The usual method of obtaining rectilinear ram motion from rotary shaft motion is by means of a crank or an eccentric'on the shaft, which revolves in the head of a pitman connected with the ram. An advantage of this form is that the eccentric not only forces the rain down, but also draws it up, ready for another stroke. The amount of stroke (or distance the rain is caused to move) will, of course, depend on the amount of the eccentricity. The same effect may be produced by cams on the shaft bear ing against rollers on the top of the rain, the ram being forced away from the shaft by the cams and drawn back to its original position by other means, such as springs, weights, or return-cams.
Toggle Coining-prem.—When a small amount of motion and a conse quent increase of pressure are desirable, the rectilinear motion can be pro duced by a toggle, which, on account of the nearly irresistible pressure exerted at the end of the stroke just when such a pressure is most required, is almost universally employed for coining-presses. Such a toggle is usu ally opened and closed by a crank motion. Coining-presses (ph 33, fi• r) usually have two massive columns, joined at the bottom by the bed and at the top by a trussed cross-beam, the columns being as close together as practicable, in order to get as much rigidity as possible.
Screw-press.—The principle of the screw is frequently used in presses to transform rotary into rectilinear motion. In this type the nut in which the screw works is a part of the frame, the end of the screw bearing against the rain (pl. 32, fig. 6). This is also an effective form of press for certain kinds of work, but it is now rarely used except for hand-power.
Drawing- and Punching-presses.—The shape and size of a press depend on the work for which it is designed. A press for cutting out large sheet metal blanks, such as sections of "pieced" tinware, requires a well-spread out frame with a large bed, containing a hole of generous proportions, through which may drop the sections cut in the dies (fig. 8). On the contrary, a press for punching out nuts (pl. 32, jig. 9), for shearing heavy bar-iron, or for other work which requires a heavy stress in a small space, must be compact, with a large reserve of metal in the frame, not only to withstand a breaking strain, but also to prevent any flexure of the parts, or, as it is called, "springing." Even if a press which springs open appreciably at every stroke should produce good work for a time, disintegration at the weakest point gradually goes on and will sooner or later make trouble.
For this reason, if for no other, a properly proportioned press-frame, with interior angles well filleted and with exterior angles neatly rounded, is always to be preferred, even if considerations of beauty are left out of the question. Presses for long and narrow work, such as buggy-axles, should
have two or more pitmans to transmit the power from the rotating shaft to the ram. Where there arc only two pitmans they should be placed near the upright columns, to obviate springing in the shaft, and the ram and the bed should be well trussed, to prevent springing. Presses for work that is cut and formed or formed only, such as tea-trays, fruit-can tops and bottoms, etc., are usually inclinable, so that the work pressed to shape in the dies may be raised out of the lower die by spring knock-outs, and be allowed to slide back by gravity through an opening in the back of the press. A style of press usually adopted for heavy punching and shearing has the shaft running from front to back, the front end of the shaft being turned into a crank-pin, while the fly-wheel is at the back, out of the way (fl. 32, jig. 9). For work in comparatively thin sheet-metals the form hav ing the shaft running from side to side is preferred (ft/. 32, figs. 7-9 ; 33, fig. 2).
The "Bottom-slide" Press (fig. 4) possesses several advantages. It has a heavy base supporting the shaft or shafts, gearing, fly-wheel, etc., while columns of considerable tensile strength support a trussed head. The ram is guided by the columns, is given an upward motion by cams on the shaft, and returns by its own weight, thus obviating the necessity of an expen sive lifting arrangement. This press is adapted for deep drawn-work. It economizes metal, which is massed in and around the base of the machine.
Although the ram is made in many forms, its adjustment, which is an important fectture, usually depends on the screw principle in some shape, but eccentrics, wedges, and other devices are also used. In some presses the bed is capable of adjustment, by which considerable latitude is pos sible between the bed and the rain; but this often affects the rigidity and accuracy of the press. The complexity of presses increases in accordance with the increase of the complexity of the work to be produced. Deep or drawn work, such as seamless cups and pans, not only is cut out but also is drawn to shape by a single stroke of a double-action press. Such presses contain two rams, one inside the other, so arranged that when the work of one is completed it is taken up and finished by the other. Triple presses are also in use. By multiplying the cams on the press-shaft the number of operations possible in a single revolution of the shaft is also multiplied.