USE/CARE OF HANDTOOLS & MEASURING TOOLS - OD1621 - LESSON 2/TASK 2
f. Use of Taps.
(1) Selecting a Tap Drill. The size of a tap is the outside diameter
of its threads; therefore, theoretically, the hole drilled for tapping could
be smaller than the tap by twice the depth of the thread if a full thread is
cut. The shape of the thread partly determines the amount to be subtracted
from the tap diameter.
Figures 136 through 139 on pages 178 through ]81
indicate the various thread dimensions of British and Metric screw thread
standards, American pipe thread standards, and the tap drill size required
for each thread. Do not use a tap drill that is too small in diameter as
this will cause tapping troubles.
A drilled hole should be of sufficient
diameter to produce a thread depth of approximately 75 percent.
percent is only an average. Actually, the percentage may vary from 50 to 53
percent for small or deep holes, to a maximum of 83 percent on any size. A
definite thread depth for all sizes, under all conditions, is not practical.
You must determine which is most suitable by analyzing the following
conditions: diameter of tapped hole, nature of material being tapped, depth
of tapped hole, and the pitch.