The word elevation, as applied to mechanical drawing, means simply a view; hence a side elevation is a side view, or an end elevation is an end view.

The word plan is employed in place of the word top; hence a plan view is a top view, or a view looking down upon the top of the piece.

A general view means a view showing the machine put together or assembled, while a detail drawing is one containing a detail, as a part of the machine or a single piece disconnected from the other parts of the whole machine.

It is obviously desirable in a mechanical drawing to present the piece of work in as few views as possible, but in all cases there must be a sufficient number to permit of the dimensions in every necessary direction to be marked on the drawing. Suppose, then, that in Figure 120 we have to represent a solid cylinder, whose length equals its diameter, and it is obvious that both the diameter and length may be marked in the one view given; hence, a second view, such as shown by the circle in Figure 121, is unnecessary, except it be to distinguish the body from a cube, in which the one view would also be sufficient whereon to mark all the dimensions necessary to enable the piece to be made. It happens, however, that a cube and a cylinder are the only two figures upon which all the dimensions can be marked on one view of the piece, and as cylindrical pieces are much more common in machine work than cubes are, it is taken for granted that, where the pieces are cylindrical, but one view shall be used, and that where they are cubes either two views shall be given, or where they are square a cross shall be marked upon the parts that are square; thus, in Figure 122, is shown a cross formed by the lines A B across the face of the drawing, which saves making a second view.

Fig. 120.

Fig. 121.

Fig. 122.

Fig. 123.

It would appear that under some conditions this might lead to error; as, for example, take the piece in Figure 123, and there is nothing to denote which is the length and which is the diameter of the piece, but there is a certain amount of custom in such cases than will usually determine this point; thus, the piece will be given a name, as pin or disk, the one denoting that its diameter is less than its length, and the other that its diameter is greater than its length. In the absence of any such name, it would be in practice assumed that it was a pin and not a disk; because, if it were a disk, it would either be named or shaded, or a second view given to show its unusual form, the disk being a more unusual form than the pin-form in mechanical structures. As an example of the use of the cross to denote a square, we have Figure 124, which represents a piece having a hexagon head, section a, a', that is rectangular, a collar b, a square part c, and a round stem d. Here it will be noted that it is the rectangular part a, a', that renders necessary two views, and that in the absence of the cross, yet another view would be necessary to show that part c is square.

Fig. 124.

Fig. 125.

Fig. 126.

A rectangular piece always requires two views and sometimes three. In Figure 125, for example, is a piece that would require a side view to show the length and breadth, and an edge view to show the thickness. Suppose the piece to be wedge-shaped in any direction; then another view will be necessary, as is shown in Figs. 126 and 127. In the former the wedge or taper is in the direction of its length, while in the latter it is in the direction of its thickness. Outline views, however, will not in some cases show the form of the figure, however many views be presented. An example of this is given in Figure 128, which represents a ring having a hexagon cross section. A sectional edge view is here necessary in order to show the hexagonal form. Another example of this kind, which occurs more frequently in practice, is a cupped ring such as shown in Figure 129.

Fig. 127.

Fig. 128.

Fig. 129.

Fig. 130.