Cast-Iron

Most operators know to change speed and feed when switching materials. Fewer know that the gun drill geometry itself should change. Running an aluminum-optimized drill in cast iron will produce poor tool life and inconsistent hole quality. Here is what changes and why. How Material Properties Drive Geometry Decisions Three material properties determine the optimal gun drill geometry: Chip formation. Materials that produce long, stringy chips (aluminum, low-carbon steel) need geometry that encourages chip breaking. Materials that produce short, broken chips (cast iron, brass) can use geometry optimized for edge strength. ...

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. ...

Castings are different from wrought material. The grain structure is not uniform, there can be porosity, and the surface has a tough scale layer. I have drilled plenty of cast iron and steel castings over the years, and I have learned that treating them like wrought bar stock is a fast way to break a drill. The main differences from wrought material are the entry surface and the internal consistency. Here is what I have learned from drilling thousands of holes in castings. ...

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. ...