Maybe he misunderstood when I talked to him about it. Yes, they use something along the line of a 3/4" (19mm) end mill, but they step over and make a second pass to get the width, which is all over the place from head to head. The problem is it makes the radius shallow, and leaves a high spot, or stepover cusp, that the shaft can rock back and forth on. Aluminum is soft, so over time the high spot flattens out and the bolt tension relaxes and allows the bolts to loosen. That's when the bad stuff starts happening.