Metal Working Tools 41 



makers being generally far too soft. The use of this tool will 

 be mainly in the cutting out of sheet metal and similar 

 material. To re-temper the chisel, it is to be made red hot 

 for some distance from the cutting edge, say i y to 2" up in 

 order to fully " draw the temper." The cutting edge is then 

 plunged under water to the extent of f" or J" for 5 seconds, 

 until it is " black," then withdrawn, and the whole flattened face 

 of the chisel rapidly rubbed with sandstone until a clear bright 

 surface is obtained. On watching this surface carefully, a 

 bluish-yellow colour will be found to creep gradually towards 

 the edge, indicating the conduction of heat from the still hot 

 upper portion of the chisel. According to the hardness or 

 " temper " required, this is allowed to proceed, the tool 

 being plunged right under the water when the correct colour 

 (pale to dark blue in this case) is seen. The whole process is 

 one that must be conducted with considerable expedition. 



The judging of the colour is the most important portion of 

 this process, and probably some time may have to be spent in 

 the acquirement of the art. There is a danger of cracking the 

 tool if it be cooled too rapidly, while, on the other hand, one 

 must quench instantly the required colour is seen. It is in the 

 judicious cooling of the tool that most skill is needed, and it 

 is a good plan to quench the whole tool slightly before rubbing 

 with sandstone, in order to make the temperature difference 

 smaller, and the colour therefore appear more slowly. Even 

 after treatment with sandstone, the whole tool may be im- 

 mersed in water several times, for an instant only, if the colour 

 shows signs of appearing too quickly. The deeper the colour 

 the softer is the resulting steel. (See also page 62.) 



Callipers. Inside and outside callipers should be 

 separate instruments. Their shapes are well known, and little 

 need be said as to the manner of their use, which is sufficiently 

 obvious. Care should be taken in their use to obtain average 

 readings, and not to rely too much upon a single measurement. 



Outside Callipers are opened until too wide for the 

 distance to be measured, slipped over the object, slightly 

 closed, and pulled free, at right angles to the axis of the object. 

 Several such readings should be taken. 



