A conical Galloway water-tube for a boiler furnace being formed by part of a cone surface comes in very conveniently at this stage of our work. A section of flue and cross-tube is shown in Fig. 112.
To set out the pattern there is need only to draw a half-elevation of the flue-tube and conical pipe, both sides being the same. In arranging the size of the Galloway tube, care should be taken that the diameter across the flange at the small end should not be greater than the diameter of hole in flue-tube at large end, as the end of conical pipe must pass through this hole. It will be noticed that the flange at the small end fits on the inside, and the flange at the large end on the outside of the flue-tube.
When going over plater's work, we explained that the thickness of metal must always be taken into account by using the centre lines on the plate section. In this case, therefore, to get a correct pattern, the section of water-tube with flanges must be accurately drawn, and the pattern developed from the middle lines of the plate section. The tube flanges are shown in Fig. 112, the centre dotted line on the shaded plate section representing the line from which the pattern would be set out.
The pattern is marked out as before explained, the line A 0 being taken as the base of cone. Laps will be allowed on the sides for riveting, the end lines of the net pattern forming the centre lines of the rivet-holes. Allowances are also put on the top and bottom of pattern for flanging, the width allowed being slightly greater than the length of dotted lines on flange section, to cover for draw. Some thought should be bestowed on the thinning of the plate corners, so that a good job may be made where the two thicknesses of plate come on the flange.
The holes in the flue-tube can be cut out in the flat, and if the plates are punched the rivet-holes for the water-tube flanges also put in.
In all conical work it should be particularly remembered that the lengths of lines required to fix points on the cut of pattern are taken from the outside line of cone. There are many more difficult examples of cut cone work in sheet and plate metal, and a few of these will be given in further chapters.