Fig. 106 represents a square bar. A is the front elevation, and shows the length and width of the bar, but not the thickness. There must then be another view. B is the plan, and shows the width and thickness of the bar. From these two views the complete form of the bar is obtained and no other views are necessary when such is the case. In all working drawings, only as many views are shown as is necessary to determine the complete form of the object being drawn.
Fig. 106. Projections of Square Bar.
Fig. 107. Projections of Round Bar.
Fig. 108. Projections of Hexagonal Bar.
Fig. 107 represents a round bar. The front elevation A, shows the width and height of the bar, but does not show that it is round. The plan B, shows the circular top of the bar and of the proper diameter. In this problem, in addition to the dotted projection lines connecting points in plan and elevation, it is advisable to put in dot and dash lines for center lines. Projection lines and center lines are construction lines, and may be erased when the drawing is finished, unless otherwise ordered.
Fig. 108 represents a hexagonal bar. In this case, center lines should be drawn. The front elevation A, shows the length of the bar, and the plan B, shows the form and the distance between faces. The vertical lines in the front elevation show the corners of the hexagonal form while both views show the distance from corner to corner of the hexagonal top.
Fig. 109 represents a hexagonal nut. Center lines should be drawn here also. The front elevation A, shows the thickness and width of the nut, and the circular hole is shown by heavy dotted lines. Holes are always represented in this way. The plan B, shows the shape of the top of the nut, and also the shape of the hole.
Fig. 109. Projections of Hexagonal Nut.
Fig. 110. Projections of Cylinder with Circular Hole.
Fig. 111. Projections of Frustum of Square Pyramid.
Fig. 110 represents a cylinder with a circular hole passing part way through. Center lines are needed here, and in fact where any circle, hexagon, octagon, or other shape except a square or rectangle occurs. The front elevation A, shows the height and width of the cylinder, and the depth and width of hole. The plan B, shows the top of the cylinder, its diameter, and the diameter of the hole.
Fig. 111 represents a block in the form of a frustum of a square pyramid. The front elevation A, shows the height of the block, and the width of the top and bottom faces. The plan B, shows the width and depth of the top and bottom faces, and also the edges connecting these faces of the frustum.
Fig. 112 represents a square bar with a portion forged to a cylindrical form. The front elevation A, shows the length and width of the bar, and also the length and width of the cylindrical portion. The plan B, shows the square top, and by the dotted circle shows the shape of the cylindrical portion. The fact that this circle is dotted means that the cylindrical portion does not come clear through to the top. A bottom view C, is also shown here, as it gives a better idea of the complete form of the bar. Enough views should always be shown by the draftsman to give the workman a clear idea of what he is to make.
Fig. 113 represents a circular ring made from a round rod. The front elevation A, shows the thickness and the diameter of the ring, and the plan B, shows the circular form.
Fig. 114 represents a block with a number of different dimensions. The block has been turned down in such a way that there are five different diameters, as shown. All these diameters, and the lengths between, may be shown in the front elevation A. From this view, only the forms of the cross-section could not be ascertained. Some might be square, some hexagonal, or some circular, but the plan B shows that all are circular.
Fig. 112. Projections of Square Bar with Cylindrical Portion.
The principles of projection which have been used so far, may be stated as follows:
(1) If a line is parallel to either the vertical or horizontal plane, its actual length is shown on that plane, and its other projection is parallel to the ground line.
Fig. 113. Projections of Circular Ring.
Fig. 114. Projections of Turned Block.
(2) A line oblique to either plane has its projections on that plane shorter than the line itself, and its other projection oblique to the ground line.
(3) No projection can be longer than the line itself.
(4) If two lines intersect, their projections must cross, and the point of crossing in the front elevation must be directly under the point of crossing in the plan.
(5) A plane surface, if parallel to either plane, is shown on that plane in its true size and shape; if oblique, it is shown smaller than the true size, and if perpendicular it is shown as a straight line.
(6) Lines parallel in space have both their vertical and horizontal projections parallel.