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Why Your Drill Bits Are Failing: A Root Cause Breakdown

Drill bits don't fail randomly. Every failure mode has a cause, and most of them are preventable. Here's what we see consistently from resharpening work and how to read what your bits are telling you.

Failure Mode 1: Chipping at the Cutting Edge

What it looks like: Small chips or notches along the cutting lip. Sometimes visible to the naked eye, always visible under a loupe.

Root causes:

• Feed rate too high. The bit is biting off more than the edge can handle per revolution. Back off the feed until chipping stops.
• Work hardened surface. If you're drilling a material that work-hardens (stainless, some aluminum alloys, inconel), dwelling at the entry point lets the surface harden before you cut through it. The answer is to drill with consistent pressure and not let the bit rub without cutting.
• Wrong point angle for the material. A 118° point biting into hardened steel or cast iron is asking to chip. Use a 135° split-point for hard materials.
• Bit already dull from previous use. A dull cutting edge generates heat instead of cutting — that heat stress causes micro-chipping that worsens with each pass.

Can it be resharpened? Yes, as long as the chips are limited to the outer cutting edge and don't extend down into the flute.

Failure Mode 2: Cratering at the Tip

What it looks like: The chisel edge or web at the tip has a crater or burn mark. Sometimes the tip looks melted or smeared.

Root causes:

• No cutting fluid. Dry drilling in steel, especially with small-diameter bits, generates tip temperatures that exceed the tempering threshold for HSS. Once the tip turns blue or black, that hardness is gone.
• Too fast RPM. Surface speed at the tip is proportional to diameter — large bits need slower RPM to stay within safe cutting speeds. A ½" bit running at 1,200 RPM in mild steel is burning itself up.
• Interrupted cutting. If you're pecking to clear chips and cool down, but not actually letting it cool between pecks, you're just spreading the heat event over more time without preventing it.

Can it be resharpened? Depends on depth. If the cratering is limited to the tip geometry, yes. If the web has been damaged or the bit has lost temper 10mm back from the tip, the resharpenability window shrinks.

Failure Mode 3: Margin Wear (Outside Diameter Taper)

What it looks like: The hole is the right diameter at entry but tapers slightly smaller as depth increases. The bit itself shows wear at the outer margins.

Root causes:

• Abrasive material without sufficient flood coolant. Cast iron, certain ceramics, and abrasive composites grind the margins instead of being cut cleanly.
• Misalignment. If the drill press spindle runs with any wobble, or if a handheld drill is walking, the margins see side load they're not designed for. They're a clearance surface, not a cutting surface.
• Oversized for the machine. Putting a ¾" bit in a drill press not designed for that diameter will produce runout-related margin wear.

Can it be resharpened? The lips can be restored, but margin wear slightly reduces the effective diameter of the bit. Usually not a problem for general work. For tight-tolerance holes, this bit has reached the end of its life for precision work.

Failure Mode 4: Spiral Fracture

What it looks like: The bit has twisted apart, usually at the first or second flute, leaving a clean helical break.

Root causes:

• Chip packing. Chips packed into the flutes bind up — the drill motor keeps turning while the bit is locked, and something has to give. The bit twists apart.
• Work piece moving. If the part isn't clamped and rotates with the drill, the resulting grab torques the bit beyond its failure limit instantly.
• Drill too small for the horsepower. Small bits have low torsional strength. A 3/32" bit in a 1HP drill press with no speed control is a broken bit waiting to happen.

Can it be resharpened? No — this is a catastrophic failure. File it.

Failure Mode 5: Bell-Mouthed Holes (Oversized at Entry)

What it looks like: The hole is noticeably larger at the entry than at depth. The surface shows a chamfer where there shouldn't be one.

Root causes:

• Drill walking at entry. On slippery surfaces (polished steel, most non-ferrous metals), an unguided bit walks before it cuts. Use a center punch or spot drill to create a starting seat.
• Point angle too shallow. A 118° bit entering steel at low feed will skate outward on hard surfaces. A split-point or pilot hole fixes this.
• Burr from previous operation. If the surface has a raised burr from a previous cut, the bit rides the burr before finding the center.

Can it be resharpened? Yes — this is a setup issue, not a damage issue. Regrind to restore point geometry, fix the setup.

What Resharpening Can and Can't Fix

Resharpening restores point geometry — the two cutting lips, the chisel edge, the relief angles. It cannot:

• Restore lost temper from overheating (the bit will feel sharp but won't hold an edge)
• Fix a twisted or fractured shank
• Recover margin wear that's reduced the bit below its labeled diameter
• Repair a bit whose flutes are worn or damaged beyond the cutting zone

When we receive a drill for resharpening, we inspect each one. If it can be returned to service, we resharpen it. If it can't, we flag it and return it with a note — we don't charge for bits we can't bring back.

Send your dull bits in before they reach the failure stage. A dull bit resharpened is a $2–$3 investment. A broken bit is a replacement purchase plus downtime.

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