the forms of which are inherently weak, provide for the first; the selection of steel or iron of high percentage elongation, and the imposition of temper, or bending tests, both hot and cold, provide for the second.
The following are the leading features of present-day methods.
It might be hastily supposed that, because plates, angles, tees, channels and joist sections are rolled ready for use, little work could be left for the plater and boilermaker. But actually so much is involved that subdivisions of tasks are numerous; the operations of templet-making, rolling, planing, punching and shearing, bending, welding and forging, flanging, drilling, riveting, caulking, and tubing require the labours of several groups of machine attendants, and of gangs of unskilled labourers or helpers. Some operations also have to be done at a red or white heat, others cold. To the first belong flanging and welding, to the latter generally all the other operations. Heating is necessary for the rolling of tubes of small diameter; bending is done cold or hot according to circumstances.
Fig. 20.—Thornycroft-Schulz Water-tube Boiler.
The fact that some kinds of treatment, as shearing and punching, flanging and bending, are of a very violent character explains why practice has changed radically in regard to the method of performing these operations in cases where safety is a cardinal matter. Shearing and punching are both severely detrusive operations performed on cold metal; both leave jagged edges and, as experience has proved, very minute cracks, the tendency of which is to extend under subsequent stress, with liability to produce fracture. But it has been found that, when a shorn edge is planed and a punched hole enlarged by reamering, no harm results, provided not less than about 116 in. is removed. A great advance was therefore made when specifications first insisted on the removal of the rough edges before the parts were united.
In the work of riveting another evil long existed. When holes are punched it is practically impossible to ensure the exact coincidence of holes in different plates which have to be brought together for the purpose of riveting. From this followed the use of the drift,—a tapered rod driven forcibly by hammer blows through corresponding holes in adjacent plates, by which violent treatment the holes were forcibly drawn into alignment. This drifting stressed the plates, setting up permanent strains and enlarging incipient cracks, and many boiler explosions have been clearly traceable to the abuse of this tool. Then, next, specifications insisted that all holes should be enlarged by reamering after the plates were in place. But even that did not prove a safeguard, because it often happened that the metal reamered was nearly all removed from one side of a hole, so leaving the other side just as the punch had torn it. Ultimately came the era of drilling rivet-holes, to which there is no exception now in high-class boiler work. For average girder and bridge work the practice of punching and reamering is still in use, because the conditions of service are not so severe as are those in steam boilers.
Flanging signifies the turning or bending over of the edges of a
