Bolts in general are of two classes, consisting of those that have no screw-parts, and those that have them. By small bolts are here signified such as are not more than an inch, or an inch and a quarter in diameter.

When a large number of small plain parallel bolts which are to be without screws require turning, all are first partly lathe-turned to their respective diameters by means of gap-gauges. The gaps in these are of proper sizes to roughly indicate the finished diameters, and to cause a small amount of metal to remain for finishing. After a lot of bolts are thus reduced, all should.

be finished to the exact sizes by using other gap-gauges, and removing the small amount of metal with springy tools.

A large number of small bolts are required to be accurately turned to some specified diameter. The turning of such commences by a first reduction to callipers or gap-gauges, previous to a final smoothing to ring-gauges. When a ring-gauge is used for this purpose, the holes which the bolts are to fit must necessarily be of the same diameter as the gauge-hole in the ring. Instead of a ring, a temporary block can be used. A block for this purpose consists of a piece of metal in which a hole is bored that shall constitute a gauge-hole to which the bolts can be fitted. A piece having such a hole is denoted by Fig. 939. The hole is exactly the same in diameter as that of the holes in which the bolts are to be placed; therefore the block can be kept close by the turner at the lathe, and all the bolts of that size be fitted to the gauge-hole. By carefully fitting a number of bolts to one hole of this character, all of them are accurately reduced to the desired diameters previous to taking them from the lathe in order to put them into their places. During the use of a soft block for such fitting, the hole must be kept quite clean and oiled, to prevent damage to any bolt that may be tried in, and to prevent damage to the hole. Every bolt must also be smooth at the time of trial; if not, the gentle hammering which is needed to drive it into the hole, will probably cut off the rough tool-marks, and make it too small. The gauge-block shown by Fig. 940, is one having a number of holes of various sizes. Some of these are taper, and some have conical mouths, for the fitting of bolts with conical heads; others are furnished with recesses for bolts with cylindrical heads.

Small bolts that are to be screwed, named screw-bolts, are turned to their required diameters with regard to the screwing process, in addition to turning their intended plain parts to fit gauge-blocks. A well-formed screw-bolt is provided with a plain part adjoining the head, which is smoothly turned to fit its place, as in the case of a bolt which has no screw. When the bolt has been thus treated, the exact required length for the plain part is ascertained and marked, by which the place for the screw is shown. This part is therefore reduced to the diameter suitable for producing the desired screw. The exact diameter of this end depends on the means to be used for screwing. If to be entirely screwed without dies, the diameter is exactly the same as that of the screw when finished. But if to be screwed with dies, it may be, in nearly every case, smaller than the finished diameter, to allow the dies to squeeze up the thread to the proper height. Gauges are employed while turning these ends; but each gauge will only suit one pair of dies at one time; and as the dies become more and more blunted by use, the bolt-ends to be screwed require to be turned smaller and smaller. Concerning this subject refer also to page 18.

Small bolts are sometimes made with square handles, similar to those mentioned for taper pins and screw-studs forged singly. Such handles are provided for both plain bolts and screwed ones; and it sometimes happens that the lathe to be used will not turn them without handles of proper length, by reason of the lathe being too large for the comparative small bolts. A bolt with a handle is seen among those shown by Figs. 935, 936, 937, and 938.