THE: most common form of double-sided screw press is shown at fig. 27, in which the top of the frame easting terminates in a boss B, which is l>ored and threaded or screw-cut to receive the screw attached to the ram. In large presses a better plan is to bore a parallel hole through the top of the frame casting (see fig. 28), and into this hole fix firmly a nut through which the screw can work. This nut has a collar D on one end, and is screwed at the other end a. It is made a tight fit in the frame, being forced into position. A lock nut b holds the main nut in position, the face of lock nut being screwed down upon the boss of casting C. When the press is l>eing operated the strain or thrust is taken between the face of the bottom of the casting at E and the collar D.

The single sided screw press is shown at fig. 29. The action of the tools in a single-sided press tends to spring the frame by an amount varying with the character of the operation performed. They are usually strengthened by either joining K to L by an arm cast in place, or by means of a rod of round iron. There are, however, certain classes of work requiring the front of the press to be entirely unobstructed, and in such cases failure of the press frame frequently occurs by fracturing at the point marked A, B. These breakages happen more frequently than many mechanics would imagine, even when employed upon comparatively light work. Where these breakages occur a good method is to cast on the press a strong rib shown at R, which, adding very little to the weight of the press, gives strength where strength is required.

Fig. 28.

Fig. 29.

Other suitable sections are shown in tig. 30, the selection in any given machine depending upon questions of manu-facture and the peculiar uses to be made of the machine. The student of mechanics should notice that this section is actually strained by a bending moment and a tensile stress, due to the load more or less suddenly applied, according to the character of the operation being executed. The stresses produced are not exactly known, because the load is a line one, and a large factor of safety must necessarily be employed, allowing for the fact that the material is being repeatedly strained by a load rapidly varying from zero up to the maximum, and back again to zero. Under these circumstances the area of the section must be ample, so that the maximum stress imposed upon the metal may be low, otherwise failure at an early date is inevitable.

Fig. 30.

Another important quality that the frame must possess is rigidity. If the frame should spring perceptibly, then there is a great risk that the dies will not meet truly, and this must result in broken tools. Hence in any press the frame should be designed on liberal lines if satisfactory work is wanted, for it is obvious that the cost of a few pounds of cast iron, suitably placed in the frame, will be a good investment, producing, as dividend, good work, few broken tools, and a valuable reputation.