A dull drill bit doesn't announce itself. It doesn't stop working — it just works worse. The machine bogs slightly, holes come out a little oversize, chips look different. By the time most shops notice something is wrong, the bit has been doing damage for a while: burning holes, generating heat that work-hardens the material, and producing parts that need to be scrapped or reworked.
The earlier you catch a dull bit, the less damage it does. Here are the five most reliable signs.
The Machine Is Working Harder
A sharp drill in steel cuts with a steady, rhythmic resistance. When a bit goes dull, you feel the change — the spindle bogs, you have to push harder on a manual drill press, or the CNC feed rate produces chatter it didn't produce before. The bit isn't cutting anymore; it's rubbing and pushing material out of the way through brute force. If a hole that used to take light pressure now takes noticeably more effort, check the bit.
Chip Color and Shape Have Changed
Chips are direct feedback from what's happening at the cutting edge. In mild steel with a sharp drill, you get tight, curled chips — silver or very light gold, clean and consistent. When the bit is dull, chips come out blue, purple, or black. Blue chips mean heat. Heat means the cutting geometry is gone and the bit is generating friction instead of clean cuts. In aluminum, watch for chips that look torn instead of curled. In either case, discolored or malformed chips are the bit telling you something is wrong at the point.
Holes Are Coming Out Oversize
A sharp drill with equal lip heights cuts a hole that's on-size or slightly undersize. When the lips are unequal — a common result of wear, and universal with hand-ground bits — one lip takes a heavier cut and pushes the drill off-center on every revolution. A 3/8" drill can easily cut a 0.390" or 0.395" hole when the lip heights are mismatched. If you're measuring holes and finding them consistently oversize, the lips are unequal. This is a regrind problem, not a machine problem.
The Bit Is Walking at Entry
A sharp drill centers itself at entry. The geometry is designed to guide the bit straight in, especially with a split point. A dull drill — particularly one with a widened chisel edge — can't center itself. It skids, walks, and produces holes that aren't where you put them. If you're seeing entry drift on a drilled hole that was dialed in on location, the chisel edge is gone. No amount of spot drilling will fully compensate for a bit that can't track its own center.
You Can See It Under Light
Hold the drill point under a bright light and rotate it slowly. A sharp cutting edge has a clean, continuous line with no visible flat. A dull edge has a flat spot at the tip — you can see light reflecting off the wear land on the cutting lips. It's not subtle once you know what to look for. A flat on the cutting lip is the optical equivalent of metal telling you: this edge is no longer cutting, it's rubbing. If you see the flat, the bit needs a regrind before it goes back in the spindle.
What To Do When You Spot a Dull Bit
The answer depends on how worn it is and what your shop's process looks like. For minor wear caught early, a single regrind cycle restores full performance. For bits that have been run well past their useful life, a regrind may reveal additional problems — a thickened web, chipped lips, or heat discoloration up the shank — that indicate retirement.
The most important habit is not running a bit you suspect is dull. Every minute a dull drill runs in your spindle is time spent generating heat, work-hardening your material, oversizing holes, and increasing the risk of breakage. Pull it and replace it with a sharp one. Regrind the dull bits as a batch.
When Resharpening Is the Right Call
For most HSS jobber drills, the answer is almost always resharpening rather than replacement — as long as the steel hasn't been overheated (look for blue-black discoloration running up the shank) and the bit still has usable flute depth. A quality HSS drill can be reground four to eight times before it's too short to clear chips. Throwing it away after one use is leaving most of its value on the floor.
MachinistPost resharpens HSS drill bits by mail from anywhere in the US. We restore chisel edges, lip relief, and lip height equality on a WinsloMatic grinder. Ship us your worn bits and get them back sharp within the week.
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