Gray-Iron

Cast iron parts often need gun drilled holes for fluid passages or weight reduction. The machining allowance — how much material to leave for finishing — depends on the casting quality and the tolerance requirement. I have drilled thousands of holes in gray iron, ductile iron, and compacted graphite iron, and the allowance decision is different for each grade. Cast iron drills differently than steel. The graphite content acts as a lubricant, so tool life is good. But cast iron can have hard spots from uneven cooling, and the surface has a tough scale layer. The graphite also affects the surface finish — the as-drilled finish is naturally smoother than in steel because the graphite flakes act as chip breakers and lubricate the cutting edge. ...

Cast Iron Deep Hole Drilling: Graphite Dust, Abrasive Wear, and What I’ve Learned Cast iron is one of those materials that looks easy on paper. It is machinable, it is relatively cheap, and the chips come off short and brittle. But after drilling thousands of holes in gray iron, ductile iron, and compacted graphite iron (CGI) for hydraulic valve bodies, pump housings, and machine tool components, I can tell you that “cast iron” is not one material — it is a family, and each member treats your tooling differently. ...