Pretty sad nomenclature here...
Cast iron is very forgiving but in difficult materials tapering the hole will prevent tearing of the threads. This tool is not a tapered drill bit but is called a pipe tap reamer like this:
https://www.mcmaster.com/pipe-reamers/In shallow bores there are no tricks required. One simply requests a short projection pipe tap like this:
https://www.mcmaster.com/pipe-taps/pipe-and-conduit-thread-taps-for-shallow-holes/How deep to go with the tap? To gauge NPT threads insert the fitting/plug hand tight and make a mark or dot. Turn counterclockwise and count turns until it lets go. 3 full turns plus/minus 1/2 turn is what you want.
I've spent every day of the last 40+ years metalcutting...kinda short of apprentices lately so there have been days I have run all these machines simultaneously.
