Why does my chamfer pass leave a rough, chewed-up edge?

Nine times out of ten a ragged chamfer like that is a feed rate problem, not a dull tool. Chamfer cutters have a very small cutting edge doing all the work, and if you run them slow they rub instead of cut, wich is exactly what smears aluminum and leaves those little burrs hanging on. Counterintuitive, but you usally want to run a chamfer pass faster than you'd think, or at least fast enough that each tooth takes a real bite. Try bumping your feed up a good chunk and see if the edge cleans up. If it gets worse instead of better, then back off and look at tooling.

The other thing that gets people is the tip of the chamfer tool. A lot of those cutters have a tiny flat or a slightly dull point right at the center, and that's the part tracing your visible line on the edge. So even when the flanks are cutting fine, the very tip drags and leaves a fuzzy line. Some folks run a tiny radius or a sharper chamfer tool for the visible pass, or they do the chamfer in two steps and take a slightly deeper final pass so a fresher part of the flute is doing the work. On the coolant question, dry is fine for a short chamfer pass on aluminum. What actually matters more is chip evacuation and getting a little lube on the edge so chips don't weld to the cutter. A quick brush of oil or a spray before the pass is plenty. Flooding a 30 second chamfer op doesn't buy you much. Machine flex is the least likely culprit here, honestly. A light chamfer pass takes almost no side load. If your pocket floor came out decent, the machine is prolly rigid enough. I'd put my money on feed rate first, tool condition second.

One easy check: run the same chamfer on a scrap piece at double your feed and compare. That'll tell you in two minutes which direction to go.

Following this because I had almost the same thing on my little router build last month, except mine was chamfering MDF so no burrs, just a fuzzy line. Turned out my collet was loose, the tool was walking a tiny bit as it cut. Retightened it and the line came out clean on the second try. So maybe worth checking the tool is actually seated and the collet nut is snug before you go changing feeds? Took me two ruined scrap pieces to think of it. Also, how long is your stickout on that chamfer tool? I had mine hanging out way further than it needed to be.

Same thing drove me nuts on my first aluminum parts, the pocket was clean and the chamfer looked like it was chewed on. What fixed it for me was a little stick of cutting wax, just rub it on the flutes before the pass. The chips stopped sticking to the cutter and the edge came out shiny right away. Cheap as anything and it lasts forever. I also started blowing the chips off with a puff of air mid-pass, the little chips were getting recut around the edge and tearing it up. Between those two things my chamfers went from hand-sanding jobs to basically done.

One thing nobody's hit yet: check the part is actually held down at the edges. I cut a lot of thin 6061 plates on my benchtop machine and even a light chamfer pass can lift a corner if the clamps are all near the middle, and that gives you exactly an uneven wavy line with fuzzy bits. On mine a 6mm plate used to do it untill I added two toe clamps right by the corners. Also worth a look, is your top face flat? If the facing op left it a touch high in spots the chamfer line will follow that.