Oblique Projection (see Fig. 6), sometimes called cavalier projection, is based on the prin ciple that one face of the object is placed paral lel to the plane of projection. The projectors make angles of 45 degrees with it in any direc tion. Thus lines perpendicular to the plane of projection will be projected in their true length. Oblique projection is similar to isometric drawing because it has three axes representing three mutually perpendicular lines; upon these lines measurements can be made, and two of the axes will always be at right angles to each other and will represent lines parallel to the plane of projection. The third axis may be repre sented at any angle, but 30 degrees is generally used. One face of the object will be repre sented without distortion. This is one of the advantages of oblique projection over isometric, particularly in representing objects with circu lar or irregular outlines. There are general rules in connection with drawing of this sort: First, place the object so that its irregular outline or contour will be parallel to the plane of pro jection; and second, if possible have the long est dimension parallel to the plane of projec tion.
Axonometric principle of isometric projection was shown to be based on the orthographic projection of a cube whose axis is perpendicular to the plane of projection. The cube might be placed in any other position with respect to the plane of projection, so that its projection would show three faces, and the angles and proportionate foreshortening of the axes used as a basis for a pictorial tion. This is known as axonometric (or axometric) projection. This is the general system and it will be noticed that isometric projection is a special case in which the axes are foreshortened equally. When two axes This is based on oblique projection. The object is placed with its vertical axes parallel to a ver tical plane of projection; and it is rotated about this axis until the horizontal axes make angles to the plane of projection, whose tangents are respectively one-third and three. The object is then projected on the vertical plane of pro jection, as shown (Fig. 8), by profile lines which make an angle to the plane of projection whose tangent is one-sixth. The result is a pleasing representation of the object.
Machine Design.— A machine is a combina tion of mechanical movements constructed to transform energy directly or indirectly from natural sources Into useful work. The design must be thought out with a view toward the economy of this transformation together with the economic manufacture of the machine. It may therefore be stated that machine design is a problem involving the proper co-ordination of the following elements: Required work; available power; mechanism or kinematic mo are equal and the third unequal the system is called projection, an example of which is shown in Fig. 7. Here the ratios are
and this makes the tangents of the angles one-eighth and seven-eighths as shown.
Other positions of the object, which would show less distortion, can be found but are not often used because there is greater difficulty in execution.
In drawing the crystal figures in mineralogy a system known as eclinographics is used.
tion; performance and limiting proportions; the nature and magnitude of the stresses acting on the various members of the machine; the materials and proportions of the several mem bers, as determined by the particular stresses: wear; stiffness; methods of manufacture and other commercial conditions, such as the inter changeability of parts to reduce the cost of both manufacture and maintenance; the general ar rangement as required for erection, easy repair_ oiling and adjustments.
There are, of course, no set rules which can be laid down for the method of procedure in completing the design of a machine, but there are right and wrong ways of attacking the problem, and since the same principles of de sign are applicable to all machines the follow ing general plan of solution is given.
1. Given— (a) Kind of work to be done.
(b) Power available, steam, gas, electric or hydraulic and the method of application of the power to the machine, i.e., whether belt or direct.
(c) Capacity and limiting proportions.
1. Determine the kinematic train with which to connect the availablepower so that the re quired work may be produced with a minimum energy loss. The design and selection of a combination of material elements so connected that motion in any one involves definite, rela tion, constrained motion of the others for a given machine, is governed by the manner in which the energy is supplied and the character of the work to be performed; for the energy may be supplied in one form of motion and the work done with quite a different one. If a mechanism which will accomplish the desired results already exists the problem is one of se lection and arrangement of parts. But if a new type of machine is desired or a new mechanism necessary, the problem may become one of the nature of invention. While it is true that the mechanism can usually be designed without ref erence to the energy transmitted it is neces sary to analyze the kinematic energy cycles be fore any definite dimensions of the parts of the mechanism can be fixed. Furthermore as stated above the methods and available facili ties of construction control the design to a cer tain extent. Lay out the centre lines of the ma chine according to the above determinations and check for limitations of motion, interfer ence, etc.