Chips are a flyin'!

[Locobob]

Sounds like you are being way too aggressive with the cutting bits if you are punching through that fast. I like to switch back and forth between cutting bits and sanders throughout the process - helps with keeping the dimensions even and the work consistant. I'll work a runner with a bit for a minute or so then try to do the same thing to others. Then I use a sanding drum to smooth out the surface. After that I measure the worked areas with a telescoping gauge to see where I'm at. Rinse and repeat.
You should be concentrating on the short side radius of the curve - not the outside. You want to straighten out the curves as much as possible. It's also an easier fix if you go through on the underside as you don't have to clean up the welds perfectly.