Cast iron is not especially hard, but it is highly abrasive and its graphite content chews up soft cutting edges fast. The best drill bits for cast iron are carbide-tipped or solid carbide for production and hard grades, with cobalt a solid choice for the occasional hole. Here is how to pick and how to drill it cleanly.
What to Look For
Cast iron produces a fine, dusty chip and wears bits by abrasion rather than the heat build-up you fight in steel. That means edge-holding is everything. Carbide resists abrasion best and keeps a sharp lip through many holes, which is why production shops reach for it. Cobalt also outlasts plain HSS thanks to its heat and wear resistance, and it is the economical pick for a handful of holes. A 118° to 135° point works well; the material starts easily, rarely work-hardens, and does not fight you the way stainless does — the challenge is simply surviving the grit.
The Best Type of Bit for Cast Iron
For repeated holes or hard grades like ductile and grey iron castings, choose carbide-tipped or solid carbide — the abrasion resistance keeps the edge sharp far longer than steel ever could. For lighter, occasional work, a cobalt bit gives good life at a lower cost.
- Production work: Solid carbide or carbide-tipped for maximum abrasion resistance.
- Occasional holes: Cobalt (M35) comfortably outlasts standard HSS.
- Drill dry: Cast iron is usually drilled without coolant — the graphite self-lubricates.
- Point angle: 118° to 135° starts cleanly with a steady feed.
Technique and Mistakes to Avoid
Wear a mask — the dust is fine, abrasive and irritating to breathe. Use a moderate speed and a consistent feed so the lips keep cutting instead of rubbing and glazing. A common error is easing off too early, then letting the bit polish rather than bite; keep it fed. Back off just before breakthrough so the bit does not grab and snatch on the far side of the casting, which can crack thin sections or wrench the workpiece loose. Because there is no coolant, let the bit cool between deep holes so the edge lasts. Our USA-made carbide and cobalt bits are built to survive abrasive materials like cast iron hole after hole.
Sizing and Tapping Cast Iron
Cast iron taps beautifully once the pilot hole is the right size, so plan the diameter before you drill. A tap drill chart gives the correct hole for each thread; get it right and the threads come out full and strong, since cast iron is not prone to the tearing you see in gummy metals. For blind holes, drill a little deeper than the thread length to leave room for chips. A drill set spanning the common fractional and letter sizes keeps the exact diameter within reach.