DEVIATION AND ITS CAUSES 13 



and nil held by springs n and Ui and plungers ^2 and Uz 

 in cylinder o. Rod mi passes up into a hollow space in 

 plunger ^2 so that when the pressure of spring n acts, pres- 

 sing plunger n2 downward, rod mi passes into the space in 

 nz. This space has a check-valve controlled upper end p, 

 which opens when the plunger ^2 descends and closes when 

 it ascends, equilibrium of pressure being effected by a fine 

 bore pi. The brackets q carry an electrical contact r 

 which is closed when plunger 712 is in its upper position 

 (Fig. 2) and broken when this descends. 



The lugs s hold the heavy pendulum t in a frame and the 

 swing of t into casing a^ is arrested by a stop u and in 

 the other direction by a stop Ui. This pendulum carries 

 a second part of the contact k of slide i so that the positions 

 of the pendulum t and slide i decide whether the contact 

 k is opened or closed. The two electric contacts k and r 

 are arranged in the circuit from the source of power d, 

 which operates worm di. These two contacts (Fig. 4) 

 are arranged in series so that the motor is stopped if only 

 one is switched out. This occurs as follows: The plunger 

 712 continues its descent by momentum after the boring 

 tool has struck its blow, and this compresses the adjusted 

 springs n and Ui, thus turning the screw spindle hi and disen- 

 gaging it from the half nut h2 so preventing slide i from 

 moving. But contact r is now broken, stopping motor 

 d and screw spindle hi. Plunger n2 can only move back 

 upward slowly, owing to the design of air valve p and the 

 hollow space nz, and this is designed so that before the spin- 

 dle can return to its working position and r close a new 

 blow — assuming regular working — with a downward move- 

 ment of the plunger takes place. In interrupted working, 

 say over 20 sec. between blows which is a maximum time 

 for springs n and rii, the mass of plunger no and the valve p 

 function ; 712 returns to its initial position, throws in spindle 

 hi and closes the r contact. This starts motor d if contact 

 k is also closed. The closure depends on the position of the 

 pendulum t, for when we have deviation of the bit to the left 

 throwing t to the right, or engaging it with stop Ui, contact 



