While the Onefinity frame itself is up to the task of light steel milling, the typical spindle choices are not (whether we’re talking Makita router or VFD type spindle).
You can cut steel with it well enough to struggle thru a one-time small project, but I definitely would not recommend planning any sort of repetitive work with steel. As mentioned, the slowest spindle speed is far too fast. Consider that even IF we had access to a proper RPM range spindle, the Z axis may still show weakness under the demands of machining steel.
That said, I did cut steel with mine: I cut the oval pocket in the front .090" deep as well as the 12mm keyways in the center of the table. On the oval, I used a 1/4" carbide 2-flute end mill @ .005" [0.127mm] depth of cut, and flooded with Cool Tool cutting/tapping fluid. I used the same DOC when cutting the keyways thru 7 ga. steel, but I used a 1/8" end mill. I was able to take a few good finish passes to achieve good dimensional tolerance once the slugs were cut out. The tip of the end mill was toast, but I still had good existing side flutes for the finish size passes. The whole process was very slow and very messy.
Now, allow me to circle back to my comment regarding the Onefinity frame. While it is up to the task, it’s a less than ideal frame for milling steel due to exposed ball screws & somewhat sealed linear bearings. While you do see ball screws & linear bearings on high-end commercial machines, the vast majority of smaller mills (Bridgeport size & smaller) typically use dovetail style guides & wipers, which are much more steel-chip friendly & cleanable than ball screws & linear bearings.
