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Cast iron is hard, brittle, and less forgiving than mild steel. The right bit depends on the casting’s thickness and condition, the hole size, and whether you’re drilling a small bracket or a thick machine part. For most ordinary holes, a sharp cobalt or high-quality high-speed-steel bit will work; carbide is for harder jobs and comes with a greater risk of chipping.
What we cover
Identify the casting before you buy
Gray cast iron is common in pipes, housings, and machine bases. Its graphite content makes chips break readily, so drilling can feel easier than drilling steel even though the material is hard and abrasive. Ductile iron is tougher and can produce longer chips. You may not know which type you have, so treat an unknown casting as hard, brittle material: support it firmly, use a sharp bit, and avoid side pressure.
Check the casting for cracks, thin edges, and curved or uneven surfaces. A hole near an edge can split the part, especially if the casting is already damaged. If you are drilling a pipe, confirm that it is safe to work on and that its contents have been drained. Do not drill a pressurized, gas-carrying, or unknown pipe.
Choose the bit by job and material
| Bit type | Best use | Trade-off |
|---|---|---|
| General-purpose HSS | Small holes in softer, thinner castings; occasional work | Lowest cost, but can dull sooner in hard or abrasive cast iron |
| Cobalt HSS, often M35 or M42 | Frequent holes, thicker sections, or unknown castings | Costs more; a practical durable choice for many DIY jobs |
| Carbide-tipped masonry bit | Some larger holes in cast iron when used carefully | Can work, but the cutting geometry is less suited to metal and the tip may wander or chip |
| Solid carbide bit | Rigid setups and demanding production work | Fast and wear-resistant, but brittle; a hand drill or loose workpiece can break it |
| Step bit | Enlarging a hole in thin sheet or a thin-walled casting | Not suitable for drilling deep holes in thick cast iron |
For a general-purpose purchase, a cobalt drill bit set is a sensible upgrade if you expect to drill several holes. For one small hole in a thin, non-critical part, a sharp HSS bit may be entirely adequate. Replace or sharpen a bit that rubs, squeals, or needs steadily more pressure; a dull bit makes heat and can leave a rough, oversized hole.
Match the drill and bit
Use a standard drill or a drill press, not hammer action. Impact and hammer modes make the bit strike the brittle casting, raising the chance of cracking the part or chipping the cutting edge. An impact driver is also a poor choice: its pulsing torque is meant for fasteners, not controlled drilling. If your cordless drill has a hammer setting, switch it off.
A drill press gives the best control, especially for holes larger than about 6 mm (1/4 inch), but a variable-speed cordless drill can handle smaller work if you can hold it square and clamp the part securely. Use a sharp, ordinary twist drill with a 118-degree point for general work. A split-point bit can start more accurately, but careful center-punching often matters more than the point style. For a range of common sizes, a cobalt jobber-length bit set keeps the bit shorter and less prone to flex than long bits.
Set up to avoid wandering and cracking
Clamp the workpiece so it cannot rotate or slide. Do not hold a casting in your hand or brace it against your leg. If its shape makes clamping difficult, use a fixture or a drill press vise with protective blocks. Support thin sections close to the drilling point; unsupported walls can vibrate or fracture.
Mark the hole and make a light center-punch dimple. On a curved surface, begin with a small pilot hole or use a drill guide to keep the bit from skating. For a larger hole, step up in stages—for example, drill a 3 mm (1/8-inch) pilot before moving to 6 mm (1/4 inch). Don’t jump from a tiny pilot to a large bit if it causes grabbing. Secure the work again between steps if needed.
Use a steady speed and light feed
Run a twist bit at a moderate speed with firm, even pressure. As a starting point for cobalt or HSS bits, use roughly 500–1,000 rpm for a 6 mm (1/4-inch) bit and 200–500 rpm for a 12 mm (1/2-inch) bit. These are practical ranges, not fixed limits: use the lower end for a hard casting or a handheld drill, and follow the bit maker’s guidance when available. A small bit spinning too slowly may rub; a large bit spinning too fast overheats quickly.
Gray cast iron is often drilled dry because its chips tend to break into small pieces and coolant can turn the graphite into a messy paste. Dry drilling is usually fine for an occasional hole. If the bit heats quickly or you are drilling ductile iron, a little cutting fluid can help, but apply it sparingly and clear chips often. Stop if smoke, discoloration, or a sharp squeal appears. Let the bit cool rather than pouring liquid onto a very hot cutting edge.
Finish cleanly and check for problems
Near breakthrough, reduce pressure so the bit does not grab or break through violently. Clear chips with the drill stopped; never brush them away with your fingers, as cast-iron chips can be sharp. Deburr the hole lightly with a countersink or hand tool. Avoid forcing a fastener into a hole that is undersized, angled, or cracked at the edge.
If the bit stops cutting, withdraw it and inspect the edge. A polished or rounded cutting edge is dull; a chipped edge usually points to vibration, side loading, or an unsuitable brittle bit. A crack radiating from the hole means the part may no longer be safe for its intended load. Do not rely on a larger bolt or washer to make a cracked structural casting sound again.