Our readers keep the lights on and my morning glass full of iced black tea. As an Amazon Associate, I earn from qualifying purchases.5 Best Drill Bit For Cast Iron | 40+ Holes in Cast Iron

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The Narwhal set stands apart because of its true industrial-grade M35 cobalt formulation paired with a fully ground 135-degree split point. Real-user data shows one bit drilling 70+ holes in thin stainless steel and remaining sharp enough to be sharpened — not discarded — after 100 holes. In cast iron, where chip hardness accelerates edge wear, this combination of cobalt content and precision grind translates directly into longer usable life per bit.

The 9/16-inch diameter with a reduced 1/2-inch shank offers stability in a drill press chuck. The 3-inch flute length provides adequate chip clearance for most cast iron applications, and the uncoated finish avoids the flaking issue that sometimes affects TiN-coated bits on abrasive materials. Users consistently note that the bits run true — no wobble, no runout — which is critical when drilling precision holes in cast iron parts like brackets or engine components.

The included plastic storage case keeps the three bits organized, though the real value is in the edge life. One user reported drilling through 80 holes in stainless with the first bit still cutting cleanly. That kind of endurance, in a material harder than most cast iron, demonstrates the metallurgy is working as intended.

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The COMOWARE 29-piece set solves a different problem than single-size packs: it provides a complete range from 1/16″ to 1/2″ in 1/64″ increments, all in M35 cobalt. For anyone who works with cast iron across multiple hole sizes — drilling pilot holes, tapping threads, or creating clearance holes — this set eliminates the frustration of needing a specific size and not having it. Users consistently note that the bits cut through 1/4-inch steel plate like “a hot knife through butter,” which suggests the 135-degree split point and cobalt formulation are performing well.

The set uses three shank styles depending on diameter: round for the smallest bits (1/16″ to 11/64″), triple-flat for the mid-range (3/16″ to 3/8″), and reduced shank with triple-flat for the largest bits (25/64″ to 1/2″). This graduated shank design prevents slippage in the chuck, a common problem when larger bits torque against cast iron’s hard surface. The included indexed storage case keeps every size organized and visible, saving time when switching between bits during a multi-hole project.

Some users noted the bits can be “grabby” in hardened steel, which is a sign of aggressive cutting geometry. On cast iron, that aggressiveness is an advantage rather than a problem — it means the bit engages the material quickly rather than skidding. Chipping reported by one user occurred on hardened steel, not on standard cast iron or mild steel, and was attributed to user pressure rather than bit quality.

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The BECOLLO 11/32-inch set is a focused solution for anyone who needs a single size drilled repeatedly in cast iron — such as when installing threaded inserts or creating precision clearance holes for machine bolts. Real-user reports indicate these bits drilled 25 holes in steel with the first bit still sharp, and one user drilled 11 holes through thick plate steel where competitor bits failed on a single hole. The 135° split point is confirmed effective at preventing walking, with users noting easy self-centering even when starting on rounded or angled surfaces.

What distinguishes this set is the fully ground processing. Many budget bits use a rolled flute that leaves surface irregularities, creating friction points that accelerate wear. BECOLLO specifies full grinding, which produces a smoother flute surface. This reduces friction and allows chips to evacuate more freely. In cast iron, where powdery chips can pack and generate heat, a smooth flute makes the difference between a bit that lasts and one that dulls after a few holes.

The set is packed in a simple plastic case with five identical 11/32″ bits. It is not a variety set — it is a consumable pack for users who know they need a specific size in quantity. One reviewer noted the bits are “good for about 3 or 4 holes in mild steel” before discarding, but that assessment likely reflects a specific application or pressure technique. The majority of users report much longer life, particularly when using cutting oil and moderate feed pressure.

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The LU&MN 10-piece set offers something rare in the M35 cobalt category — ten identical 1/4″ bits at a price point that makes replacement affordable. This is a consumable approach: buy a pack, use them up, and repurchase. For shop environments where bits get dulled or broken regularly, this eliminates the pain of sharpening or the cost of expensive single bits. Users report that these bits cut through stainless steel and John Deere bucket steel (high-strength plate) without dulling after eight holes, with the first bit remaining sharp through the entire job.

The titanium nitride (TiN) finish is worth noting. TiN adds surface hardness and reduces friction, which helps in cast iron where abrasive chips would otherwise wear the flute surface over time. The 135° split point is standard for this category, but the fully ground construction gives these bits a consistent cutting geometry that users appreciate. One user who drilled into steel tube after standard bits failed noted that the LU&MN bits “cut right through like you expect a metal bit to do,” which speaks to the effective tip geometry and sharpness out of the box.

One user reported that the smallest bit in a different set from this brand broke after a minute of drilling, which suggests that feed technique matters — excessive pressure or lack of lubrication can snap any 5% cobalt bit. The majority of reviewers, however, rate these highly for their intended use in cast iron and stainless steel.

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The HYMNORQ 13/32-inch set enters the mid-range tier with a specific advantage: a bronze finish that provides a thin wear-resistant layer on the cobalt steel substrate. Unlike TiN, which adds significant surface hardness, the bronze coating is primarily a cosmetic and mild anti-corrosion treatment. The real value here is the M35 base material and the DIN338 standard compliance, which ensures consistent shank diameter and overall geometry. Users who redecked a car hauler trailer — predrilling through steel and occasionally hitting old screws — found these bits durable enough to survive accidental contact with hardened obstructions.

The 135° split point is fully ground, and the dual-twist flute design is optimized for chip evacuation. One user reported drilling 40+ holes in hard angle iron with a single bit that remained sharp throughout — a strong indicator of effective heat management and edge retention. In cast iron, where the abrasive chip load is high, this kind of endurance matters. The set includes five identical 13/32″ bits, making it a consumable pack for users who need that specific size for repetitive work.

One user noted that a bit twisted off after about 10 holes, and another dulled prematurely. This variability suggests that while the metallurgy is solid, user technique — feed rate, lubrication, and drill speed — plays a significant role in bit life. The majority of users report good results, especially when using cutting oil and a drill press rather than a hand drill, where wobble can cause uneven loading.