Threaded Fasteners & Screw Threads:
"n" = pitch. Edit: threads per inch. For metric it would be 4*n instead of 4/n.
Thread Depth:
Thread depth (C & A) is 2x nominal diameter (d) for Aluminium and 1.5x nominal diameter for Steel.
Thread Clearance:
Thread Clearance (B) is at least 2x thread pitch. This is because you assume that the last two threads of a tap will not result in a full thread of engagement, so you need to "overshoot" the tap in order to get the actual full-thread depth. This Wikipedia page has a good picture of a typical tap, and you can see the chamfer that will not create full threads.
Example:
I'm using a .250-20 UNC. There are 20 threads per inch, or .05 inches per thread.
.05 x 2 = .1 inch drill depth minimum beyond your full-thread depth.
Drill Depth:
The drill depth (E) is the same as the clearance above. In order to get the full-thread depth per the requirement, the chamfered tip of the tap must have room to extend past the threaded portion.
=====================================================
Imperial:
=========================================================
=========================================================
=========================================================
