How to drill a washer exactly

Those who engaged in plumbing, various repairs of cars and motorcycles, probably got into a similar situation when it was necessary to drill the washer exactly to the desired diameter. It would seem, take it and ream, what's the problem? But not everything is as simple as it might seem at first glance. If the washer is narrow or of thin metal, then to hold it, somewhere it can not be squeezed.