时间：<2020-08-06 03:13:55 作者：3U大尚国际eUr 浏览量：9777
Irregularity of cooling may be the result of unequal conducting power in different parts of a mould or cores, or it may be  from the varying dimensions of the castings, which contain heat as their thickness, and give it off in the same ratio. As a rule, the drag or bottom side of a casting cools first, especially if a mould rests on the ground, and there is not much sand between the castings and the earth; this is a common cause of unequal cooling, especially in large flat pieces. Air being a bad conductor of heat, and the sand usually thin on the cope or top side, the result is that the top of moulds remain quite hot, while at the bottom the earth, being a good conductor, carries off the heat and cools that side first, so that the iron 'sets' first on the bottom, afterwards cooling and contracting on the top, so that castings are warped and left with inherent strains.
Most of the difficulties which formerly pertained to drilling are now removed by machine-made drills which are manufactured and sold as an article of trade. Such drills do not require dressing and tempering or fitting to size after they are in use, make true holes, are more rigid than common solid shank drills, and will drill to a considerable depth without clogging.
CHAPTER XXIX. FITTING AND FINISHING.It is strange, considering the simplicity of construction and the very important office filled by machines for cutting on plane surfaces, that they were not sooner invented and applied in metal work. Many men yet working at finishing, can remember when all flat surfaces were chipped and filed, and that long after engine lathes had reached a state of efficiency and were generally employed, planing machines were not known. This is no doubt to be accounted for in the fact that reciprocal movement, except that produced by cranks or eccentrics, was unknown or regarded as impracticable for useful purposes until late years, and when finally applied it was thought impracticable to have such movements operate automatically. This may seem quite absurd to even an apprentice of the present time, yet such reciprocating movement, as a mechanical problem, is by no means so simple as it may at first appear.
There is no use in entering upon detailed explanations of what a learner has before him. Shafts are seen wherever there is machinery; it is easy to see the extent to which they are employed to transmit power, and the usual manner of arranging them. Various text-books afford data for determining the amount of torsional strain that shafts of a given diameter will bear; explain that their capacity to resist torsional strain is as the cube of the diameter, and that the deflection from transverse strains is so many degrees; with many other matters that are highly useful and proper to know. I will therefore not devote any space to these things here, but notice some of the more obscure conditions that pertain to shafts, such as are demonstrated by practical experience rather than deduced from mathematical data. What is said will apply especially to what is called line-shafting for conveying and distributing power in machine-shops and other manufacturing establishments. The following propositions in reference to shafts will assist in understanding what is to follow:】【A pair of spring calipers will illustrate this principle. The points are always steady, because the spring acting continually in one direction compensates the loose play that may be in the screw. In a train of tooth wheels there is always more or less play between the teeth; and unless the wheels always revolve in one direction, and have some constant resistance offered to their motion, 'backlash' or irregular movement will take place; but if there is some constant and uniform resistance such as a spring would impart, a train of wheels will transmit the slightest motion throughout.Chasing produces screws true with respect to their axis, and is the common process of threading all screws which are to have a running motion in use, either of the screw itself, or the nut.
This failure of laws to regulate apprenticeship, which facts fully warrant us in assuming, is due in a large degree to the impossibility of applying general rules to special cases; it may be attributed to the same reasons which make it useless to fix values or the conditions of exchange by legislation. What is required is that the master, the apprentice, and the public should understand the true relations between them—the value of what is given and what is received on both sides. When this is understood, the whole matter will regulate itself without any interference on the part of the law.
In working to regular scales, such as one-half, one-eighth, or one-sixteenth size, a good plan is to use a common rule, instead of a graduated scale. There is nothing more convenient for a mechanical draughtsman than to be able to readily resolve dimensions into various scales, and the use of a common rule for fractional scales trains the mind, so that computations come naturally, and after a time almost without effort. A plain T square, with a parallel blade fastened on the side of the head,  but not imbedded into it, is the best; in this way set squares can pass over the head of a T square in working at the edges of the drawing. It is strange that a draughting square should ever have been made in any other manner than this, and still more strange, that people will use squares that do not allow the set squares to pass over the heads and come near to the edge of the board.A safe rule will be to assume that machinery mainly used and seen by the skilled should be devoid of ornament, and that machinery seen mainly by the unskilled, or in public, should have some ornament. Steam fire engines, sewing machines, and works of a similar kind, which fall under the inspection of the unskilled, are usually arranged with more or less ornament.
CHAPTER IX. WIND-POWER.To sum up, it is supposed to have been determined by inductive reasoning, coupled with some knowledge of mechanics, that a steam hammer, to give a dead blow, requires the following conditions in the valve gearing: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—a 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  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.
There were three kinds of strain mentioned—torsional, deflective, and accidental. To meet these several strains the same means have to be provided, which is a sufficient size and strength to resist them; hence it is useless to consider each of these different strains separately. If we know which of the three is greatest, and provide for that, the rest, of course, may be disregarded. This, in practice, is found to be accidental strains to which shafts are in ordinary use subjected, and they are usually made, in point of strength, far in excess of any standard that would be fixed by either torsional or transverse strain due to the regular duty performed.Slotting machines with vertical cutting movement differ from planing machines in several respects, to which attention may be directed. In slotting, the tools are in most cases held rigidly and do not swing from a pivot as in planing machines. The tools are held rigidly for two reasons; because the force of gravity cannot be employed to hold them in position at starting, and because the thrust or strain of cutting falls parallel, and not transverse to the tools as in planing. Another difference between slotting and planing is that the cutting movement is performed by the tools and not by movement of the material. The cutting strains are also different, falling at right angles to the face of the table, in the same direction as the force of gravity, and not parallel to the face of the table, as in planing and  shaping machines.