The lead for this form of seam is prepared, as its name indicates, by lapping one sheet ½ to ¾ inch over the other sheet. The under edges are to be shaved clean, as also the upper edge. The lead required to make the seam is melted from the upper lap and is fused on the lower sheet. There is no reason why the lead at the point of juncture cannot be made as thick as the original lead. This is the point aimed at in practicing, and the only accurate way to determine the relative strength of the seams is to cut squarely across a finished seam, then bend the beam slightly. The thickness can then be noted. A cross section of a perfect lap seam is shown at a in Fig. 23. The beginner should practice the different seams until the thickness of the joint can be told by the looks of the lead. A few days' diligent practice at the bench will soon train the eye to note any imperfection that may arise.
The different seams will be taken up serially. A description of how the seams are prepared and the several positions of the blow pipe, as well as the little difficulties that may arise, is the extent of the instruction that can be given. The rest must come with practice and the application of a little common sense. There is no royal road to this business; but practice, and practice hard, is the only way to satisfactorily master the blow pipe and flame, and in practicing remember that all this work has been done before, and can be easily done again, and by you. Do not get discouraged by failure to make a perfect seam at the first application, but stick to it for a short time and it will be found to be a most fascinating pastime, for which the persistent student will eventually be well repaid.
Flat Butt Seam.
For practicing I would recommend the beginner to use pieces of sheet lead about 12 inches long, as strips of that length are much easier to prepare. The edges are straightened with a fine rasp which is held lengthwise of and parallel to the edge to be trued, in the manner shown in Fig. 22. The rasp must be used lightly, or it will be apt to tear the lead and so leave it in worse condition than before using it. The edge should then be gone over with the shave hook and cleaned. Then shave the top surface a distance of ⅛ inch each way from the edge, which will make a seam ¼ inch wide when finished. Then butt the edges together and secure the sheets firmly to a board with a few tacks. The extra lead that is necessary to add to make a butt joint full must be obtained from a strip of lead, which should be about ⅛ inch square and shaved clean.
After regulating the flame to the proper size and shape the burning should be begun at the end of the seam nearest the operator. With the point of the inner flame melt off a drop from the lead strip and have it fall squarely on the seam just slightly in advance of the point of fusion. Follow it up with the flame, placing the point of the inner flame directly over the edges of the seam, which is almost under the lead drop. As soon as fusion commences on the lead seam the melted drop will flow to the bright spot and immediately unite with it. The flame must then be quickly removed and the drop be allowed to set.
In order to avoid any misunderstanding regarding the time required for the lead drop to set I would say that the drop will cool immediately upon the flame being removed from contact with it. It is not necessary to wait for any specified time, but if the flame is allowed to play constantly on the sheet it is apt to get overheated, and when in that condition it takes very little heat to set the lead running like water. To avoid this the flame should be lifted clear of the seam for an instant after each drop has been fused into place.
Fig. 24.—Burning a Flat Butt Seam.
These remarks apply to all seams that are made by the blow pipe process and should be noted, as this particular point will not be referred to again. Now melt off another drop and let it fall as before, only it should lap on the previous drop about one-half its diameter. Secure it to the seam as before. This operation should be repeated until the seam is completed, and if the seam is correctly done a section will appear as a in Fig. 24. This form of flat seam should be practiced until perfectly familiar with the blow pipe flame and until the beginner can approach the lead with the flame without burning holes through it, which will probably be the first thing to happen.