Extrusion in imported mesh body


I have an imported STL with mesh bodies. I am trying to blow a hole through one of the mesh body side faces. I’m using extrusion but I only get an addition to the mesh (cylinder) and not a subtraction of what I have sketched. Boolean operations also do nothing.

I’ve done this before successfully but it’s been some time (I.e. a few versions back).

I know that there are limitations on imported mesh objects but this 100% worked before (I’m staring at a printed box I imported and then manipulated).

What a I doing wrong? Thanks for the help


Fixed it myself. The Boolean subtract operation is the way to go provided you do not select to keep originals…