Shears are instruments of a character quite different from any of those hitherto described, as the cutting edges of shearing tools are always used in pairs, and on opposite sides of the material to be sheared or severed. In many cases the shears are constructed after the manner of pincers and pliers, or as two double-ended levers united at the fulcrum by a pin, but other modes of uniting the two cutting parts of the instruments are also employed, as will be shown.
The general form and position of the cutting blades of shears, was adverted to in the elementary diagram fig. 316, at the beginning of this volume, and the sections of some varieties of this instrument are represented by a, b, c, of the annexed fig. 005, from which it will be seen that the edges of shears and scissors meet in lateral contact, and pass close against one another, severing the material by two cuts, or indentations, or thrusts, which take place in the same plane as that in which the blades are situated and are moved.
Some of the largest shearing tools of the kinds used by engineers, such as c, serve to divide bars of iron, 4, 5, or 6 inches wide, and 1 to 2 inches thick, then requiring the greatest possible solidity and freedom from elasticity.
On the other hand some of the finest scissors of the section a, such as are used by ladies in cutting lace, will cut with the greatest cleanness and perfection, the most flexible thread or tissue of threads, or the finest membranes met with in animal or vegetable structures. But this latter kind of shears, unlike the engineer's shears, is altogether useless unless possessed of a considerable share of elasticity, to keep their edges in accurate contact at that point in which the blades at the moment cross each other, as will be explained, otherwise such thin materials arc folded down between the blades instead of being fairly cut. The tran-sition from the elastic to the inelastic kinds of shears is not, as may be supposed, by one defined step, but by gradual stages, making it as difficult in this, as in other classifications, to adopt any precise line of demarcation.
In addition to the above, or to shears properly so considered, there are a few tools known as cutting pliers or nippers, in which the blades meet in direct opposition, but do not pass each other as in the legitimate kinds of shears; this kind is represented by the section d, fig. 905, and it is proposed to consider these several tools as nearly as may be under four heads, namely, -
Cutting nippers for wires.
Scissors and shears for soft flexible materials.
Shears for metal, worked by manual power.
Engineer's shearing tools, generally worked by steam power.
Cutting pliers, if they admit of being classed with shears, are certainly the most simple of the group, and are used for cutting asunder, small wires, nails, and a few other substances. Their edges are simply opposed wedges, exactly as shown in the above diagram at d; and as respects the remainder of the instruments by which their wedges are compressed, the most simple kind exactly resembles carpenters' ordinary pincers for drawing out nails, except that the cutting pincers are made with thinner edges; and figs. 906 to 909, overleaf, represent different kinds of cutting pliers and nippers.
When cutting nippers are compressed upon a nail or a piece of wire, they first indent it on opposite sides, and when from their penetration, the surfaces of the wedges exert a lateral pressure against the material, the latter eventually yields, and is torn asunder at the moment the pressure exerted by the wedges exceeds the cohesive strength of the central metal yet uncut. Consequently the divided wire shows two bevilled surfaces, terminating in a ridge, slightly torn and ragged. The quantity of the material thus torn instead of being cut, will be the less, the softer the metal and the keener the pliers, but experience shows an angle of about 30 to 40 degrees to be the most economical for the edges of such tools.
Little remains to be said on the varieties of cutting pliers; most of these used by general artizans and clockmakers, are smaller than carpenters' pincers, and the extremities of the jaws are bevilled as in waich-nippers, fig. 906, that they may cut pins lying upon a flat surface. Other cutting pliers called side-nippers are oblique as in fig. 907; those used for the dressing-case, and known as nail-nippers, are concave on the edge to pare the nails convex; and another kind known as nipper-pliers, bell-hangers or bottler's-pliers, have flat points at the end for grasping and twisting wires, and cutters on the sides for removing the waste ends, as shown in fig. 908.
Surgeons also employ cutting nippers, for dividing small bones, such as those of the fingers and toes, and for removing splintered and dead portions of bone. They assume the forms already explained, and also some others as will be seen on consulting the work before quoted in the foot note, pages 801, 2, namely, Seerig's Armamentarium.
The edges of cutting nippers are apt to be notched, if used upon hard wires, or if wriggled whilst the cutting edges are buried in the wire, and they scarcely admit of being reground or repaired. This inconvenience led to a modification of the instrument fig. 909, by the enlargement of the extremities, to admit of loose cutters fitted in shallow grooves being affixed by one screw in each, as shown detached at c, so that the cutters may admit of removal and restoration by grinding, which end is effectually obtained although somewhat to the prejudice of the instrument, by increasing its bulk.*
* H. Bursill, a youth only 12 years old, was rewarded for this contrivance by the Society of Arts in 1845.