How did you hold the fork legs while drillng from 15mm to 17 mm?

[CutterBill]

You also need to know that end mills are not self-centering like a drill.  If the end mill isn't centered exactly over the existing hole, it will quite happily "drill" an off-center hole.  You might want to use a 15mm pilot to line it all up, clamp the fork leg, then swap out the 15mm pilot for the 17mm end mill and go for it.  Let us know how it turns out.