Lap Seams.
The lap seams are the seams commonly used on all classes of work. When the beginner becomes proficient with the blow pipe no trouble will be experienced in making lap seams that will show when cut a joint equal in thickness to the sheets that are joined. The lead sheets for the flat lap seam are prepared by shaving clean the exposed edge; also, shave the sheets where they touch each other. The upper edges can then be shaved for a distance of ⅛ inch each side of the lap, which will make the finished seam ¾ inch wide, as shown at a in Fig. 23. The sheets should be lapped ½ to ¾ inch, according to the weight of the stock. It is very evident that light weights would not require as large a lap as would heavier sheets, as the object of lapping the sheets is to leave the sheets practically as one piece, and the lead, to accomplish this object, is to be melted from the upper sheet.
Fig. 27.—Burning an Inverted Lap Seam.
From the foregoing it will be seen why a 12-pound sheet requires a ½-inch lap, while a 24-pound sheet would require a ¾-inch lap. The flame, when regulated to the work, is brought to bear squarely on the edge of the upper sheet, slanting slightly in the direction of the lower sheet, as shown. When the edge has brightened almost to the fusing point the blow pipe should be drawn quickly to one side and from the upper sheet to the lower sheet. If the metal is sufficiently hot the melted drop will follow the point of the flame and instantly fuse with the lower sheet, and if properly done the seam will resemble b in Fig. 23. This process is repeated, advancing about ⅛ to ¼ inch each time. Do not attempt to fuse a large surface at a time. Experts cannot do such a thing satisfactorily, so why should a beginner try to? Rather, try to fuse small surfaces quickly and strongly, as better work and more of it can be accomplished in that manner.