This section is from the book "The New Metal Worker Pattern Book", by George Watson Kittredge. Also available from Amazon: The new metal worker pattern book.

In Fig. 320, let E M L I H K N F be the elevation of a three-piece elbow. The drawing of a three-piece elbow, at any angle whatever, should be so constructed that the middle section or portion bears the same angle with reference to the two arms. Since the two arms in the present instance are at right angles (90 degrees) to each other, the middle section must therefore be drawn at an angle of 45 degrees to both. Make its diameter the same as that of the two arms, and draw the miter lines M N and L K. Draw the profile A B C in line with the arm from which the pattern is to be taken, as shown, and divide it into any convenient number of equal parts. Place the blade of the T-square parallel to this arm of the elbow, and, bringing it against the points in the profile, drop corresponding points upon the miter line L K. At right angles to L I draw a stretchout, as R S, through the divisions in which draw measuring lines in the usual manner. Placing the T-square at right angles to L I, and bringing it successively against the points in the miter line L K, cut the corresponding measuring lines, as shown. Then the line T U V, traced through the points thus obtained, forms in connection with S R the pattern of an end section.

Fig. 320. - A Three-Piece Elbow.

Place the 45-degree set square against the blade of the T-square so that its oblique or long side shall coincide with the lines of the middle section of the elbow, and, bringing it against the points in L K, drop like divisions upon M N. At right angles to L M lay off a stretchout of the profile A B C, as shown by P O, through the points in which draw measuring lines in the usual manner. Reversing the position of the set-square so that its long side shall come at right angles to M L, or, what is the same, parallel to the stretchout line, bringing it successively against the several points in the miter lines M N and L K, and cut the corresponding measuring lines. Then lines traced through these points, as shown by D X Y and G W Z, will be the pattern of the middle section.

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