From the images in the first post it looks like the machine slightly lost position after a few passes, then continued as normal. Tricky to explain, but what I think happened is at the depth of the poor finish the machine lost position and moved further away from this cut, but not by much, such that on the subsequent passes the cutter didn't quite touch the first bit that was cut, but did stick hot swarf to it, hence the poorer finish. Meanwhile the cutter would have cut more from the waste piece, over the full height - hence the better finish on that part. I've seen this happen on my machine when a part hasn't been clamped well, so it moves ever so slightly giving the same effect. Could be the same here...


Quote Originally Posted by Tenson View Post
Slowing the spindle down shouldn't be an issue if I keep the feedrate proportional, right? The chipload is the important thing I think.
Spot on...

Precisely what feedrate, spindle speed and depth of cut were you using?