170 Chapter 7
tap drill size, then using the drilling-machine chuck to hold and align the tap while it is turned by hand.
There are typically three types of taps used: a tapered tap to facilitate initial thread cutting; an interme-
diate type, used to progress the thread after it has been started; and a “bottoming” thread, which is used
to obtain the full thread depth when cutting partway through the piece.
Finish Marks.
Many metal surfaces must be finished with machine tools for various reasons. The acceptable rough-
ness of a surface depends upon how the part will be used. Sometimes only certain surfaces of a part
need to be finished while others are not. A modified symbol (check mark) with a number or numbers
above it is used to show these surfaces and to specify the degree of finish required. The proportions of
the surface roughness symbol are shown in Figure 7.9. On small drawings the symbol is proportionately
smaller.
Figure 7.7A A typical drill with the terminology related to it and two holes—one 0.75 deep and the other
through.