Cylindrical gauges of this character are made of good iron which is hardened, or of steel which is not hardened. To make them of steel without hardening is the cheapest plan ; such a course avoids the tedious grinding and filing after the gauges are swelled or injured in some other manner during the usual hardening processes. When it is intended to make them of iron, the metal selected should be a hard crystalline iron, which has been thoroughly welded and hammered to erase all appearances of cracks, sandy streaks, limy and other earthy accumulations, these rendering the metal unsolid and unsuitable for such gauges, although it might be of excellent quality. The proper metal being selected, the forging of the plugs commences by heating in a clean fire and steam-hammering a rod or bar and welding it with rounding tools having angular gaps, in order to close the particles. After the iron is thus prepared the plugs are roughly shaped while attached to the bar, each one being made singly and cut off to be afterwards separately trimmed to the proper form. A plug thus shaped and ready to be cut off is shown at one end of a piece denoted by Fig. 510. Previous to forging, it is often necessary to consider the sizes and conveniences of the lathes that are to be used for the turning; in small four or six inch lathes one plug is easily turned without any additional handle, but if a large lathe is to be used for small plugs they are forged with handles which are solid with the work, and so remain until of no further use, when they are cut off in the lathe. For some lathes two plugs may be forged with their handles solid together, as shown by Fig. 511; these are turned while together, the cutting apart being the final process. The convenient method to suit most lathes is to forge the holder at the cylindrical end of the plug, as indicated in Fig. 512 ; by this arrangement the necessity for gripping the cylindrical end of the gauge is avoided. Steel plugs are forged with similar holders, if required, a cheap close-grained cast steel being selected for the purpose.

The forging of the rings is effected by two principal processes, whether the metal used is iron or steel. One process is suited to all small rings not exceeding two inches at their outer diameters; these are forged by cutting slices from a rod of solid close-grained metal, and afterwards cutting off the rugged portions near the centres of the intended rings. The other process should be resorted to for making the larger sizes; these are produced from slices or properly welded cakes, that have holes made in their middles, and are finished separately on a mandril which is of the same diameter as that of the intended hole when forged. In such a ring the hole is first made with a punch, and afterwards enlarged with taper drifts. If the ring being made is of iron, a welding heat is given to the work after a hole is made, and the ring is welded with a hammering given to its entire surface, by which it is rendered solid, and any injury that may have resulted from punching is remedied. To punch steel, a round punch having an oval cutting-end is used, that the punch may gradually enlarge the hole from its original form of a slit, indicated in Fig. 513, to the desired circular shape. After punching, the hole is drifted until of sufficient diameter to admit the mandril's end which is to hold the ring while being hammered for stretching or welding. The final forging of the ring consists in cutting off superfluous pieces, and curving the outer circumference termed the edge, and this is done with half-round top and bottom tools; after which the ring is softened for turning, by allowing it to cool slowly in cinders.