Z axis circle cut

Is there a G code or M code to make a Z axis
circular cut?

I'm trying to make a rectangular box with a
semi-circle cut on one side.

In this cut, there are spaced gouges within
the main, semi-circular surface.

These don't have to be there, but this is the
project.

I can make something that will work by turning
the thing on its side, but the little lines in
the surface I cannot think of how to make.

I'm using a sherline table top mill with the
Tmill program, ( a basic program that sequences
through g&m codes, some of them)

The part is made from nylon.


Thanks.