Have a unique application that requires a tap for photography threads, and nowhere on earth can I find this tap for sale, so I am going to try and make it. I have successfully had a helical boring function on my home made CNC that works, but I have always wanted to make a tap.

I will be starting with a 2" OD 1045 steel bar and harden it as best as I can. It will only be used on 6061 aluminum, so I am not worried too much about the hardness yet. After cutting I will take a propane torch to the teeth to add some hardness to see if the tap works before going through the hardening process for real. (meaning it taps a 1/8" sheet and my part threads on nicely.

The question I have is about tap geometry. Every generic tap I see has slots dropped dead center on the axis of the tap. I wonder why they are never offset to increase the rake geometry? for example:
Attachment 472806

Using a 1/2" ball end mill dropped in like normal I get something like a 76° rake angle, vs just offset I can get 73°. If I drop the end mill way closer to the edge, I have a 48° rake angle. I am genuinely curious why I have never seen a tap of this configuration before? I am guessing the center will give the teeth the most strength? Or is this to cut in both directions vs only in the threading direction? This will be the first tap I have made, so i am open to others thoughts.

I do plan on dropping the end mill along the axis, but I am curious if its to cut in both directions (cutting threading, chip breaking reversing) Keep in mind these threads are probably 0.5mm in height. SUPER find threads. I have that drill rod-kit thing to measure the pitch of the threads coming to make sure its done right, but to be honest so long as my part threads, I am happy. That is the go/no-go test.

Just look at the difference in geometry:
Attachment 472808
It looks like a much sharper tap. (blue cross is the axis of the part, and the U is the 1/2" end mill dropped along that axis.