One type of these machines is shown in Figs. 241 and 242. Plates are held flat for shearing or punching in chain slings carried on the hook of the crane. Two men usually hold the plate to guide it under the machine at the direction of a third man who sets the machine in motion by means of a conveniently placed foot or hand-lever.
Punch and shears are often in one double machine, or, by a change of equipment, a punch may be changed for shearing, or vice versa.
Fig. 243. - Hand Shears and Punch.
That part of a plate operated on by punch or shears is necessarily strained to a point of disruption. Metal on each side of a shearing line or around a punched hole is strained more or less according to its proximity to the line of disruption, and some of this metal is strained beyond its elastic limit, and hence is less strong than it was before. For this reason, when boiler plates are punched or sheared, a punched hole must be drilled larger, and the sheared edge must be planed or chipped away sufficient to remove the metal strained beyond the elastic limit.