What most people do not know is that the deeper you tap or the thicker the material that you are tapping, the larger the drill you should use.
There is a chart in your Machinery's Handbook, I don't have my latest edition with me, so I can't give you the page. I have my antique 1943 edition and it does not have the chart.
Drill and tap charts are only good for about 1 diameter of thread. For example, the chart will call for .201" drill for a 1/4-20 tap at 75% full thread. But, this is only for about a 1/4 inch deep tapping. As you go to 1 1/2, 2, and 3 times the diameter of the tap, the hole to drill gets larger. If memory serves me correctly and it seldom does at my age, if you tap 3/4 inch deep with a 1/4-20 tap, the holes can be about .218" in diameter and still considered as 75% full thread. I will find the information in my latest book and post it.
I worked at a shop where they tapped a lot of stainless, mostly 304, and they always wanted these deep holes even if they were not necessary. People were breaking taps all of the time and I spent hours removing broken taps. When I printed out the minimum and maximum hole sizes for our most commonly used taps, the breakage was mostly eliminated.
I'll find it and post it today.
Mike
Two Haas VF-2's, Haas HA5C, Haas HRT-9, Hardinge CHNC 1, Bother HS-300 Wire EDM, BobCAD V23, BobCAD V28