258 APPLIED SCIENCE 



or " nicks" the bolt deeper, and therefore the bolt is not so 

 strong as when threaded United States standard. 



301. Taps and Tap Drills. A tap is a tool for cutting 

 inside or internal threads in holes so that the holes will hold 

 tightly the bolts, screws, or studs which may be screwed into 

 them. Taps are generally made from hammered round bar 

 steel. After being drawn nearly to size, they are heated to 

 a low, red heat, and covered with lime or ashes, that they 

 may cool slowly. This process softens the metal and takes 

 out the strains, which occur in iron or steel after it is ham- 

 mered. The outside surface or skin, where the hammer 

 blows affect the iron most, is subjected to the greatest strain, 

 or t as it is called " initial tension." There are many styles 

 of taps, the most common being standard hand-taps, boiler 

 taps, stay bolt taps, pipe taps, and machine screw taps. 



Tap drills are drills used to make the proper sized hole 

 for a standard tap, leaving the hole small enough in diameter 

 to permit of threads being made by the teeth of the tap. 

 For example, the size of tap drill for a & in. screw is .24 

 in. in diameter; for a J^ in. screw tap it is .4 in., leaving 

 .1 in. for the diameter of threads on both sides of the 

 hole. The size of a drill's or reamer's outside diameter and 

 the size of a tap is the diameter outside of the threads, and 

 not the size at the bottom of the threads. 



Standard hand-taps are found in sets of three. Figure 

 119a is called a taper tap and is used to start the thread in 

 the drilled hole; Fig. 119b is called a plug tap, and Fig. 

 119c, a bottoming tap. The plug tap will finish the thread 

 if the hole goes through the piece, but if the hole " bottoms" 

 or only goes part of the way through, the bottoming tap must 

 be used to cut a full thread the entire depth of the hole. 



