When should I replace a drill bit or blade?
Reading the symptoms that mean blunt, which items are worth resharpening, and the false economy of pushing on.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
- Safety
- warning
Short answer
Replace it when it stops cutting under normal pressure, produces dust rather than chips, burns or smokes the work, or wanders off the mark. Pushing harder on a blunt bit damages the workpiece, the tool and occasionally you — a new bit costs less than any of them.
Consumables get treated as permanent, and the cost of pushing on with a blunt one is nearly always higher than replacing it. Recognising the signs takes a minute to learn.
Safety
Step by step
- Use the pressure test.A sharp bit or blade cuts under light, steady pressure. If you are leaning on it, it is blunt. This applies to everything from a drill bit to a circular saw and it is the most reliable single sign.
- Look at the waste it produces.Sharp tools produce chips, curls or shavings. Blunt ones produce dust and powder, because they are abrading rather than cutting.
- Watch for burning and smoke.Scorch marks around a hole or along a cut, or a burning smell, mean friction rather than cutting. In wood it marks the work permanently.
- Notice wandering and grabbing.A bit that will not start where you put it, or a blade that pulls off the line, has lost its cutting geometry. On a circular saw or router that is a kickback risk.
- Check the tips on a saw blade.Carbide teeth chip and go rounded. Look along the edge — a missing or chipped tooth means replace it now, since it unbalances the blade.
- Look at the cutting edges of a drill bit.The two lips should be even and sharp with a clean point. Rounded, chipped or unevenly worn means it will wander and overheat.
- Know what is worth resharpening.Large twist bits, spade bits, good carbide saw blades, planer blades and chisels — yes. Small twist bits, masonry bits, jigsaw blades and cheap blades — no, they cost less than the time.
- Match the consumable to the material.A wood blade in metal, a masonry bit in tile, or the wrong tooth count all destroy the item in minutes. Most premature blunting is actually a wrong-tool problem.
- Let it cool.Heat is what destroys cutting edges. Ease off, withdraw regularly to clear waste, and use cutting fluid on metal. A bit that has turned blue is finished.
- Clear the waste.A clogged flute or gullet stops the tool cutting and generates heat. Withdrawing a drill bit periodically to clear it doubles its life in deep holes.
- Buy fewer, better ones.A good bit set outlasts several cheap ones and cuts better throughout. Cheap masonry and spade bits in particular are a false economy.
- Store them so they do not blunt each other.Loose in a drawer, edges knock together. A case, a block or a rack keeps them sharp between uses.
Tips
- If you are pushing hard, it is blunt. That single test covers drill bits, saw blades, planer blades and knives.
- Dust instead of chips is the clearest early sign, and it is visible before the tool starts burning the work.
- Blue or discoloured steel means the temper is gone. That bit will not cut properly again whatever you do to it.
Common mistakes
- Pushing harder when a bit stops cutting — It generates heat that finishes the bit off, damages the work, and causes grabbing and kickback. Stop and change it.
- Using the wrong bit for the material — A wood blade in metal or a masonry bit in tile is destroyed within minutes. Most premature blunting is actually the wrong consumable rather than wear.
If it doesn't work
Drill bit spinning without cutting
Cause: Blunt, or wrong type for the material — Fix: Replace it, and check it is the right bit — masonry, wood, metal and tile all need different geometry.
Burn marks along a saw cut
Cause: Blunt blade, or feeding too slowly — Fix: Change or resharpen the blade and feed at a steady rate. Check the blade is clean of resin as well.
Cut wandering off the line
Cause: Blade dull or bent, or teeth damaged — Fix: Inspect for chipped teeth and replace. A bent blade cannot be corrected.
Bit turned blue
Cause: Overheated — Fix: It is finished — the temper has gone. Reduce pressure, clear waste more often and use cutting fluid on metal next time.
Questions people ask
How do I know when a drill bit is blunt?
It needs pressure to cut, produces dust rather than chips or curls, wanders instead of starting where you put it, and scorches or smokes the work. Any one of those means replace it.
Is it worth sharpening drill bits?
Large twist bits, spade bits and good carbide saw blades, yes. Small twist bits, masonry bits and jigsaw blades cost less than the time it takes, so replace those.