Small bolts sometimes require square holders, by which the bolts are held while being turned in a lathe, or while being ground after being hardened. The holder is similar to that shown in Fig. 59. In small bolts the holder is drawn down from the head; for larger bolts the holder is reduced from the stems, to avoid the longer process of reducing from the heads.

The ordinary vertical steam-hammer is very efficient for bolt-making. By its powerful aid, bolts are quickly made by reducing the iron to form the stems, instead of upsetting iron of smaller diameter to form the heads. Large bolts for engines of all kinds demand extra care, because of their important uses; and also because much time is needlessly consumed in the lathe process with bolts that are badly made or forged too large. The tough Low Moor iron is exceedingly good, and should be used for all bolts of importance, such as connecting-rod bolts, main-shaft bolts, and coupling bolts.

Large numbers of small bolts are forged to a gauge, that they may be screwed easily by dies, the bolt being neither too large nor too small. A bolt to be screwed by dies need not be forged larger than the finished diameter of the screw; but frequently it is necessary to forge it smaller. The precise amount smaller depends upon the kind of screwing-dies used at the time, which will be demonstrated in its place. The safe method of proceeding is by carefully rounding one or two bolts and screwing them, previous to forging the total number necessary. These are rounded to the diameter of the one that was found to be correct, a gauge being made thereto. If all dies were so constructed as to cut equally and similar to each other, it would be convenient to make standard gauges by which to round the bolt-ends, and the result would be good screws without the bolts being too large.

All bolts, large and small, that are to be turned in a lathe require the two extremities to be at right angles to the length of the bolt, to avoid waste of time in centring previous to the turning process; and connecting-rod bolts, and main-shaft bolts require softening, which makes them less liable to break in a sudden manner; and it is important to remember that hammering a bolt while cold will make it brittle and unsafe, although the bolt may contain more iron than would be sufficient if the bolt were soft. Great solidity in a bolt is only necessary in that portion of it which is to be formed into a screw. The bolt is less liable to break if all the other parts are fibrous, and the lengths of the fibres are parallel to the bolt's length. But in the screw, more solidity is necessary, to prevent breaking off while the bolt is being screwed, or while in use. However good the iron may be, the bolt is useless if the screw is unsound; and it is well to apply a pair of angular-gap tools (Fig. 64) to the bolt-end while at welding heat.

When Bessemer iron or steel is selected for bolts, it is particularly necessary to reject all the brittle varieties, of which there is a large number. It is much safer to reject Bessemer product altogether for bolt-making, until the process shall have become more capable of producing a tough, reliable metal, sufficiently tenacious to resist the vibration and straining to which all bolts are subjected. Although Bessemer product will sustain a greater tensile strain than Low Moor iron, it would be highly improper to use such product for bolts unless it would bear the same amount of bending, twisting, or vibration as Low Moor iron. We know by experience that this iron, generally, is far superior to any kind of Bessemer product for bolts ; consequently, we must continue to use Low Moor iron until the Bessemer process can supply our requirements at a cheap rate.

Small connecting-bolts, not more than two or three inches in diameter, are made in an economical manner by drawing down the stems by a steam-hammer. Those who have not a steam-hammer will find it convenient to make a collar (Fig. 91) to be welded on a stem, in order to form a head, as shown by Fig. 92. After being welded, the head may be made circular or hexagonal, as required. The tools for shaping hexagonal heads are indicated by Fig. 93, and also by Fig. 94, which is the more convenient of the two. Such an apparatus may be adapted to a number of different sizes by fixing the sliding part of the tool at any required place along the top of the block, in order to shape heads of several different diameters. The movable or sliding block is denoted in the figure by S.

Fig. 8 represents a bolt with circular head and stop. This kind of bolt is used principally for piston-rods and connecting-rods, and requires proper fitting by being turned in lathes, and the stops are not forged solid with the heads, but are tightly fitted in the holes or slots, which are made for the purpose after the bolts are turned; consequently, the smith forges the bolts as if no stops were intended.

Small bolts with six-sided heads (Fig. 9) may be quickly made by means of a small heading-tool similar to Fig. 86, if the recess for shaping the heads is slightly tapered to allow the bolts to be driven out easily from the tool. Large bolts with six-sided heads are made of two pieces; one to be formed into the head, and the other piece the stem. The piece for the head, previous to being welded to the stem, is named a collar. By referring to Fig. 92, it may be observed that the collar is short enough to form a gap or opening between the two ends, after being wrapped tightly round the bolt-stem. By this means the collar, when welded by a pair of angular-gap tools, is both closed together and united to the bolt-stem at the same hammering; and if the collar is of a suitable thickness previous to welding, the bolt-head, when produced, will be of the desired dimensions.

To make a bolt-head by such means, it is not necessary that the bolt should be at welding heat to the centre; it is sufficient if the outside of the bolt-stem is at welding heat at the time the collar is in similar condition. To promote an easy weld, the iron of which the collar is formed should be of the same tendency to fusibility as the iron for the stem, both pieces being of tough Low Moor iron. The collar will thus arrive at welding heat about the same moment as the outside of the bolt-stem. Low Moor iron requires a very great heat for welding; and if attempt should be made to weld a collar of impure fusible iron to a bolt of fibrous Low Moor iron, the collar would fuse and burn to clinker before a welding heat could be obtained in the superior metal.

But there exists no absolute necessity for welding a head to the bolt; for many kinds of work it is sufficient if the two ends of the collar are welded to each other. The bolt-head is permanently fixed by upsetting about an eighth of an inch of the bolt-stem at the outside end of the collar. Previous to welding, the bolt-stem is allowed to protrude only an eighth of an inch beyond the collar; and during the welding, the eighth of an inch is upset or riveted, and the shape of that part of the bolt within the head becomes conical, and the larger diameter of the cone is outwards, so that the strain upon the bolt-head while in use tends to tighten instead of loosen it, the head being hindered from slipping off by the conical shape of the bolt-stem, although the one may not be, and frequently is not, welded to the other.