Why Drill Bits Break: 10 Common Causes and How to Prevent Them

Drill bit failure is one of the most frustrating and costly problems in any workshop. Whether you're running a CNC machine, hand-drilling components, or managing a fabrication shop, a broken drill can mean lost time, scrapped parts, and unexpected tooling costs. The worst part? Most drill failures are preventable.

In this guide, we'll walk through the 10 most common causes of drill bit breakage and show you exactly how to prevent them. By understanding what causes drills to break, you'll spend less time replacing tools and more time producing quality parts.

The Real Cost of Drill Breakage

A single broken drill doesn't just cost you the price of a new tool. It costs you:

• Tool replacement costs – carbide drills and specialty drills aren't cheap
• Machine downtime – stopping production to change the tool and clear debris
• Scrap parts – components ruined by a broken drill or poor hole quality
• Lost productivity – time spent troubleshooting instead of machining
• Potential spindle damage – in worst cases, a broken drill can damage your machine

A workshop that experiences frequent drill bit failure can lose thousands of dollars per month in downtime and scrap. The good news is that most of these failures follow predictable patterns—and once you know what to look for, they're easy to prevent.

Cause #1: Incorrect Speeds and Feeds

This is the single most common cause of drill failure. Running a drill at the wrong speed or feed rate creates excessive heat, poor chip formation, and uneven cutting loads.

Too high RPM: Excessive spindle speed generates heat faster than the drill can dissipate it. The cutting edges soften, wear rapidly, and eventually fail.

Too low RPM: Running too slowly causes the drill to rub rather than cut. This generates heat through friction, dulls the edges, and creates poor chip formation.

Excessive feed: Pushing the drill too hard overloads the cutting edges. The drill flexes, vibrates, and eventually snaps.

The solution: Use the correct speeds and feeds for your material, drill size, and machine. Harder materials need lower speeds; softer materials can run faster. Start conservative and adjust gradually based on results.

Cause #2: Poor Chip Evacuation

Chips that don't clear from the hole create a cascade of problems. They pack around the drill, increase friction, trap heat, and eventually cause jamming and breakage.

Chip packing: When chips can't escape, they compress around the flutes. This increases cutting forces, generates heat, and eventually locks the drill in the hole.

Deep hole drilling: The deeper you drill, the harder it is for chips to travel back up the flutes. Long, stringy chips from aluminium and stainless steel are especially problematic.

The solution: Use peck drilling for deep holes—retract the drill frequently to clear chips. Ensure adequate coolant flow. For materials prone to long chips, consider using a chip-breaker drill or reducing feed to create shorter chips.

Cause #3: Inadequate Coolant

Coolant removes heat and lubricates the cutting edges. Without it, drills fail fast.

Heat build-up: Drilling generates intense heat at the cutting edges. Without coolant, the drill softens and the edges dull.

Edge wear: Coolant lubricates the cutting edges and reduces friction. Without it, the edges wear rapidly and become prone to chipping.

The solution: Use adequate coolant for your material and application. For steel and stainless, flood coolant is essential. Ensure coolant is fresh, properly mixed, and delivered effectively to the cutting edges.

Cause #4: Excessive Runout

Runout is the wobble or eccentricity in your drill setup. Even small amounts create uneven cutting loads, cause one edge to cut more than the other, and lead to rapid wear and breakage.

Toolholder problems: A worn or damaged toolholder, or a drill that's not seated properly, introduces runout.

Spindle runout: If your spindle itself has runout, even a perfectly held drill will wobble.

The impact: Runout causes one cutting edge to take a heavier load. This edge wears faster, becomes chipped, and eventually breaks. Runout also produces oversized, out-of-round holes.

The solution: Check and maintain your toolholders regularly. Ensure drills are fully seated and chucks are tightened evenly. Use a dial indicator to measure spindle runout. Replace worn toolholders and bent drills. Acceptable runout is typically less than 0.005" (0.13 mm) for general work.

Cause #5: Incorrect Drill Selection

Using the wrong drill for the job is a recipe for failure. Different materials and applications require different drill types, coatings, and geometries.

The solution: Match your drill to the material and application. For steel, a standard HSS or coated HSS drill works well. For stainless steel, use a drill designed for stainless with a 135° point angle. For aluminium, consider a 90° point angle to reduce rubbing. For production runs, carbide drills offer speed—but only in rigid setups.

Cause #6: Poor Machine Rigidity

A machine that vibrates or flexes under load creates unstable cutting conditions. The drill bounces, loads become uneven, and breakage is almost inevitable.

Machine vibration: Worn spindle bearings, loose components, or an unbalanced spindle cause vibration.

Workholding issues: A workpiece that's not held securely can shift or vibrate during drilling.

Long tool overhang: Using a long drill or holding the drill far from the chuck reduces rigidity. Keep overhang as short as practical—ideally no more than 3–4 times the drill diameter.

The solution: Maintain your machine regularly. Check spindle bearings, tighten loose components, and balance the spindle. Use solid workholding—vices, clamps, or fixtures that grip securely. Keep tool overhang short.

Cause #7: Drilling Stainless Steel Incorrectly

Stainless steel is notoriously hard on drills. It work-hardens rapidly, generates intense heat, and produces long, tough chips that pack easily. More drills fail in stainless than in almost any other material.

Work hardening: Stainless hardens as you cut it. If your speeds aren't aggressive enough, the drill rubs rather than cuts, work-hardens the material, and makes the problem worse.

Heat generation: Stainless generates more heat than steel. Without adequate coolant and correct speeds, the drill softens and fails quickly.

The solution: Use a drill designed for stainless steel with a 135° point angle and special flute geometry. Run at moderate to high speeds (faster than you'd use in steel) to cut aggressively and avoid work hardening. Use plenty of coolant—flood coolant at minimum. Peck frequently to clear chips. Monitor the drill closely and replace it as soon as wear becomes visible.

Cause #8: Peck Drilling Mistakes

Peck drilling—retracting the drill frequently to clear chips—is essential for deep holes. But done incorrectly, it can cause more problems than it solves.

Excessive pecking: Retracting too frequently creates thermal stress. Each retraction and re-entry heats the drill, and repeated cycles weaken the material.

Insufficient chip evacuation: If you don't retract far enough or frequently enough, chips still pack and jam the drill.

The solution: Peck at intervals that match your hole depth and material. For most applications, retract every 1–2 times the drill diameter. Retract fully to clear chips. Use adequate coolant to cool the drill between pecks.

Cause #9: Worn Drills Kept in Service Too Long

A worn drill doesn't just produce poor holes—it's a breakage waiting to happen. Worn drills are stressed, unstable, and prone to sudden failure.

Wear indicators: A drill is ready for retirement when you see flank wear (wear on the cutting edges), margin wear (wear on the outer edge), or chipping.

Surface finish degradation: If holes are becoming rough or oversized, the drill is worn. Replace it before it breaks.

The solution: Inspect drills regularly. Replace them as soon as wear becomes visible. Keep a log of how many holes each drill has drilled. For production runs, replace drills on a schedule rather than waiting for failure. A worn drill that breaks mid-hole can damage your machine and ruin a component.

Cause #10: Poor Hole Entry Conditions

How a drill enters the workpiece sets the tone for the entire hole. Poor entry conditions create uneven loads, deflection, and often immediate breakage.

Curved surfaces: Drilling into a curved surface causes the drill to deflect and skate across the material before it bites.

Angled surfaces: An angled entry point causes the drill to walk or deflect. The cutting edges load unevenly.

Lack of spot drilling: A small pilot hole (created with a spot drill or centre drill) guides the main drill and prevents walking.

The solution: Always spot-drill before drilling the main hole. A spot drill creates a small conical depression that guides the main drill and prevents walking. Ensure the workpiece is held securely and the drill is perpendicular to the surface.

Master Troubleshooting Table

Warning Signs Before a Drill Breaks

Most drills don't fail without warning. If you know what to listen and look for, you can catch problems before they become breakage.

• Increased spindle load: If the spindle is working harder, something is wrong. The drill may be dull, speeds may be too low, or feed may be too high.
• Poor chip formation: Chips should be small, curled, and easy to clear. Long, stringy chips or powder-like chips indicate a problem.
• Noise and vibration: Chatter, squealing, or unusual vibration indicates runout, poor rigidity, or a dull drill.
• Hole quality deterioration: If holes are becoming oversized, out-of-round, or rough, the drill is wearing.
• Heat discolouration: If the drill or workpiece is discoloured (blue or brown), the drill is overheating.

How to Maximise Drill Life

• Use correct speeds and feeds: Invest time in finding the right speeds and feeds for your materials and drills.
• Optimise coolant delivery: Use adequate coolant, keep it fresh, and deliver it effectively to the cutting edges.
• Reduce runout: Maintain your toolholders, check spindle runout, and ensure drills are properly seated.
• Select the correct drill: Match your drill to the material and application.
• Monitor wear: Inspect drills regularly and replace them before they fail.
• Improve workholding: Secure your workpiece firmly. Movement during drilling is a common cause of breakage.

Quick Prevention Guide

Frequently Asked Questions

Why do carbide drills break suddenly?

Carbide is harder and faster than HSS but more brittle. It doesn't flex like steel, so when it hits an unexpected load (runout, vibration, or a hard spot), it shatters rather than bends. Carbide drills require rigid setups, correct speeds, and stable workholding.

Why do drills break more often in stainless steel?

Stainless steel work-hardens rapidly and generates intense heat. If your speeds are too low, the drill rubs and work-hardens the material, making it harder to cut. The heat builds up, the drill softens, and failure follows. Run faster and use plenty of coolant.

What is chip packing and why does it cause breakage?

Chip packing occurs when chips can't escape from the hole. They compress around the drill flutes, increasing friction and heat. Eventually, the drill jams and either breaks or stalls the spindle. Peck drilling, adequate coolant, and chip-breaker drills all help prevent it.

What is acceptable runout?

For general drilling, runout under 0.005" (0.13 mm) is acceptable. For precision work, aim for under 0.002" (0.05 mm). Measure runout with a dial indicator on the drill shank near the chuck.

When should I replace a drill?

Replace a drill as soon as you see visible wear on the cutting edges, chipping, or margin wear. If holes are becoming oversized or rough, the drill is worn. Don't wait for breakage—a preventive replacement costs far less than a broken drill that damages your machine.

Can I use the same drill for steel and stainless steel?

Not ideally. A standard steel drill works in stainless but wears much faster. For best results, use a stainless-specific drill with a 135° point angle and special flute geometry.

Is dry drilling ever acceptable?

Yes, for some materials. Cast iron can be drilled dry because it produces short, brittle chips that don't pack. However, for stainless steel, aluminium, and deep holes, coolant is essential.

Conclusion

Drill bit breakage is frustrating, but it's also one of the most preventable machining problems. Most failures come down to a handful of causes: incorrect speeds and feeds, poor chip evacuation, inadequate coolant, excessive runout, or wrong drill selection. Address these fundamentals, monitor your drills, and maintain your machine—and breakage becomes rare.

The workshops that run smoothly aren't the ones with the fanciest machines. They're the ones where operators understand their tools, follow best practices, and catch problems before they become failures. By applying the principles in this guide, you'll join that group.

Ready to reduce drill breakage and improve your productivity? True Tooling Australia stocks a comprehensive range of high-performance drilling tools—carbide drills, HSS drills, stainless-specific drills, through-coolant drills, and specialist geometries for every application. Browse our drill range and find the right tool for your next job.