Z-Axis Milling problems

[Kbartok]

Hello,

I have inherited an F1 in Schools Project, and I have just setup the machine for the first time following the tutorials on the Denford YouTube channel. I tried machining the LHS and RHS, but the material kept blowing through the cartridge hole. So I decided to machine the top and bottom instead. I obtained better results, but I can see there is a problem with the Z-axis. In the CAD model, the cartridge hole has a consistent thickness all around, but after machining, there is 1mm of material above the cartridge hole and 6mm below as seen in the photos attached. Any Ideas what I am doing wrong? I set the datum correctly to the centre of the hole.