Measured the shank of the cutter just above the flute. Converting from imperial to metric (only have an imperial mike) that came out at 3.08mm. Plunged a hole straight down and checked size with the shanks of twist drills and that came out between 3.0 and 3.1mm. Pretty conclusive, then.
I was going to measure backlash, leadscrew float, etc., but when I did a quick "shove it and see if it clunks" test, I realised that the whole thing bends so much that it wasn't worth the effort of digging out the dial gauge, etc. As a further demonstration of general bendiness, I changed from two passes of 1.8mm DOC in MDF to a single 3.5mm pass, and the dimensional error changed significantly to the point that the pieces didn't fit any more. The machine works fine for engraving nameplates, or anything involving light cuts where there's no need of great dimensional accuracy, but ask it to machine two components that will slot into each other, and you're on a loser. Those of you who can machine aluminium on a CNC router are allowed to laugh. I can't machine 3mm MDF...