This kind of pan (Fig. 63) is of the baking-tin order; but the method of forming the corners can be adopted in all cases where it is necessary to have a pan that will be liquid-tight at temperatures above the melting point of solder.
A pan may have an equal overhang all round, or its ends may overhang the bottom more or less than the sides do. We will set out a pattern for each case, taking a pan with equal taper for sides and ends first.
Suppose a pan is 12 in. by 9 in. at the top, 9 1/2 in. by
6 1/2 in. at the bottom, and 2 in. deep. The distance that the top projects over the bottom will be found by deducting the length of the bottom from the length of the top and dividing by two. Thus -
Overhang = 12 - 9½ / 2 = l¼ in.
To get the length down the side all we need do is to set out a right-angled triangle (Fig. 64) with height 2 in. and base 1¼ in.; the third side, or hypotenuse, will then give us the length down the side of pan. Or, without setting out the triangle, the required length can be calculated thus -
Side length = = 2 3/8 (nearly).
The size of the bottom is first marked out, and the side length added by marking A B on the pattern (Fig. 64) equal to a b on the triangle. The overhang is then set along the sides - that is, B C is marked off equal in length to b c. The points C C are joined up to A, and what we might call the net pattern is now complete; for if the piece
C A C be cut out and the sides of the pan bent up the two lines AC, AC will coincide; hence, whatever method of forming the corner is adopted the allowance for jointing must be additional to the net pattern. In this case we want to keep the corner solid by doubling up the sheet to form a flap, which will be folded over on to the end of pan. For the flap to turn over on the end and come flush with the top edge of pan, it is manifest that the angle of the flap must be equal to the angle of the end, and whatever construction is followed to obtain the cut of the corner is with the object of arriving at this result. Two methods can be used, and we will show both - one in this case and the other in connection with a pan of unequal overhang.
Again referring to Fig. 64, bisect the angle C A C, which, in this case of equal overhang, is simply done by drawing the diagonal line A E. With centre A and radius A B, describe the arc of circle marked B D; then, if a line be drawn from C to touch the arc, the point F on A E will be determined, and thus the shape of the top of flap. To accurately draw the line C F, it is not a bad plan to take centre C and radius C B, and thus mark the point D on the arc; then join D to C, and so obtain F.
The allowance for wiring must be added on as shown, and if the sheet is fairly strong, it will be as well to cut the top of the flaps a little lower, and thus avoid the wiring being lumpy where it runs over the flaps.
To obtain the shape of the part to be cut away at the other three corners, without the trouble of marking each out separately, a good plan to follow is to cut out the shaded part as shown, and use this as a template to mark off the other corners.