Gas permeable mold steel can be machined by conventional methods using cutoff saws, end mills, grinders, EDM, stoning, etc. It is important to remember that this material is honeycombed with thousands of microscopic holes, which are affected by metal removal. Oil coolant is recommended.
Grinding and milling closes the pores of this porous steel. EDM is the best way to re-establish permeability. EDM will burn away the metal fragments that are crushed over the pores by milling and/or grinding. Because of its powdered metal origin, when using a wire EDM, it is important to increase the wire speed about 10 percent to prevent arcing from the particles produced. Because a wire EDM machine uses water, the insert must be dried in an oven after EDM, or rust and pore closure will ensue.