Fig. 180 shows a sketch of an internal angle for an O.G. gutter, and patterns for both internal and external angle-pieces. A section of the gutter is set out as shown on the pattern for an external angle. This is then divided into seven parts - 0 to 1, 1 to 2, 2 to 3, etc. - and these lengths measured and set out to give the girth of gutter or width of pattern. From the points on the section, lines are drawn down, and from the corresponding points on the girth line, lines are drawn across. Where these meet give points on the pattern curve, as will be seen. The points are joined up, and thus the cut of pattern obtained. In joining up it should be remembered that where the line on the section is straight, the corresponding part on the pattern will also be straight. Thus 5 to 6 is seen to be straight on the section; hence on the pattern curve the line joining these
Internal Angle Gutter two points will also be straight. For heavy sheet iron the laps will be as shown by the dotted lines, and as in former cases flanged over when hot. For light galvanised sheet iron laps will be allowed on the straight parts of cut only; the edges of the curved part butting together, and being soldered from the inside. Laps will, of course, only be needed on one arm of the elbow.
The pattern for an internal elbow can be struck out as above, or, which is much better, when the pattern for the external angle is cut out can be marked off it as shown in the lower figure of Fig. 180. The cut of the end of pattern will be exactly the same as in the external angle, but used in the reversed manner. Laps will be as shown in the figure. Holes can be punched in the laps that will remain straight after the plate is bent, as these will not interfere with the part that has to be flanged over. The pattern for the inside arm should be slightly tapered, as in the half-round gutter angle, and this is shown by the side dotted lines.
It might be as well to here explain that an external angle-piece is an elbow which is supposed to fit on a corner, and that an internal angle-piece is an elbow which is made to fit into a corner.
Obtuse or Acute Elbow for O.G. Gutter.
An obtuse elbow is one whose arms are extended to an angle which is greater than a right angle (90°), and an acute elbow one whose arms are opened out less than a right angle.
The method here given will apply to either case, and, indeed, might have been used for the square elbow instead of that shown in Fig. 180; but for that particular angle-piece the method illustrated by Fig. 181 will not be so good as the one previously explained.
As this problem of jointing together two pieces of gutter or moulding to form a mitre or bevel joint is important, we will fully explain it by means of Fig. 181. To take a concrete case, let us suppose that the arms of the elbow make an angle of 100° with each other.
The exact shape of the section must first be set out. The double curve of this being drawn by dividing the straight line 6 to 10 into four equal parts. Through two of the points, as shown, draw perpendicular lines. Produce line 11 to 10 up, and line 5 to 6 down to meet these lines, thus obtaining points C C, the centres of the arcs. Before drawing the curves in, it is as well to join the two centres by the line C C, and where this crosses the line 6 to 10, will be the meeting point of the two arcs. (Particular notice should be taken of this construction, as double curves are often required in sheet metal work.) Now to set out the patterns. First a plan of the joint line must be drawn, and as the angle of the elbow is 100° the joint line will make 100 / 2 = 50° with the outside line, as shown. For construction purposes, however, it will be easier to set the joint-line angle from the back of the section, and a general rule for obtaining this angle will be: "Deduct half the elbow angle from 90°." Thus, in this case the angle will be -
90 - 100 / 2 = 40°, and this will be set out as shown in Fig. 181.
The section is divided into parts 0, 1, 2, 3, 4, etc., up to 13, and lines drawn down through each point on to the joint line. From the end of the joint line a girth line is drawn, as indicated, and the exact lengths of 0 to 1, 1 to 2, 2 to 3, etc., from the section set along this line. Lines are now drawn up through each of these points square to the girth line, and where they intersect the corresponding dotted line will give a point on the pattern curve. Thus, for instance, the dotted line which is drawn from the joint line at the foot of the line drawn down through point 9 on the section will intersect the line drawn up from point 9 on the girth line. So with each other pair of lines. If the pattern curve be carefully cut along, the upper portion of the figure will give a pattern for an external angle or elbow, and the lower part a pattern for an internal angle.
It should not be forgotten that, whilst the bending up of this class of work is simple, the highest degree of accuracy in striking out the patterns and in forming the moulding or guttering to the exact shape of section is essential if the parts are to fit together properly. All sheet metal work of an ornamental character, if it is to look well, must be made as neatly as possible, having neither lumps nor hollows nor superfluous solder about the joints.