Have bored holes to a unilateral 0.0002 on parts square, parallel and to size of the same tolerance. Time was that was my everyday job. The company was great. They knew those tolerances take time and they understood no one could do it every time. I remember throwing away a part I had worked on for a week when I blew a tolerance and the foreman just shrugged and said to make another. Attitudes like that make precision work a joy.
I also am a believer it throwing away a tap before it breaks. A lot cheaper in the long run.
