COMMON FASTENING AGENTS 



251 



good holding power. Screws are usually made of steel, and 

 are finished in many ways, so that we have on the mar- 

 ket blued, brassed, and bronzed screws. Wood screws are 



FIG. 110. Kegs of Nails. 



specified by their length, and by a number which is the 

 gauge number of wire from which they are made. They 

 are sold by the gross. The screw is capable of resisting a 

 much greater force than the nail, and is therefore a much 

 better fastening agent. It is, however, 

 more expensive than the nail, and cannot 

 be driven into wood so rapidly. 



In addition to being used as fastening 

 agents, screws are also used for communi- 

 cating motion, as is the case of the lead 

 screw of a lathe or the screw of a jack 

 screw. These screws are produced by a 

 cutting process in which the thread is 

 formed from solid pieces of stock; that is, 

 a single-pointed cutting tool, harder than 

 the stock, cuts it, or it is cut by means of taps and dies. 

 The tap and die are tools of hard steel used to produce in- 

 ternal and external threads respectively. 



FIG. 111. Wood 

 Screws. 



