SCREW-CUTTING. 



95 



)ther. Objects, the thickness of which is to be measured, are placed 

 between the beaks ; the diameter of apertures, -wider than the breadth 

 of both beaks together, may be measured by inserting the beaks into 

 the aperture, drawing them apart as far as the aperture permits, and 

 adding the width of the two beaks, usually 15 mm , to the number of 

 millimetres read off on the scale. The piece of wire to be made into 

 a screw is fastened in a vice, while its end is placed in the die ; the 

 two halves of the die are at first farther apart than necessary, but 

 capable of being brought together by regulating the pressure-screw 



ixed in the die-stock. The die- stock is now turned, so as to worm 

 the die on the bolt. Pressure downwards is only required in the 

 beginning ; as soon as the operator is satisfied that the shallow 

 thread first produced is a screw line, the stock is turned as far as 

 the screw is required to extend ; it will do so without pressure. The 



I stock is then turned backwards and taken off, the die screwed up a 

 little closer, and again applied in the same manner ; the process is 

 repeated, closing the die a little after each operation, until the worm 



i is cut to the required depth. The portions of metal which are re- 

 moved fall through the notches of the die, but should the latter cease 

 to proceed smoothly, it is generally owing to minute scales which 

 clog the die and which must be removed by a pointed instrument 

 before continuing the work. The die must be greased with the 

 same materials as are used for the tap. In cutting metal rough 

 edges are often produced ; these must be removed by filing. 



The tools required for boring holes in metal are drills, the drill- 

 stock, and the centre-punch ; these the student may easily make 

 himself. The centre-punch is a piece of steel, either square or 



