A dull drill bit doesn't just drill slower — it leaves clues about exactly what went wrong. Machinists who know how to read wear patterns can diagnose problems fast, fix the root cause, and get far more life out of every bit in the crib. Here's what the four main wear patterns actually mean.
Most shops either toss a bit the moment it starts squeaking or run it until it snaps mid-hole. The smarter move is recognizing why a bit failed and fixing the process — because if the same bit keeps wearing the same way, you're going to keep replacing it at the same rate.
A 1/4" bit shouldn't be single-use. Resharpened properly, it can run for thousands of holes. But only if you understand what's happening at the cutting edge.
What it looks like: The chisel edge (the blunt, non-cutting center of the tip) is worn down, rounded, or chipped. The cutting lips may still look okay.
What it means: The bit is being forced rather than cutting. When the chisel edge takes excessive pressure, it's usually because feed rate is too high, the point angle is wrong for the material, or dull lips are transferring load to the center.
Fix: Check your feed rate first. If the lips look sharp but the chisel is beat, you're over-feeding. If both are worn, start with the lips.
What it looks like: The sharp outer corners of the cutting lips wear down faster than the rest of the edge. The center of the tip may still look decent.
What it means: This is the most common wear pattern in general shop use. Outer corners are the fastest-moving part of the bit — they see the highest surface footage. Factors that accelerate corner wear: speed too high, hard spots or interrupted cuts, insufficient coolant or lubrication, scale or work hardened surface.
Fix: Drop your RPM 10–15% and use cutting fluid. Outer corner wear is almost always a speed problem.
What it looks like: The relief surface behind each cutting lip is worn flat. The edge itself may appear intact but the back angle behind it has been rubbed away.
What it means: The lip is still "sharp" geometrically but the relief is gone — so the back face is rubbing the workpiece instead of clearing away. Causes include insufficient relief angle (especially on bits reground incorrectly), running too slow with too much feed, or drilling soft gummy materials where the edge bites then drags.
Fix: Inspect relief angle. A properly ground HSS bit has 7–15° of relief behind each lip. If that's gone, the bit needs resharpening — not just touching up the cutting edge.
What it looks like: The shiny margins (the narrow lands running along the length of the flute) are worn, scuffed, or showing heat discoloration.
What it means: The margins are rubbing the hole wall. This indicates oversized holes where the bit is slightly undersize, insufficient chip clearance causing chips to drag the margins across the wall, too-deep holes without pecking, or the wrong drill for the material.
Fix: Peck drill anything deeper than 3x diameter. Clear chips regularly. If you're seeing margin wear in shallow holes, check your chip load and material compatibility.
Color matters too. Blue-black discoloration on the cutting edges is always bad — it means the steel reached temperatures that begin to affect the temper. HSS can handle a lot, but those deep heat colors mean the bit was running too hot and the hardness at the cutting edge has been compromised. A bit with heat damage still looks sharp but will dull in a fraction of the time.
Straw/gold coloring is borderline. It got hot but may still be okay depending on how far back it goes. No discoloration means the process was right — this is normal mechanical wear.
Resharpen when: wear is concentrated at the cutting edges and corners (normal attrition), chisel edge is intact or just slightly worn, no heat damage (no blue coloring), flutes are clean and margins look normal, shank is straight.
Retire when: significant heat damage at the tip (blue-black), chipped or cracked flutes, shank deformation from being caught in a chuck, margin wear so severe the OD is measurably undersize.
Most wear patterns — especially corner wear and lip wear — are fully correctable through resharpening. The bit isn't done, it just needs fresh geometry.
Read the wear. Fix the process. Get more holes per bit.
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