How to Drill Cast Iron? | Bits, Speed & Step-by-Step

Cast iron is brittle, dusty, and unforgiving — a drill bit that works fine on steel can skate across its surface or snap trying to cut it. The difference between a clean hole and a cracked workpiece comes down to the right bit, the right speed, and controlled pressure applied throughout the cut. Knowing how to drill cast iron the right way means matching those three variables to the material’s hardness and grain.

What Tools Do You Need to Drill Cast Iron?

Bit choice matters most. Cobalt (M42) or carbide bits handle cast iron’s abrasive dust without dulling quickly. A 135-degree point angle is widely recommended because it bites without wandering; a 118-degree point works well in a drill press. High-speed steel bits will cut but wear faster — fine for occasional small holes if kept sharp.

You also need a center punch for a deep starting dimple, sturdy clamps, eye protection, and a dust mask — iron powder should not be inhaled. A drill press gives the steadiest results, though a handheld drill with a firm grip works for smaller bits. For a full comparison of top-rated options, our guide to the best drill bits for cast iron covers cobalt, carbide, and coated choices in detail.

Step-by-Step: How to Drill Cast Iron

Speed and feed are the two variables that separate a clean hole from a broken bit or cracked workpiece. Cast iron must be drilled slowly with enough pressure to make the bit cut continuously — rubbing at low pressure dulls the bit fast. The table below summarizes recommended speeds from multiple metalworking guides.

The drilling sequence:

1. Clamp the workpiece firmly. Any movement during drilling can crack the part or snap the bit.
2. Mark the spot and center-punch deeply. A shallow dimple lets the bit wander, ruining placement and raising fracture risk.
3. Start with a pilot hole for any bit over ¼ inch. A small pilot guides the larger bit and reduces the force needed at the final diameter.
4. Drill at low speed with firm, consistent pressure. Keep the bit cutting — if you see fine powder instead of chips, you’re in the right zone. Clear dust periodically by lifting the bit.
5. Ease pressure near breakthrough. Cast iron is brittle on the exit side; letting up slightly prevents blowout and chipping.
6. Deburr the hole with a file or deburring tool. The edges will be sharp.

Lubrication note: Practice varies by cast iron type. Many sources recommend drilling grey cast iron completely dry — coolant mixes with iron dust to form an abrasive paste. Other guides still advise light cutting oil for heat reduction. When in doubt, drill dry for grey cast iron and use a drop of oil for harder alloyed varieties.

Common Cast Iron Drilling Mistakes

• Drilling too fast. High RPM overheats the bit tip and can work-harden the surface. Slow speed is safer every time.
• Skipping the center punch. One missed dimple lets the bit walk, ruining location and scratching the surrounding area.
• Forcing the bit through at exit. Pushing hard as the bit emerges creates a rough breakout or a crack. Reduce pressure and let the bit cut its way out.
• Not clamping the work. Cast iron is heavy but can shift under torque — even a slight shift can crack thin sections.
• Using a dull or wrong bit. A standard wood or masonry bit will not cut cast iron. Cobalt or carbide is the reliable choice.