You may think all you need is a countersink with a router in the middle, but I think you're wrong. They are very useful for enlarging existing holes, starting holes that need to be very small or very thin. A thousand times they succeeded in their task and others did not. Every toolbox should have a set of these. They work with aluminum, although too much aluminum can dull them. It is great! My only complaint is that they are easy to lose. LAUGH OUT LOUD