Newbie - poor auminium cutting, but why?

[Dodgy Geezer]

Hi,

I have just started with an eShapeoko, which I have been using successfully to cut balsa and styrene shapes using a cheap motor for a spindle.

I am now starting to cut aluminium, but my first attempts leave an unsightly ridge around the edge of the cut. This was not a problem before with wood or plastic.

I assume that this is going to be something to do with feed rates or speeds, but I don't know whether to go higher or lower. Perhaps it's the end mill I'm using...

I enclose a picture to show what is happening. This is cutting at depths of 0.3 to 0.5mm, using a 1mm twin-flute mill. Feed speed was between 20 and 10, and spindle speed was around 13,500rpm. I put a bit of cutting oil on the mill and aluminium surface. Can anyone advise what my next settings ought to be? What I am doing wrong? Or what I should do next to diagnose the problem?