The frame screws of my reels are 4-40 UNC. There are 10 such holes (foot & pillars) to tap for each reel. On repetitive tasks, it is good to have the best possible tool. McMaster-Carr sells “Vanadium Steel Hand Taps for Aluminum” which I have used for several months.
The threaded length on this 4-40 tap is just 0.3 inch; behind that is a reduced diameter shank so that a deeper hole may be tapped.
Note 7 Jan 2020: It did not last. I think that this is only the second tap that I have broken since I started reel making.