TOOLS FOR DRILLING. 97 



jieces of iron sticking between the teeth should be removed by a 

 )ointed instrument. 



Fig, 69, A, B, C, JD, shows the successive shapes to be given to the 

 )oint of the centre-punch. After producing the octagonal form D the 

 >unch is clamped between a tail- vice, which is a small tool resem- 

 bling a vice, but used by the hand, and the point placed upon a 

 ;quare piece of wood, which is fixed between the vice in such a 

 nanner that a side perpendicular to the fibres is horizontal and 

 ippermost. To hold the punch more securely, a small groove may 

 )e filed in the wood, in which the punch fits. The file is now guided 

 )y the right hand alone, while the punch is pressed upon the wood 

 ind turned in a direction opposite to the file, until the point is 

 ounded off. To give a better form to the punch, the portion next 

 jo the point may be made cylindrical, as shown in F, G, H. The 

 aoint mast now be hardened. This is done by making it red hot 

 ind immersing it in water. 



The dark layer produced on the surface of steel by heating may 

 oe removed by emery-powder. This is powdered corundum, a mineral 

 }f great hardness, which is to be had in various degrees of fineness, 

 ft is used in the laboratory not only for giving a smooth surface to 

 metals but also for grinding glass, and several sorts ought to be 

 sept. Stout paper, coarse linen, or square pieces of soft wood are 

 covered with glue, and over the glue a thin layer of the powder is 

 spread. After drying the rough surfaces are used like files. 



Red-hot steel cooled suddenly in water becomes so hard as to 

 scratch glass, but also so brittle that it breaks very easily. This ex- 

 :reme brittleness and hardness may be diminished to any required 

 legree by the process of tempering ; this consists in heating the steel 

 moderately and then allowing it to cool. Pieces of steel, of the size 

 }f the centre-punch, and also larger pieces, are held by the pincers 

 over a spirit- or gas- flame and continually turned until they are heated 

 :o the required degree ; smaller objects must be heated upon a piece 

 )f sheet iron, as otherwise the heating would not be uniform. The 

 steel, when thus heated, assumes successively different colours, first 

 i light straw-colour, then a full yellow, then brown yellow, purple, 

 )lue, and finally a greyish black is produced. Tools for working 

 netal are heated to a full yellow, and then immersed in water. Tools 

 leated to a light yellow are too brittle ; when heated to a brown 

 yellow, they are too soft and do not preserve their edges when in use. 



Drills are made of steel wire 3 or 4 mm thick, of which about 100 

 )r 20Q8r (1 or 2 m long) are bought, in order that the drills may be 

 )f equal thickness at the end which fits into the aperture of the drill- 

 stock. The drill-stock, fig. 70 A (page 98), consists of a wooden 



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