I need to make a 20mm deep hole into a round steel bar that is 8.5mm thick at the entrance and 8.0mm thick at it's deepest end. My carbide inser tools are too thick to fit inside the hole. I have extra thin bits that would fit inside the hole but they are not stiff enough (PAULIMOT HSS-Drehmeissel-Set 5-teilig 3,5 mm : Amazon.de: Gewerbe, Industrie & Wissenschaft). So the idea is to get drills from 8.0mm to 8.5mm in 0.1mm increments and drill with each at its respective depth to form the taper. Instead of a steady slope I would have several steps 0.05mm each. What tool could I use to smooth it out further? Unless someone has a better idea how to do it.