Cast Iron Plays by Its Own Rules
If you learned to drill on steel, most of what you know transfers. Speeds, feeds, coolant, chip handling — the principles carry over with adjustments for hardness and machinability. Then you pick up a cast iron job and the rulebook changes.
Cast iron is drilled dry. It runs at lower surface speeds than steel. It produces dust, not chips. The optimal drill geometry is different. And if you've been treating it like mild steel, you've been burning through drills faster than necessary and wondering why.
Why Cast Iron Is Drilled Dry
This is the one that surprises machinists the most: coolant actively hurts when drilling cast iron. The reason is the chip form. Cast iron doesn't produce continuous chips — it fractures ahead of the cutting edge into fine particles and graphite dust. Add water-soluble coolant or cutting oil to that fine abrasive dust, and you create a paste that packs into the flutes and accelerates wear faster than running dry does.
The paste effect is particularly bad in deep holes where chips can't evacuate freely. The abrasive slurry works against the drill's margin and flute surface with every pass, grinding away the HSS or carbide. Drills that would last for dozens of holes dry fail in a handful of holes when coolant is applied.
The exception is compressed air. Air blast is beneficial — it evacuates the graphite dust, keeps the cutting zone clear, and provides some cooling without the paste problem. If your setup allows it, air is worth using. But coolant stays off.
Gray Iron vs. Ductile Iron: They're Not the Same
Cast iron is a family, not a single material, and the two most common variants behave differently under the drill.
Gray cast iron (the most common) is characterized by graphite flakes in the microstructure. The graphite acts as a natural lubricant, making gray iron relatively easy to machine. It machines freely, produces the dusty chips described above, and is forgiving of modest parameter variations. Most older machine bases, engine blocks, and general castings are gray iron.
Ductile iron (also called nodular or spheroidal graphite iron) has graphite in spherical form rather than flakes. The result is a tougher, more ductile material that machines more like steel than like gray iron. Ductile iron produces more chip-like material, can be somewhat stickier on the cutting edge, and generally requires more cutting force than gray iron of similar hardness. The "drill it like steel with no coolant" rule still applies, but expect harder work per hole.
Chilled cast iron and white iron are another class entirely — extremely hard, often requiring carbide tooling or pre-grinding before drilling. If you're drilling iron that's eating HSS drills in seconds, check whether you have a chilled zone or a white iron specification.
Speed and Feed for Cast Iron
Cast iron runs slower than steel for HSS drills. A common rule of thumb: 40–80 SFM for gray cast iron with HSS, versus 80–100 SFM for mild steel. In practical terms, for a 1/2" HSS drill:
- Mild steel: roughly 600–750 RPM
- Gray cast iron: roughly 300–500 RPM
The lower speed matters because cast iron is abrasive. The fine graphite particles actively wear the cutting edge, and higher surface speed accelerates that abrasion. Slowing down extends tool life meaningfully — not by a few percent, but by a factor of two or more in many cases.
Feed rate can be moderate to aggressive compared to steel, because cast iron is relatively brittle and doesn't require the same chip-thinning technique. Excessive dwelling (light feed with high speed) tends to work-harden the surface slightly ahead of the drill and should be avoided.
Point Angle and Geometry
Standard 118° point angle works acceptably for gray cast iron. Some machinists prefer a slightly flatter angle — 130–135° — for harder cast irons, as it distributes the cutting load over a shorter chisel edge and produces a more compressive entry into the brittle material. The flatter angle also tends to reduce chipping at the drill exit, which is a real concern with brittle iron.
Helix angle is worth considering for deep holes. A lower helix angle (slow helix, around 12–18°) evacuates the powdery cast iron chips more reliably than a standard or high-helix drill. Low helix angles are a specialty item — you won't always have them — but if you're drilling deep holes in cast iron with chip packing problems, they're worth sourcing for that application.
Lip relief angle should be moderate. Too little relief and the margin rubs; too much and the thin cutting edge chips. For cast iron specifically, a slightly more conservative lip relief than you'd use for steel gives the cutting edge more support against the abrasive material.
Dust Management
This is practical rather than technical, but worth noting: cast iron dust is a real nuisance. It goes everywhere, it stains, and graphite particles are slippery underfoot. It's also worth noting that cast iron dust (particularly finer particles) is a respiratory irritant with long-term exposure concerns.
A vacuum or air collection near the work area is worthwhile for production work in cast iron. At minimum, avoid blowing the chips around with high-pressure air — that spreads graphite dust across the shop rather than collecting it.
Resharpening Cast Iron Drills
Because cast iron is abrasive, drills used on it wear faster than drills used on mild steel at the same cutting conditions. Flank wear — the degradation of the relief surface behind the cutting edge — progresses steadily with each hole. The good news is that the wear is usually uniform and predictable, which makes cast iron drills good candidates for resharpening.
A drill used exclusively on cast iron typically needs resharpening after fewer holes than a steel drill would, but it doesn't need heavy material removal to bring the edge back. Light, frequent resharpening is better economics than waiting until the drill fails.
MachinistPost resharpens HSS twist drills starting at $3.00 each, with web thinning included. If your shop runs any cast iron work — automotive, pump housings, machine repair, valve bodies — maintaining your drill edges is worth doing on a schedule rather than waiting for failure. Mail in your worn drills from anywhere in the country; we ship back fast from Perry, Oklahoma.
Quick Reference: Cast Iron Drilling
- Coolant: None. Dry only. Air blast acceptable.
- Speed: 40–80 SFM (HSS) — roughly half of mild steel
- Point angle: 118° standard, 130° for harder/chilled iron
- Helix: Standard or slow helix; avoid high helix in deep holes
- Gray vs. ductile: Ductile requires more force, behaves more like steel
- Wear pattern: Abrasive flank wear — resharpen frequently