Page:Sheet Metal Drafting.djvu/224

210 91. Square to Round Split Header.—This type of fitting is often used where a fan with a rectangular outlet must supply two round pipes running in different directions.

This problem presents a case wherein development both by triangulation and by sections may be employed in order to obtain the pattern.

The pattern is in reality two square to round transitions mitering upon each other. Where the two transitions come together a miter line is produced. A true section on this miter line must be developed. The practice of assuming a section, common to some drafting rooms, results in a more or less distorted fitting, according to the experience the draftsman has had in designing such fittings. The workman in forming and assembling the fitting has difficulty in that he must compel the assembly to take an unnatural shape.

The plan, Fig. 301, is first drawn according to the dimensions taken at the job. The profiles are then divided into the same number of equal spaces. After the order of triangulation has been determined, base lines are carried to the corners M and N of the rectangle.

The elevation is drawn in its proper location by means of extension lines carried from the plan. The miter line is drawn in both plan and elevation and should pass through the intersections of the base lines, Fig. 301, at points A, B, C, D, and E; it should also pass through the elevation of the elements, Fig. 302, at points F, G, and H.

A true section on this miter line should now be developed by drawing a horizontal line equal in length to twice the distance AE of Fig. 301. The center point of this line. Fig. 303, should be lettered A, and a perpendicular center line erected. Upon each side of this center, points B, C, D, and E should be located exactly as they appear on the miter line of Fig. 301. Perpendiculars are now erected at each of these points. Point B in Fig. 301 falls on line N-1, and point H falls on line N-1 of Fig. 302; therefore, the perpendicular height of point H from the base line of Fig. 302 should be set off on perpendicular B of Fig. 303. For the same