What exactly is it that you are trying to do?
Generally, in steel, you check the tables (like this one
) and pre-drill to the required size. The drill sizes in the table will yield a typical 75% thread, which, for all practical purposes, is almost as strong as a full depth thread, yet much easier to cut. Easier to cut means you are less likely to break the tap off in the hole.
Just recently, I retapped a French threaded (15mm x 1.25?) crank thread to the standard 9/16-20. They are pretty close, yet just far enough apart that modern pedals will not fit. A few drops of cutting oil, five minutes light effort, and the job was done.
Be careful to start the tap square to the work. Back tap (1/4-1/2 turn) every revolution to break the chip.