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172项重大水利工程累计开工143项

类型:奇幻地区:莫桑比克剧发布:2020-09-28 01:09:37

《重庆时时开奖结果预测》剧情介绍

Draught, or the taper required to allow patterns to be drawn readily, is another of those indefinite conditions in pattern-making that must be constantly decided by judgment and experience. It is not uncommon to find rules for the draught of patterns laid down in books, but it would be difficult to find such rules applied. The draught may be one-sixteenth of an inch to each foot of depth, or it may be one inch to a foot of depth, or there may be no draught whatever. Any rule, considered aside from specified conditions, will only confuse a learner. The only plan to understand the proper amount of draught for patterns is to study the matter in connection with patterns and foundry operations.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.

Examining machines made by the best designers, it will be found that their dimensions bear but little if any reference to calculated strains, especially in machines involving rapid motion. Accidents destroy constants, and a draughtsman or designer who does not combine special and experimental knowledge with what he may learn from general sources, will find his services to [89]be of but little value in actual practice.

CHAPTER II. MECHANICAL ENGINEERING.

These divisions of machinery will next be treated of separately, with a view of making the classification more clear, and to explain the principles of operation in each division. This dissertation will form a kind of base upon which the practical part of the treatise will in a measure rest. It is trusted that the reader will carefully consider each proposition that is laid down, and on his own behalf pursue the subjects farther than the limits here permit.

CHAPTER XXIV. TRIP-HAMMERS.Tempering may be called a mystery of the smith-shop; this operation has that attraction which characterises every process that is mysterious, especially such as are connected with, or belong to mechanical manipulation. A strange and perhaps fortunate habit of the mind is to be greatly interested in what is not well understood, and to disregard what is capable of plain demonstration.

When we look at a steam-engine there are certain impressions conveyed to the mind, and by these impressions we are governed in a train of reflection that follows. We may conceive of a cylinder and its details as a complete machine with independent functions, or we can conceive of it as a mechanical device for transmitting the force generated by a boiler, and this conception might be independent of, or even contrary to, specific knowledge that we at the same time possessed; hence the importance of starting with a correct idea of the boiler being, as we may say, the base of steam machinery.Let the reader compare a hammer with a wheel and axle, inclined plane, screw, or lever, as an agent for concentrating and applying power, noting the principles of its action first, and then considering its universal use, and he will conclude that, if there is a mechanical device that comprehends distinct principles, that device is the common hammer. It seems, indeed, to be one of those provisions to meet a human necessity, and without which mechanical industry could not be carried on. In the manipulation of nearly every kind of material, the hammer is continually necessary in order to exert a force beyond what the hands may do, unaided by mechanism to multiply their force. A carpenter in driving a spike requires a force of from one to two tons; a blacksmith requires a force of from five pounds to five tons to meet the requirements of his work; a stonemason applies a force of from one hundred to one thousand pounds in driving the edge of his tools; chipping, calking, in fact nearly all mechanical operations, consist more or less in blows, such blows being the application of accumulated force expended throughout a limited distance.

The expense of forming pattern-moulds may be considered as divided between the foundry and pattern-shop. What a pattern-maker saves a moulder may lose, and what a pattern-maker spends a moulder may save; in other words, there is a point beyond which saving expense in patterns is balanced by extra labour and waste in moulding鈥攁 fact that is not generally realised because of inaccurate records of both pattern and foundry work. What is lost or saved by judicious or careless management in the matter of patterns and moulding can only be known to those who are well skilled in both moulding and pattern-making. A moulder may cut all the fillets in a mould with a trowel; he may stop off, fill [94] up, and print in, to save pattern-work, but it is only expedient to do so when it costs very much less than to prepare proper patterns, because patching and cutting in moulds seldom improves them.The feed adjustment of the tool being intermittent and the amount of cutting continually varying, this movement should be performed by hand, so as to be controlled at will by the sense of feeling. The same rule applies to the adjustment of the rack for spacing; being intermittent and irregular as to time, this movement should also be performed by hand. The speed of the cutting movement is known from ordinary practice to be from sixteen feet to twenty feet a minute, and a belt two and a half inches wide must move two hundred feet a minute to propel an ordinary metal cutting tool, so that the crank movement or cutter movement must be increased by gearing until a proper speed of the belt is reached; from this the speed of intermediate movers will be found.

The engineering apprentice, as a rule, has a desire to make drawings as soon as he begins his studies or his work, and there is not the least objection to his doing so; in fact, there is a great deal gained by illustrating movements and the details of machinery at the same time of studying the principles. Drawings if made should always be finished, carefully inked in, and memoranda made on the margin of the sheets, with the date and the conditions under which the drawings were made. The sheets should be of uniform size, not too large for a portfolio, and carefully preserved, no matter how imperfect they may be. An apprentice who will preserve his first drawings in this manner will some day find himself in possession of a souvenir that no consideration would cause him to part with.[100]3. The force of the water is greatest by its striking against planes at right angles to its course.

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Power-hammers operated by crank motion, adapted to the lighter kinds of work, are now commonly met with in the forging-shops of engineering establishments. They are usually of very simple construction, and I will mention only two points in regard to such hammers, which might be overlooked by an apprentice in examining them.

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