Bolts are centred for turning by several modes, either of which may be adopted to suit the sizes and shapes of the bolts in hand. If a bolt has several dimensions differing considerably from each other, such as a conical one denoted by Fig. 1080, a little care is requisite to measure it before centring, that the portion nearest to the desired size may be known and centred accordingly.

Short bolts shaped like Figs. 1081 and 1082 can be held in a vice and their ends dotted without any preliminary turning; but all bolts that are large enough or long enough, to be troublesome while being moved about, should be lined. Lining will indicate the places for the dots, and in many cases will entirely obviate the lifting of heavy pieces during the whole time, from the beginning of their centring to the conclusion.

Fig. 1083 represents a holdfast bolt or nut-head bolt, the article consisting of a straight rod screwed at each end, and having a screw-nut to serve instead of a head. Such bolts should be so forged as to require little or no treatment in the lathe, except the screw-cutting. It is, therefore, necessary to select iron or steel rod, the diameter of which is exactly equal to the diameter of the intended screwed ends. If steel bolts are required, the proper lengths of rod are cut oft' while red hot, and each extremity should be pared with a trimming-chisel until somewhat like the intended form; and this trimming while red-hot is especially necessary if curved extremities like those in the Fig. 1083 are desired. Each piece is next carefully straightened, and the ends softened, that the threads may be easily cut, and the bolts be not so likely to break. This completes the forging and prepares the piece for the lathe process.

Iron bolts also may be forged in exactly the same way as steel ones, if the iron rod used is of solid texture; but nearly all sorts of iron require to be forged after its first making, to properly prepare it for screw-cutting. This forging consists in thickening and welding each end of each bolt piece, until all are rendered solid, and of a larger diameter than the specified diameter of the screws. An amount of superfluous metal is thereby provided which will be turned off with the lathe. When a large number of pieces are to be thus thickened, it is

2 y 2 necessary to first completely forge one of each size, that the forger may know the exact length of iron required for each bolt, by which means the cutting off waste pieces will be avoided.

When all the bolts are forged and straightened, they are ready for centring by the aid of vee-blocks and scribing, while situate on a lining-table as indicated in Fig. 1084. It will be seen that the comparative straight and smooth intermediate part of the bolt is in contact with the vee-block, and the marking with the scriber-block will therefore indicate the centres of those portions which touch the vee-gaps. If the blocks are put near the thick ends, as in the Figure, the entire middle part will be centred, if it is exactly straight; but if it is not, only the ends will be centred, and it will therefore be necessary to straighten the mid-portion afterwards, if found to be bent enough to require it.

The centring of steel bolts is effected by using the vee-blocks in the same manner as for iron ones, with the difference of placing them nearer to the extremities - in contact with the portions to be screwed, instead of near the thread-junctions, because it is specially requisite to find the centres of the ends, by reason of no metal existing thereon to be turned off. Consequently, such a bolt is first accurately centred to cause its ends to rotate truly on the pivots, and the intermediate part is afterwards carefully straightened, if necessary.

In order to centre a connecting-bolt it is put upon vee-blocks, as seen in Fig. 1085. This bolt resembles, in some respects, the one in Fig. 1084, being forged so that the end for the screw is thicker than the remainder of the stem. It is made of iron, the diameter of which allows little or nothing to be turned off; consequently, the vee-blocks are put beneath the smooth parallel part of the stem; and the scribing will show the places for the centre recesses, although these places may not be in the centre of the bolt's extremities. The heads of such bolts when forged, are usually of comparative large diameter, so that no attention need be given them during centring. But when it is specially requisite to turn a bolt head, or a number of them, to a specified diameter, and but little metal is to be removed, one of the vee-blocks must be put beneath each bolt-head for scribing; and not, as in the Figure, beneath the part of the stem adjoining the head.

When the head of a bolt is tolerably concentric with the stem, and the bolt's entire surface is to be about equally reduced, the marking should be done with a callipers. Each extremity of the bolt is chalked, and the marking performed resembles that shown on the middle of the bolt-head in Fig. 1087, in the centre of which is the required centre. This point is easily found and dotted, because it is only about a sixteenth of an inch from either of the four lines.

Figures 1086, 1088, and 1089, relate to the mode of centring by means of a centre-finder devised by the author, termed a rectol. The instrument alone is denoted by Fig. 1086, and consists of a sort of gap straight-edge, in one end of which is a gap large enough to admit bolt-heads of several sizes. The lower edge of the tool is its principal straight part; and the edge of the short portion at the left hand of the gap, is in line with the edge of the longer portion at the right hand. The entire tool consists of a thin piece of steel, that it may be easily moved about.

A rectol in use is denoted by Fig. 1088. The implement is in contact with a bolt-stem, which is forged tolerably parallel, and therefore allows the edge to touch at several places along the stem. While on the bolt, the rectol is slid along a short distance until the edge of the short end touches the outer surface of the head. In this condition it appears in the Figure, and is thus held while the operator scribes a mark upon the head exactly at the place touched with the rectol. This mark is in line with the stem of the bolt, because both portions of the rectol are in line with each other. The implement is next shifted a quarter of a revolution around the bolt-stem, and again put into position for scribing another short line, in the same manner as before, which mark is analogous to the first one because of being in line with the bolt-stem. After this, two more marks are made, by shifting the rectol twice more; consequently, four marks are shown, which represent four points of a circle, in the centre of which is the centre required for the bolt-head. To plainly and easily show this point, it is only necessary to place a divider with one point in each of the four marks, and scribe short arcs across the middle of the surface, which produces four lines similar to those in Fig. 1087.

It will be easily perceived that a rectol is very useful for centring heavy bolts, and other pieces which may not be very heavy, but are too long to be conveniently moved about. By employing this instrument a lining-table and surface-blocks can be dispensed with ; and the article can be centred wherever it may be, either on the floor or on the lathe-bed ready for turning.

In order to centre a large bolt having its end thickened, a rectol with a shallow gap may be used. One of this sort is shown in Fig. 1089, and can be quickly made, in emergency, of any piece of sheet iron which is available. In such a case only one edge of the tool is made straight, and the gap is but roughly formed with chiselling, no filing being performed except upon the straight edge of the implement.