about tap drill sizes and thread depth.

I ran across this the other day and when Ron asked about tapping, I started looking for it again. Google wasn't very helpful, but finally found this in my collection of mostly boring books. I've also made what appears to be a mistake of boring the holes a bit on the smallish side so I would end up with what I thought would be plenty of threads when I ran the tap through.

Why is the tap drill diameter important ?

Tapping troubles are often caused by using tap drills that are too small in diameter. For ordinary manufacturing not more than 75 or 80 per cent of the standard thread depth is necessary, and for some classes of work not more than 50 per cent is required. Tests have demonstrated that a full depth of thread in a nut is practically no stronger than 75 per cent depth of thread, and that 75 per cent depth of thread is only 20 per cent stronger than 50 per cent depth of thread. From 75 per cent depth of thread, the power required to tap rapidly increases as a full depth of thread is approached, and the danger of tap breakage is much greater.
Tap manufacturers often find that users of taps actually punch or drill the holes to be tapped of a smaller diameter than the root diameter of the thread, so that the end of the tap must act as a reamer before the thread can be cut. Frequently the tap will ream clear through the nut.
Depth of tapped hole to obtain strength equal to or greater than stud is as follows: For steel, depth d=1 1/4D (D=diameter of stud) ; for cast iron, d= 1 3/4D; for aluminum, d=2D; for magnesium, d=2 1/2D.

Is the tap drill size affected by the material to be tapped ?

Tap drill sizes for machine screws, in particular, should be varied according to the material to be tapped and the depth of the tapped hole. In general, for holes where the screws enter more than one and one-half times the diameter, one-half of the full thread is usually sufficient. Soft, tough materials, such as copper, Norway iron, drawn aluminum, etc., should have a larger hole for the tap than the hard, crystalline materials, such as cast metals. If the tap drill size is not increased, the tops of the threads of the softer materials will be torn off, thus actually decreasing the effective depth of the thread of the tapped hole. On the other hand, if the hole is originally drilled large, the tap will, when cutting tenacious materials, especially after the keen edge has been slightly dulled by use, reduce the size of the hole by drawing the metal at the top of the thread, thereby increasing the depth of the threads.