Generally, these files should be only seen as idea. The milling was far from perfect.
For example, the speed for the milling is much slower than recommend for the used material,
resulting in chips sticking to the cutter.

CNC Drilling for a case.
1.A FreeCAD 0.17 uses            *.fcstd
                                  |
                                  v
  B FreeCAD exported into        *.step
2.A Heekscad 1.1.0 imported from *.step
                                  |
                                  v
  B Heekscad uses                *-*.heeks
                                  |
                                  v
  C Heekscad exported            *-*.ngc
3.A Candle 1.1.7 opens           *-*.ngc
                                  |
                                  v
  B Candle sends the data over   USB
4.A GRBL 1.1 gets the data from  USB
  B GRBL controls the three stepper of the CNC with a cutter

The lower and top case were drilled with a 3mm cutter.
The Plexiglas was drilled with a 2mm cutter.

On the top* files, manual changes were done in Heekscad and manually copied back to the Freecad file top5.fcstd.
top4.fcsd -> modified top4-1.heeks -> modification copied back to top5.fcstd.
So they might not fit perfectly together and there is no intermediate .step file.