[platonas1]

Hello all,
I am trying to machine a half cylinder but as you can see in pictures the result is not smooth, there some lines on the part (marble), I tried various toolpaths and using 4.5 mm of endmill.

Any ideas please?

[routalot]

Try changing the cut direction so that it goes across the job.

[Superman]

try using surfacing strategy..."Flowline" allows to define a cusp height, even better if you also use a bull or ball mill... I suggest a bull ie 5mm cutter with R2 corners.
ballnose cutters are not running at correct speeds when cutting at the tooltip ( zero diameter )

surface contour will always give steps...

plus.... do not rely on backplot or simulation to show actual finished part... it is so dependant on PC specs & screen scale it not funny. A lot comes down to trial & error ( read this as experience )