to either of these objects singly, or to all of them together, or these objects may vary in their relative importance in different operations; but in all cases where machines are profitably employed, their action can be traced to one or more of the functions named.