Electro chemical etch can have close tolerances, is basically the opposite of electro plating, no harsh chemicals, not complex set up.
I am thinking of thin of printing 3d on a thin plate as the electrode, tap the bottom. Then etch out the exposed metal leaving the shap of the printed plastic on top. Remove the tap and drill down through the fitting or thread for a metal fitting. Attach a wire to the plate, could be in an area covered with tap on the top. Heavy tin foil may work as the plate. For this you will need a proper flux to wet the surface with solder and then add the wire. Add water with salt. Follow the information in the articles. Turn everything upside down so the weight of the metal to be cut is pressing against the tool, add a backing plate so the part is still floating at the end. use the water with salt to lift the metal to be cut up floating on a film of water which is also carrying away the metal oxide produced by electro chemical etching.