im working on a design that will need a very precise tapered hole in hardened steel, 58-60hrc. Doing it pre heat treat is definitely an option, but i would prefer to do it after if possible for best accuracy and finish. The hole is quite small, 1/4" at the top, 10:1 taper (2.86 deg per side or 5.72 included), and only 1/8" deep, through hole. The reaming operation will be removing very little material, about 6 thousandths off the wall at top edge. This will be done on cnc.

So first question, can i do this in hardened steel with a carbide reamer? From what i understand about tapered reaming, it can be challenging because the whole length of every flute is cutting and it has to take a very small chip. I would imagine especially in hard steel, it will resist cutting and try to rub. should i even attempt this in 60hrc material?

Next question, where do i get such a tool? Ive been searching around without much luck. Morse taper and taper pin reamers are very common, but im not finding other angles, especially not in carbide and not this small. This doesnt need to be exactly 10:1 taper, but something at least close to that for the geometry to work. I will be turning the tapered pin that drops into this hole so the exact angle is not important. I would say between 5 and 6.5 degree included angle should work. Ive searched ebay and found various used taper carbide reamers, but nothing that will work for me. Also i will need to be able to replace this tool when needed, so even if i happen to find the right one used on ebay, thats not gonna help me much. Am i looking in the wrong place, or Is this going to be a custom made tool?

Im also open to other suggestions for making this hole. It needs to have a diameter tolerance of about 5 tenths, locational tolerance of about a thousandth and have a very good finish. I know that "very good finish" is vague, but i dont know well enough to give it an ra. I would say definitely better than what i would get by doing a helical bore with a ball endmill. The mating tapered pin will drop into this hole with about 20oz of pressure and it needs to easily release with absolutely no sticking. To me it seems reaming will be the easiest way to hit these requirements. If its not gonna work, i do have the option of turning this hole on the lathe, but it will be a big awkward fixture and an extra op requiring a lot of setup. I would very much prefer not to do it that way.

Im also not stuck on the idea of doing it hard, theres a good chance it can be done pre heat treat and keep the necessary tolerance. I can measure the change in heat treat and adjust for that. It would just be better to do afterwards if possible.