时间：<2020-06-04 15:42:39 作者：gU本田思域怎么样V3Z 浏览量：9777
Finally, it must constantly be borne in mind that what will be learned is no less a question of faculties than effort, and that the means of succeeding are closed to none who at the beginning form proper plans, and follow them persistently.
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.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.
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.
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.Machines and tools operating by percussive action, although they comprise a numerous class, and are applied in nearly all mechanical operations, have never received that amount of attention in text-books which the importance of the machines and their extensive use calls for. Such machines have not even been set off as a class and treated of separately, although the distinction is quite clear between machines with percussive action, and those with what may be termed direct action, both in the manner of operating and in the general plans of construction. There is, of course, no lack of formul? for determining the measure of force, and computing the dynamic effect of percussive machines acting against a measured or assumed resistance, and so on; but this is not what is meant. There are certain conditions in the operation of machines, such as the strains which fall upon supporting frames, the effect produced upon malleable material when struck or pressed, and more especially of conditions which may render percussive or positive acting machines applicable to certain purposes; but little explanation has been given which is of value to practical men.If it were not for securing vent for gas, moulds could be made from plastic material so as to produce fine castings with clear sharp outlines.
Following this proposition into modern engineering examples, we find how practice has gradually conformed to what these properties in belts suggest. Wire and other ropes of small diameter, to avoid air friction, and allowed to droop in low curves to avoid too many supporting pulleys, are now in many cases employed for transmitting power through long distances, as at Schaffhausen, in Germany. This system has been very successfully applied in some cases for distributing power in large manufacturing establishments. Belts, among which are included all flexible bands, do not afford the same facilities for taking off power at different points as shafts, but have advantages in transmitting power to portable machinery, when power is to be taken off at movable points, as in the case of portable travelling cranes, machines, and so on.In regard to the character or quality of wood patterns, they can be made, as already stated, at greater or less expense, and if necessary, capable of almost any degree of endurance. The writer has examined patterns which had been used more than two hundred times, and were apparently good for an equal amount of use. Such patterns are expensive in their first cost, but are the cheapest in the end, if they are to be employed for a large number of castings. Patterns for special pieces, or such as are to be used for a few times only, do not require to be strong nor expensive, yet with patterns, as with everything else pertaining to machinery, the safest plan is to err on the side of strength.
5. The lubrication of surfaces working under air pressure, such as the pistons and valves of engines.Designing machines must have reference to adaptation, endurance, and the expense of construction. Adaptation includes the performance of machinery, its commercial value, or what the machinery may earn in operating; endurance, the time that machines may operate without being repaired, and the constancy of their performance; expense, the investment represented in machinery.Assuming, for example, that a machine will cost as much as the wages of an attendant for one year, which is not far from an average estimate for iron working machine tools, and that interest, wear, and repairs amount to ten per cent. on this sum, then the attendance would cost ten times as much as the machine; in other words, the wages paid to a workman to attend a machine is, on an average, ten times as much as the other expenses attending its operation, power excepted. This assumed, it follows that in machine tools any improvement directed to labour saving is worth ten times as much as an equal improvement directed to the economy of first cost.
One pair each of three and one-half inch and five inch compasses, two ruling pens, two pairs of spring dividers, one for pens and one for pencils, a triangular boxwood scale, a common rule, and a hard pencil, are the essential instruments for machine-drawing. At the beginning, when "scratching out" will probably form an item in the work, it is best to use Whatman's paper, or the best roll paper, which, of the best manufacture, is quite as good as any other for drawings that are not water-shaded.There is not a more generous or kindly feeling in the world than that with which a skilled mechanic will share his knowledge with those who have gained his esteem, and who he thinks merit and desire the aid that he can give.