Originally Posted by
awerby
I don't want to disturb your dreams or anything, but I don't see that design as working very well for steel (or anything else, for that matter). Where does the part you're machining on fit? You've got the spindle assembly attached to that red box, and unless the vise is attached to the bottom of the box, the tool won't reach it. But although the box can move around, there's no way the spindle can move in relation to a workpiece, which is what milling is all about. At best, this would be able to drill a hole in something. You'd have to refixture your workpiece to drill another hole.
To change this into something with a chance of working, you need to eliminate that red box, and attach your Z axis to a big "G"-shaped iron casting, or the equivalent in heavy welded steel plate. Put that assembly of moving stages on the horizontal part of the "G" and you've got an approximation of a milling machine. How well it would cut steel would depend on how rigid you could make it, among other things like the accuracy of the slides and screws, and the construction of the spindle.