🔥重庆时时彩10月29日开奖号码: It is not assumed that shaded elevations should not be made, nor that ink shading should not be learned, but it is thought best to point out the greater importance of other kinds of drawing, too often neglected to gratify a taste for picture-making, which has but little to do with practical mechanics.。Considering material progress as consisting primarily in the demonstration of scientific truths, and secondly, in their application to useful purposes, we can see the position of the engineer as an agent in this great work of reconstruction now going on around us. The position is a proud one, but not to be attained except at the expense of great effort, and a denial of everything that may interfere with the acquirement of knowledge during apprenticeship and the study which must follow. 已获得职业资格证书
🔥重庆时时彩10月29日开奖号码First. Motive machinery for utilising or converting the natural forces.
Judged upon theoretical grounds, and leaving out the mechanical conditions of operation, it would at once be conceded that a proper plan would be to move the lightest body; that is, if the tools and their attachments were heavier than the material to be acted upon, then the material should be moved for the cutting action, and vice versa. But in practice there are other conditions to be considered more important than a question of the relative weight of reciprocating parts; and it must be remembered that in solving any problem pertaining to machine action, the conditions of operation are to be considered first and have precedence over problems of strain, arrangement, or even the general principles of construction; that is, the conditions of operating must form a base from which proportions, arrangements, and so on, must be deduced. A standard planing machine, such as is employed for most kinds of work, is arranged with a running platen or carriage upon which the material is fastened and traversed beneath the cutting tools.  The uniformity of arrangement and design in machines of this kind in all countries wherever they are made, must lead to the conclusion that there are substantial reasons for employing running platens instead of giving a cutting movement to the tools.
For implements procure two drawing-boards, forty-two inches long and thirty inches wide, to receive double elephant paper; have the boards plain without cleets, or ingenious devices for fastening the paper; they should be made from thoroughly seasoned lumber, at least one and one-fourth inches thick; if thinner they will not be heavy enough to resist the thrust of the T squares.
A planing machine with a running platen occupies nearly twice as much floor space, and requires a frame at least one-third longer than if the platen were fixed and the tools performed the cutting movement. The weight which has to be traversed, including the carriage, will in nearly all cases exceed what it would be with a tool movement; so that there must exist some very strong reasons in favour of a moving platen, which I will now attempt to explain, or at least point out some of the more prominent causes which have led to the common arrangement of planing machines.
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.
Machines may be defined in general terms as agents for converting, transmitting, and applying power, or motion and force, which constitute power. By machinery the natural forces are utilised, and directed to the performance of operations where human strength is insufficient, when natural force is cheaper, and when the rate of movement exceeds what the hands can perform. The term "agent" applied to machines conveys a true idea of their nature and functions.
Eighth.—Rapping plates, draw plates, and lifting irons for drawing the patterns out of the moulds; fallow and match boards, with other details that are peculiar to patterns, and have no counterparts, neither in names nor uses, outside the foundry.
(1.) Why are machine frames constructed in sections, instead of being in one piece?—(2.) Why must parts which have contact on opposite sides have specific dimensions?—(3.) What are standards of measure based upon in England, America, and France?—(4.) How can weight be measured by time?—(5.) Has the French metre proved a standard admitting of test reference?。
In a half century past all has changed; the application of the sciences, the utilisation of natural forces, manufacturing, the transportation of material, the preparation and diffusion of printed matter, and other great matters of human interest, have come to shape our laws, control commerce, establish new relations between people and countries—in short, has revolutionised the world. So rapid has been this change that it has outrun the powers of conception, and people waken as from a dream to find themselves governed by a new master.。
CHAPTER XXV. CRANK-HAMMERS.。