Extropic, good questions. You are correct, I could have done a through hole with no significant consequences. I did want to retain a smooth plain on the top of the press plate so that if I was forming metal it would not flow into any indentations such as the through hole. For the most part, that excuse is not worth its weight in dog food but I figured that since I only had 2 holes to do, the time expended to remove the tap and clean out the hole was not significant. The tap was a 3/8-16, 4 flute bottom tap. Why? Other than needing to finish with a bottom tap, my decision was mostly driven by price. This tap was $30 delivered, I saw many others that were close to $200. Also, I didn't think I was going to get very far with a 3/8" taper tap or gun tap since my depth was .700 (even if I ground off part of the tip). I expected a massive change in cutting performance like switching from HSS to carbide when drilling the hole. The carbide produced a beautiful chip with both the drill and the end mills when drilling. Thought the tap was going to do the same. It did not, the swarf it left behind was more like dust instead of "C's" and "6's". However, the threads were very smooth and polished. I wasn't hobbled by starting with a bottoming tap, I had the luxury of not moving the part after drilling the hole so my tap guide was directly over center allowing me to apply force straight down into the hole until the cutting threads could pick up the feed.
