Problem 55
COAL HOD
95. The Coal Hod.—The coal hod is made in many different designs, and since there is no standard the manufacturer draws the design to suit his own ideas.
Figure 324 shows a half-profile of a coal hod, and since the center line divides the object into two equal parts the pattern can be developed by sections.
Figure 322 shows the plan of the coal hod after it has been cut by an imaginary plane and so placed that the cutting plane assumes a horizontal position.
Figure 323 is a half-profile of the bottom located in its relative position by means of extension lines.
Both profiles should be divided into equal spaces and each division numbered or lettered as shown. It is evident that line 1–10 of Fig. 324 is longer than line 1–10 of Fig. 322 (since a straight line is the shortest distance between two points). Because of this fact the distances between points in the profile, Fig. 324, are not true lengths.
Figure 325, which is sometimes called an extended section, must be developed in order to ascertain the true distances between these points. A tangent parallel to the bottom line of the coal hod is drawn through the plan as shown in Fig. 322. Since point 4 is the lowest part of the curve the tangent must pass through this point. Extension lines are then carried from each point in the curve until they meet the tangent as shown in Fig. 322.
Upon any straight line, Fig. 325, the exact spacing between points 1 to 9 of Fig. 324 are laid off. Perpendiculars are erected at each point, and upon each perpendicular a distance equal to that from a correspondingly numbered point in the curve to the tangent of Fig. 322 is set off.
A curve traced through the points thus obtained will give the exact distance between these points, and these must be used in developing the pattern.
The order of sections must now be determined, such as will be found on the drawing, and a diagram of sections, Fig. 326, constructed. The base lines for this diagram are taken from the plan, Fig. 322. The altitudes 1 to 10 are taken from Fig. 324, and the altitudes A to K from Fig. 323.
