So I make stuff as shown in the large photo (in this case, an aluminum carburetor actuator to replace the steel one that doesn’t work right.) It looks okay from the top but when you look at the side, it’s really rough. Partly that’s a worn-out bit, partly it’s that my machine isn’t rugged enough to do the job in a single cut, but what I’d like to do is do an oversize multiple pass, then come back and do a last full-depth pass to the right size. How much should I leave previous to the finishing cut? Should I run a higher spindle speed for finishing? Higher feed rate? I’m using HSS on 6061 mostly, on a Sherline mill.
Nothing is gonna save you from a worn out bit. Best bang-for-the-buck upgrade will be good quality, sharp cutters, designed for aluminum.
Last pass should be set to get thick enough chips to actually cut rather than smear. For aluminum, I’d climb mill, and generally select feed rate and rpm to advance 2 or 3 mil bite for each flute on the cutter. Use lube, especially at high cutter rpms. I don’t know what cutter diameter or rpm you’re currently running. I’m used to thinking of ideal cutter speed as the velocity of the cutting edge.
If you want really fine finish, and advance smaller amounts, sharp cutters are a requirement. Then again, is a well-adjusted machine and good workholding. Especially on aluminum, copper, and plastics.
Thank you: I’ll institute as much of that as I can. The machine can do close to 10Krpm, and this was using a 0.125" two-flute HSS… but generally, over 3Krpm the machine vibrates badly because it’s just too lightweight.
How much to leave for a finishing pass is a matter of experimentation. You want you finishing pass to cut the entire depth and leave a flawless surface. 5-10 thou is usually enough. Slow your feed down, and increase the spindle RPM and you should have a nice looking part.
ok as i see it there are several opportunities why it failed / worn out bit (an option) / wrong speed on spinle and move / to much enthusiasm by starting the job 
but to be serius there are so many probs you can fail the first is normaly software and control / 2 cond level user fail / if you post all parameters it would be easier to sort out what has happend ( i have frikeld around a problem like this for ages but i have instandly ignored that the tool had only 3 intead of four cutters )