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Dennis, yep, your description of 2-1/2D milling is how I cut contours. To the touch, I can feel no ridges on the rounded surfaces I've cut, and they look great. I'll likely cut the Dreadnaught end from a block of machinable wax to ensure the tiny cutters don't snap all over the place.
My problem is how to CAD draw a square corner (early) 4/4 Dreadnaught end (is EDE correct?) so that I can extract cutting paths from it. I'll have to make an attempt from the 1943 CBC drawing, I guess, since I wouldn't know where to get an End Detail drawing of the early 4/4. BTW, thanks much for helping me out, always much enjoy reading about your model-making processes. I learn a ton from your postings.
Andrew, CamBam, I will take a look at it. I've got FlashCut hardware and software for machine control, works great for me. Just now checked . . . FlashCut expects to offer a 3D version "soon." Among the offerings on its page are Rhino Cam. Since I have Rhino CAD, maybe I should break down and get Rhino CAM. Hate to see what it will cost. And what it will cost in time learning.
Sure do appreciate your mentioning CamBam though, thank you much.
Cedar Rapids, Iowa