In highly coloured drawings it is usual to show holes by a light wash of black.

In drawings where no parts are shown in section, it may be necessary to use a little colour to denote the metals, and this is best done by a narrow band of colour inside the outer lines. Generally speaking, all colours should be used in as light washes as possible; but in drawings where parts both in section and not in section are coloured, it is usual to indicate the sectioned parts by using the colour of a darker tone. All sectioned parts should be coloured, either by washing all over or by hatching the parts by broad diagonal lines of colour. The latter is usual

for drawings of larger scale, as large patches of colour are to be avoided. Drawings which are not coloured should have their sectioned parts shown by ink lines drawn diagonally, as in the following figures (see p. 119).

The shading of inclined or circular parts is affected either by "line shading or "colour shading," in the former a flat incline is shown by drawing a number of equally spaced parallel lines (Fig. 102a), and a circular part by lines which are drawn closer

together as they approach the outside lines of the diameter (Fig. 1026). Colour shading consists of graduating the tone of the colour from light to dark to produce the same effect, this is best done by a wash of black or Indian ink, and afterwards washing over with a light uniform tint of the required colour. General arrangement drawings are frequently highly coloured and shaded.

The student should occasionally practise these methods of shading, in order that he may be able to use them if required. In drawing offices the practice as to colouring and shading varics

considerably, and generally depends upon the amount of time at the disposal of the draughtsman. The same remark applies to the title of a drawing; this should always be clearly written or printed in a conspicuous place, but the question of whether it shall be plainly or elaborately done also depends upon special circumstances.

(9) Design. The first principle of design is to arrange the form of a machine or structure according to the work it is required to do, and the second to proportion its different parts in accordance with the known forces it will have to resist, and the resistance of the material of which it is made. The former of these is probably a matter of personal intuition, and the latter is considered as shown more fully in later pages. But in addition to these, most engineering machines have to be designed by considerations of practical workshop conveniences and possibilities, of the cost of construction, of convenience in repairing, and of simplicity and symmetry of form. The full importance of these points cannot be recognised except by those possessed of practical workshop experience; but if the student will carefully consider the following remarks, he will be materially assisted in working through the remaining sections of this book, and in approaching the problem in an intelligent manner.

(10) Cost of Construction. - First of all it should be understood that the design generally most approved by the engineer is that which is the most easily and cheaply constructed, and which offers the best facilities for repair. There are many arrangements of forgings and castings which may appear at first sight very suitable, but which are more difficult to forge or cast than some other design not apparently so convenient and neat, and in the same way a part may be designed in order to permit of machining in a certain way, although by so doing it may even be wanting in good proportions and form. It is well known that "turning" is the cheapest kind of machining, and, as a result, there are a large number of parts which are designed with the chief object of getting in as much turning and as little of other machining as possible, although other considerations would seem to suggest quite another form of construction. Designers also aim at keeping the amount of machining as small as possible, for which object the parts of castings which require facing for fitting together or to some other piece, are made with projecting bosses, lugs, or strips which allow of machining without touching the main body of the casting. It is only by considerations such as these that many of the designs in engines and machines can be explained.

(11) Proportions of Parts.-Then, again, with regard to the

proportions of different parts. It may often be possille to determine exactly the forces which will act upon any given piece of a machine, and then by knowing the safe working stress 10 which it may be subjected to arrange the proportions accordingly. The strength and ordinary working stresses of different materials are given in the following pages, but if any special material is used the value of its resistance and its safe working stress should be determined before proceeding to the design.

But there are undoubtedly many parts of machines where it is practically impossible to find the stresses which act upon them when working, as, for instance, in lathes and other shop tools, or in spinning and weaving machinery, and, similarly, there are other parts, such as brackets and pedestals for shaft bearings, in which, although the forces acting upon them may be measurable, yet their resistance to these forces is far too difficult for ordinary calculation owing to their complex form.

Such parts have then to be proportioned in accordance with practical experience, by knowing what has been allowed in similar cases before, and also by such assistance as can be gained by a general consideration of symmetrical proportion and convenient arrangement. It is probable that all such parts are really abnormally strong for the work they have to do, but it must also be remembered that stiffness and solidity may be absolutely necessary, to obtain which much metal is required.

(12) Castings.—And, still further, it should be noticed that castings are invariably much heavier and stronger than forgings, on account of practical foundry difficulties, which soon reach a limit of possible thinness. The metal must also be more equally spread over the different parts on account of the stresses produced when cooling, and in order to ensure an equal cooling of the whole mass. Notice also that the corners of castings are always

well rounded.

These are, after all, but a few of the many points which have to be considered in engineering design, but so far as they go, they should be grasped by the student. He will then, perhaps, be better able to understand why so many machines, which come under his notice, appear so abnormally strong, heavy, and ugly, and to see how impossible it is to lay down hard and fast rules for the proportions of each part, on account of the different degrees of importance which may be given to the various points which have been mentioned. But he should also recognise the necessity of approaching the question of design in as scientific and thoughtful a manner as possible, of bringing to bear upon it all the knowledge he possesses of the strength of materials and their most efficient use, and of seeing that he should not neces

sarily be tied by any particular proportion or arrangement when several ways are open, unless his choice be directed by sound and sensible considerations. It would be a most invaluable result if every student of machine design would take care never to draw any part, no matter how unimportant, without having fully considered the whole "why and wherefore" of the part, and being prepared to sensibly justify his design if challenged.

THE scope of this work is not sufficient to require any detailed or advanced treatment of the strength and properties of engineering materials. But the obvious fact that the strength of machines and structures must bear some relation to the strength of the materials of which they are made, necessitates at least a clear understanding of the different forces which act upon materials, their classification and recognition, and the power possessed by the materials to resist them.

(13) Stresses in Materials.-Generally speaking, all materials of construction are called upon to resist stresses caused by their own mass as well as those produced by external forces. In the case of heavy structures, such as bridges or arches, these stresses may be very great, and in some instances may even exceed the external forces, hence they cannot then be neglected, but in most machines the masses of the parts themselves need not be considered, except when their momentum produces stress; and we only have to deal with the external forces acting upon the material, caused generally by the work the machine does, and capable of a sufficiently exact measurement.

The forces which act upon materials may be classified as follows:

(a) Tensile forces.-Those which tend to separate the particles of a body from each other by direct pulling.

EXAMPLES.-Forces in ropes, chains, belts, bolts in flanges and covers, connecting-rods during inward stroke, tie bars of roofs and bridges.

(b) Compressive forces.-Those which tend to press together the particles of a body by direct pushing.

EXAMPLES.-Forces in supporting columns, rams of pumps and presses, connecting-rods during outward stroke, struts in bridges and roofs, jibs in cranes.

(c) Shearing forces.-Those which tend to cause the particles of a body to move one over the other by direct sliding.

EXAMPLES.-Forces on rivets in riveted joints, bolts in shaft couplings, pins and cotters, shafts transmitting power.

(d) Bending or Transverse forces.-Those which tend to make a body assume a position at an angle to its former position.

EXAMPLES.-Forces acting on beams, hooks of cranes.

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(e) Torsional forces. Those which tend to separate the particles of a body by direct twisting. When a body is twisted apart, its particles separate, due to a shearing force.

EXAMPLES.-Forces in shafts transmitting power.

(14) Measurement of Forces.-It is clear that the effect produced by tensile, compressive, or shearing forces depends upon the magnitude of the forces, and the area across which they act, and is completely measured when we know, for instance, that the force is equal to, say, 5,000 lbs. or 5 tons on every square inch. But it is equally clear that the effect of a force producing, bending, or twisting depends not only upon the magnitude of the force, but also upon the distance at which it acts from the point, about which the bending or twisting is taking place. Hence, we must consider the combined effect of the force and the distance, and following the laws of mechanics we thus speak of bending moments (B. M.), and twisting moments (T. M.) Bending and twisting moments are due to external causes, and produce in the