kwoswalt99- wrote:A lot of the inaccuracy of twist drills is caused by them not being sharpened perfectly.
After some searching, I still couldn't find what type of machine or fixture they drill the plugs and cylinders with.
That's a good point. Especially if a bit gets dull on an off shift and a foreman gives it a quick swipe on a grinder instead of sending it back to the tool and die shop.
I don't think it could be done with a gang drill (that woud drill all holes at once) as the holes are too close together. That would point to a single drill being used with a fixture holding the workpiece that indexes to each place to drill. Some of the problem will be with basic drill problems and positional error in the fixture / indexing mechanism. Guess would be that the drill bits are special made with a stepped cutter that would drill the hole and counter sink to debur at the same time. The condition of the cutting fluid could have an impact also.
If I ever get around to getting a surface plate I might have some fun by taking my surface gage and doing some measuring on different brands of lock to reverse engineer what tolerances they use.