Let A B C D E F G in Fig. 375 be the elevation of a three-piece elbow to any given angle, as G F E, the middle piece of which, B C H, forms a gore extending around one-half the diameter. The lines H B and H C are drawn parallel respectively to the ends of the two outer pieces, therefore the patterns for the end pieces will be straight from H to C and H to B and mitered from H to F. To obtain the pattern for one of the ends, as F H C D E, divide N K L into any convenient number of equal parts. With the square at right angles to N L, carry lines from the points in

N K L, cutting the miter line F H C, as shown. O any line, as E D extended, lay off a stretchout of M K L, as shown by e d, through the points in which draw the usual measuring lines, as shown. With the T-square brought successively against the points in F H C, cut corresponding measuring lines, as shown. A line traced through the points of intersection, as shown by fh" c, will be the half pattern of end, as represented in elevation by F H C D E, or in profile by N K L. The other half of the pattern can be obtained by duplication.

Since H C is drawn parallel to E D, the distance H J is less than one-half the diameter of the normal profile, or less than O L, therefore it will be necessary before obtaining the pattern of B H C to obtain a correct section on line H J of elevation. To do this, first place the T-square parallel with B C and carry lines from the points in H C, cutting H J and H B. Next draw any line, as K' M' of the section, and erect the perpendicular H' J'. From H', on H' J', set off the spaces in H J of elevation, transferring them point by point. Through the points thus obtained draw lines parallel with K' M', as shown. With the dividers take the distance across K O L or L O M, on the several lines drawn parallel with K M, and set off the same distance on lines of corresponding number drawn through H' J'. Thus H' M' and H' K' are the same as O K and O M. A. line traced through the points thus obtained, as shown by K' J' M', will be the section desired.

For the pattern of H B C, lay off on H J extended, as h h', a stretchout of K J' M' of section, through which draw the usual measuring lines. With the T-square placed parallel with H J, and brought successively against the points in B H and C H, cut corresponding measuring lines drawn through h h', as indicated by the dotted lines. Lines drawn through these points of intersection, as shown by h b h' c, will be the pattern of the part shown in elevation by B H C.

Fig. 176. - Pattern for a Three-Piece Elbow, the Middle Piece Being a dure.