Screw-nuts are principally made of forged iron or steel, and cast gun-metal. After a nut has been either cast or forged, it requires three principal shaping processes ; these are, screwing the hole, turning the two faces, and shaping the six sides or planes. Nuts are distinguished into sizes with regard to the thicknesses of the bolts or screws for which the nuts are made; therefore a nut to fit a screw one inch thick is termed an inch-nut, although it may be two inches or more in diameter.

After forging, nuts next require screwing. This is effected either by means of a long taper tap, shown by Fig 312 or 317, while the nuts are held in a vice, and the tap rotated with a spanner; or is effected by means of a tapping-machine. It is to be here noticed that whether hand-tapping or machine-tapping is adopted, a properly shaped long tap should be used, in

Sreference to two or three short ones, such as are denoted by Figs. 309, 310, and 311; or 'igs. 314, 315, and 316. Nuts that have been properly forged are furnished with holes of proper diameters for the respective taps; and the holes are also tolerably square to the broad surfaces of the nuts, termed faces. Consequently, when they have been tapped by allowing them to travel freely along without improper hindrance, the screws are also square to the faces, as intended.

Gun-metal nuts are usually cast without holes, unless they are of comparative large sizes, such as inch and a half or two inches in diameter. Some classes of gun-metal nuts are furnished with flanges, and resemble those denoted by Figs. 942 and 943. A nut without a hole is shown by Fig 943; and on the top of this a couple of lines are shown for indicating the centre, in order to mark it for drilling. Two cross-lines of this sort are sufficient to show the centre, supposing that the nut is regularly formed ; in which case, two straight lines from four corners will intersect at the centre, as represented in the comparative large Fig. 944. At the centre a deep dot is put with a coning-punch, and from it a circle is scribed of the same diameter as the intended hole; this is dotted as seen in the Figure, and becomes a gauge-circle to which the drill-point is made central for drilling. The opposite or flange side of the nut is easily marked with a similar circle, by means of an outside calliper, as before described. When each nut is thus lined it is ready for drilling.

The centre of a very irregular face belonging to a nut cannot be found by marking cross-lines. The lining for such a nut is effected with callipers. Fig. 945 denotes an irregular nut-face, at the middle of which six arcs intersect each other, and in their midst is the centre required. To mark the arcs, a calliper is opened until the distance between its points is a trifle greater than half-way across between two opposite flat sides; one point is then put to about the centre of one of the six edges, while the other point is extended across the nut-face and a short arc scribed. The calliper is next shifted to the opposite side, and another arc scribed to intersect the first one. The point midway between these arcs is the centre of that portion of the surface over which the calliper was extended while marking; and by next shifting the calliper to the other four sides or edges, four more arcs can be marked; so that the mean centre of all the arcs, and therefore of the entire surface, is clearly shown. This being found, a dotted circle is marked, similar to any other required for drilling, and the lining is completed. In the Figure the six centres from which the arcs are marked are denoted by the letters C. Those nuts that are irregular along the entire lengths of their six sides are properly lined by placing them into vee-blocks on a lining-table.

After lining, the nuts can be drilled so that the holes are square to the faces, by employing a suitable chuck for holding each nut. Such a chuck may consist merely of a parallel block in which holes are bored of different sizes. When this is to be used it is put beneath the drill with one of the holes concentric with the drill, and it is then fixed with holdfast plates, or with little screws belonging to the chuck. A nut which is properly lined is now put upon the chuck and held with a spanner, or with ledges situate on the chuck, to prevent the nut rotating while being drilled.

When several thousand nuts are to be drilled, a chuck should be used which has a couple of vee-grips. These are caused to slide either towards each other or apart, by being attached to a screw which is both left-handed and right-handed. The action of this screw is like that of one belonging to a lathe-chuck, and causes the vee-grips to hold nuts of several different diameters. Each nut is also fixed concentric with the drill by the act of tightening it between the grips, supposing that the chuck is well made and fixed at the proper place on the drilling-table. Consequently, with such a chuck no adjustment of the nut after fixing is necessary.

Whenever it is specially desirable to drill a number of long nuts so that their holes shall be as nearly as possible square to the nut-faces, it is necessary to centre both faces of each nut, and drill half-way through from each face, instead of entirely through from one face.