This section is from the book "Machines And Tools Employed In The Working Of Sheet Metals", by R. B. Hodgson. Also available from Amazon: Machines and tools employed in the working of sheet metals.

When considering the power required to perform work in either a press or stamp difficulties arise under the usual working conditions which make it impossible to obtain reliable results by any known formula. One reason for this is that the effect at the face of the hammer, or at the point of the punch, as the case may be, depends so much upon the nature of the resistance, the result being that it renders the ordinary formula unreliable, so far as practical workshop operations are concerned. In a previous article attention has been drawn to the fact that when punching or shearing metals approximate results, which are governed by the ultimate shearing strength of the metal, may be obtained.

As a rule some ordinary formula would be used to calculate the work stored up in a fly press or stamp hammer, after which a rough estimate of the resistance offered-according to the nature of the material, its thickness, and the time taken to perform the work-would be obtained. In works where a large number of presses or stamps are used it is usual to carry out a series of experiments to find the amount of work that can be done in a certain press, having a screw of known pitch, a fly lever of a certain length, and by placing balls of different weights upon this lever. Again, experiments are frequently carried out to enable some idea to be formed as to the amount of work that can be done by a certain weight of hammer falling through a given height, and in this way approximate data may be obtained which will greatly assist in estimating what size machine would be required to make some specific metal article. By working out a few examples it will be instructive to see how these unknown quantities are continually having to be assumed when dealing with problems connected with the press and stamp; at the same time the working out of these problems will enable one to obtain some approximate results by the principle of work.

Work in the Copying Press.

The screw-press, fig. 281, is used extensively for copying letters. It consists of a frame D, bored and screw-cut to form a nut to receive the square-threaded screw S, an iron slab E connected to one end of the screw, and a lever handle A C B fixed to the other end. In operating the press a couple comes into action which rotates the screw. The relation of P to W may be found as follows: Let P equal the push applied at the end A, and also the pull applied at the end B, of the lever handle, and W the resistance overcome-i.e., the pressure that can be put upon any object O-and let p equal the pitch of the screw thread.

Work of W per revolution = Wp

Motion of P per revolution = 2 π r

Then from the principle of work we have in one revolution (the power applied being 2 P)-

Wp = 2Px2πr, or P: W:: p: 4 π r.

Example.-In the screw press, fig. 281, the lever handle is 16 in. long, P is 20 lb. at each end of the lever, and the pitch of the screw is 1/4 in. Find the pressure that can be obtained at the slab E.

Fig. 281.

The distance travelled by 2 P in one revolution = 2 x16 π inches, and distance moved by W = \ in.

Then

w | = | 2 x 20 x 16π | = | 8042.49 lb. pressure. |

1/4 |

From the equation

P/W = p/4πr it is evident that any of the unknown terms may be found when the other three terms are given.

The work done by the screw-jack when used for lifting weights may be treated in the same manner as the screw press.

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