Gauges of this type are stamped with the words "go on" and "not go on" for external work, "go in" and " not go in" for internal work (see fig. 11), and as the two ends are of different shape the operator quickly learns to distinguish the large from the small end without referring each time to the legend stamped upon the gauge, after having satisfied himself that he has the proper one for his particular job. In making or using limit gauges the degree of accuracy required governs the variations between the two ends of the gauge in thousandths or fractions of a thousandth of an inch, as the case demands.

Fig. 11.- External.

Fig. 11.- Internal.

The object of the metal worker is to produce a certain shaped article from a fiat thin plate, and in consequence the material of the plate must be squeezed, forced, or stretched, as the case demands, into the given form. When the operation is carried out by hand the principal tools made use of are the hammer, some form of anvil, together with some templates giving the peculiar outlines called for by the design, to which the workman refers from time to time in the progress of his work. The action of the hammer upon the material is to stretch the plate locally-that is, where the blow is given; there the plate is slightly thinned, and there will be in consequence a change of form. In the case of sheet iron or steel the act of hammering must be carried on upon the hot plate only, and if the hammering is carried on after the plate has cooled down below a certain limit the material will be damaged. Particularly is this the case with some qualities of steel. Some other metals may be worked to a certain point cold, but must be annealed at intervals to remove the brittleness produced by the hammering. To facilitate this process of shaping cast-iron blocks are used, suitably formed, upon which the sheet is laid and gradually worked into its hollows by means of the hammer, etc. But the operations by hand labour must of necessity be tedious, requiring much skill on the part of the workman, which can only be acquired after many years' constant effort, with the result that the costs of production must be very heavy.

Under the stimulus of increasing competition the aid of mechanical tools has been sought, initially possibly mainly with a view to reducing the heavy costs of assistant labour required by those engaged in the working of such heavy plates as are required in the manufacture of the steam boilers, The weight of these plates is very great, even in the case of small boilers, and the operations of Hanging and shaping them makes it necessary to provide each workman with a squad of assistants to manipulate heavy hammers, and to move the plate from time to time into the furnace and back again into the required position for flanging, etc. From causes the tools for pressing the flanges into shape, for punching holes, and rolls for bending are absolutely necessary in every well-equipped boiler shop to-day.

But whilst in the case of heavy work the use of tools has been common for some time past, the case is somewhat different in workshops where the work is of a much lighter description. The better knowledge that we possess of the

"laws of flow"' of metals under pressure has shown engineers the limitations under which materials can be successfully forced in to special shapes by pressure.

One of the earliest machines which the metal worker used was probably one which enabled him to stamp a pattern upon metal blanks at one operation, instead of slowly cutting the same by hand. The operation of squeezing or pressing gave the name to the tool which now embraces an innumerable number of special operations.

The term"press,"as applied to metal work, may be any machine capable of altering the form of any body by squeezing, forcing, or crushing. The machine naturally takes a great variety of forms, and is often classified according to its construction and the source and nature of the power required for its working. For instance, the terms single-sided press, double-sided press, lever press, foot press, hand press,screw press, fly press, toggle press, crank press, power press,. hydraulic press, etc., and numerous other names are in general use.

Then, in other cases, the name of the press depends upon the special operations that it is intended to perform, and of these shearing, punching, forming, bending, embossing, coining, etc, are familiar examples. Thus the press is classified in actual practice on several lines, each of which has its advantages. It is the intention of the author to take for the first example the simplest form and the most commonly used of all the presses as the starting point from which the student and mechanic alike may study the development of the modern machines.