Elbow patterns for all forms of pipe

The importance of a quick and accurate method of securing patterns for elbows, has induced the writer to lay before the Sheet Metal Worker in the following pages, methods which may be employed in securing the patterns for elbows of any angle or number of pieces in all forms of pipe. Methods are here pointed out which admit of the least display, and the greatest accuracy, without that study of geometrical work usually recommended.

In other words, if we possess a pair of compasses and straightedge, we can produce patterns for elbows in round pipe of any size, angle, or number of pieces. With very little additional attention, we are enabled to produce the patterns for elbows of all forms of pipe with the greatest accuracy and in the least possible time, which is an important factor in these days of sharp competition.

Fig. I illustrates a four pieced 90 degree elbow, and upon reducing this to its simplest form, we find it contains three elbows at an angle of 30 degrees. When it is stated that it contains three elbows at an angle of 30 degrees, it should be remembered that the term 30 degrees is to an extent shop phraseology, since angles are measured by the position the two lines of which they are formed occupy as radii of a circle. This places what is usually termed 30 degrees at 150 degrees, unless the axis of one arm of said angle or elbow is presumed to be parallel to a right line in space, then will the axis of the second arm be at an angle of 30 degrees to said right line. The writer has in this work used the usual shop term or the number of degrees the elbow varies from a right line While the illustration Fig 1 shows only an elbow of four pieces at an assumed angle of 90 degrees, the reader can readily conceive how the above principles may be adapted to elbows of any angle or number of pieces. In every case the required number of component elbows is dependent upon the required number of pieces, the angle of which is also dependent upon the required angle of the finished elbow. For example, let it be presumed that an elbow is required to be made in three pieces at an angle of 72 degrees, we then have two component elbows at an angle of 36 degrees: and had the specification called for this elbow in five pieces, then four component elbows at an angle of 18 degrees would have been required.

Thus in every instance, we may deduct one from the number of pieces required in the finished elbow, then will the remainder represent the number of component elbows required: and by dividing the specified angle in degrees of the finished elbow, by the number representing the component elbows, we find the required angle of each component.

A graphical solution may be arrived at by the use of our compasses and straightedge as shown at Fig. 2, by drawing an indefinite right line as a b, and from any point on this line as center as at c, describe an arc of convenient radius.

Presuming the bevel to have been set to the required angle of the finished elbow, (in this instance 80 degrees) place it in position as shown, i.e., one arm parallel to line a b, with vertex of angle at point c, when point d may be located. Upon dividing the arc b d, into one less number of equal parts than pieces required in the finished elbow, the point e is established in the first division from b: and by drawing a line from point c, intersecting point e, the required angle of the component elbow is shown at a c e, i.e., 16 degrees.

From what has been shown, it is evident that the use of the protractor will greatly facilitate the work, but it is by no means necessary. It is also evident that when called upon to produce the patterns for an elbow of any angle, or number of pieces beyond two, we have only to secure the curved line upon which one piece of the component elbow is cut, and duplicate for the whole. Therefore the important factor to be determined is, at what angle to the base of a right cylinder must a plane be presumed to cut its curved surface to produce a line, whose development upon a flat surface will constitute the curved line of the required pattern: presuming the diameter of said cylinder is equal to the diameter of pipe for which the elbow is to be made.

The above statement is exemplified in the so called adjustable elbow, inasmuch as its sections may be so revolved as to form either a piece of straight pipe, or an elbow, whose angle is dependent upon the number of sections, together with the angle each section is cut to its axis.

Fig. 3 illustrates an adjustable square elbow, made in four pieces which have been so revolved as to form a piece of straight pipe, and as each section is cut at the same angle to its axis, when we determine the angle of one, we know the angle of all.


DOWNLOAD OLD SHEET METAL BOOK:
Elbow patterns for all forms of pipe