96 



THE FILE. THE CENTRE-PUNCH. 



round, about l cm thick and from 6 to 7 cm long, tapered off at one end 

 into a short round point, which must be very sharp, but not thin. 

 It is used for making small marks or cavities in the metal, such as 

 those in the dies and the screw-stock. 



A short piece of bar steel may be bought of the required length 

 and thickness, but before filing it, it must be softened by making 

 it red hot and allowing it to cool very slowly. The heating must 

 be done over a charcoal or coke fire ; common coals injure the 

 steel. A piece of wire may be wrapt partially round it, leaving a 

 long portion to be used as a handle for withdrawing the hot steel 

 from the fire, and also for afterwards suspending it in order to cool 

 it slowly ; or better still, the hot bar may be laid upon ashes, which 

 are a bad conductor of heat. Two kinds of files are required, double- 

 cut and single-cut ; the former for giving to the material the re- 

 quired shape, the latter for smoothing the rough surfaces produced 

 in the former operation. Brass requires keen files, but does not 



FIG. 69 (| real size]. 



wear them so much as iron and steel, which may be worked witli 

 coai^r files. It is best to have double sets of files, of each kind. 

 Large files are not suited for small vices, like those in fig. 48 and 49 ; 

 files to be used with these must not exceed a weight of 250? r ; such 

 a file is about 25 cm long, 25 mm broad, and in the middle 6 mm thick. 

 The utmost care should be taken in filing to produce flat surfaces ; 

 the surfaces turned out by bad filing are always rounded. Pressing 

 down is only required during the forward stroke; for the file does' 

 not cut during the backward stroke and pressure makes the file 

 blunt. The file-handle is grasped with the right hand, and the ex- 

 tremity of the file is held between the thumb and the first twc 

 fingers of the left hand. The surface to be filed should be placed 

 horizontally, the object being fixed in the vice so as not to project 

 too much above it. Files should be kept clean by a wire-brush 



