I am trying to make an EDM electrode model. I have subtracted my part from my electrode material and would now like to split the solid along the faces highlighted. I need to extend the faces to the cylindrical face behind them then split the solid on the resulting faces. I am unable to find a way to extend the faces. I have tried creating surfaces on them and then extending those but it would not work. I can't find any other function that would allow me to do this. Any suggestions?

Thanks, Glenn