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When to sharpen a twist drill bit
A sharp bit cuts with steady pressure and produces chips. A dull one needs more force, squeals, rubs, or leaves a rough, oversized hole. In wood, it may burn or wander; in metal, it may polish the surface without biting. Stop if the bit is hot enough to discolor, because overheating can soften its cutting edges.
Sharpening makes sense for quality high-speed steel (HSS) bits, especially common sizes you use often. For a cheap set with several broken, badly worn, or very small bits, replacement is usually faster. Carbide masonry bits and specialty bits such as brad-point, spade, and step bits have different cutting geometry; the method here is for standard two-flute twist bits.
Tools and safety
A bench grinder is the usual choice. A belt sander can also work if its platen is firm and the belt is fine and in good condition. A dedicated drill-bit sharpener is easier to repeat accurately, but it costs more and works only within its stated size range. For occasional sharpening, a bench grinder with a fine-grit wheel is often adequate.
Wear safety glasses. Tie back long hair, remove loose clothing, and keep gloves away from rotating wheels. Do not wear gloves while grinding: they can catch and pull your hand in. Set the grinder’s tool rest close to the wheel—about 1/8 inch or less—so the bit cannot slip into the gap. Check that the wheel is sound and rated for the grinder before use.
Keep a cup of water nearby for cooling. Grinding removes steel quickly, so use brief touches rather than pressing hard. If you have a drill-bit angle gauge or a drill sharpener with an angle guide, use it; otherwise, make a simple angle guide before starting.
Know the cutting geometry
A standard general-purpose metal bit usually has a point angle near 118 degrees. That means each cutting lip meets the center line at roughly 59 degrees. A 135-degree split-point bit is flatter and tends to start more reliably in metal, but it is harder to reproduce freehand. For ordinary wood and metal work, preserve the bit’s existing point angle instead of changing it.
The two cutting lips should be the same length and meet at the center. Behind each lip is a relief surface that slopes away from the cutting edge. That relief lets the edge cut rather than rub. Too little relief makes the bit skate and heat up; too much makes the edge weak and easy to chip. The flutes should remain clear of grinding damage.
Sharpen the bit step by step
First, clean the bit and inspect it under good light. If a cutting lip is chipped deeply, the web at the center is damaged, or the bit is bent, sharpening may not restore a reliable hole. For a sound bit, mark the cutting lips with a permanent marker; the ink helps you see where the wheel is removing metal.
Hold the shank firmly between thumb and forefinger, close enough to the tip for control but clear of the wheel. Rest your hand against the tool rest if that improves stability. Bring one cutting lip to the wheel at the bit’s original point angle. Use a light touch for about one or two seconds, then withdraw and cool the bit in water. Do not quench a glowing-hot bit; the aim is to keep it cool enough that it never glows.
To make the relief, keep the cutting edge at the wheel and lower the shank slightly as you sweep the bit across the wheel. This leaves the surface immediately behind the edge lower than the edge itself. Avoid grinding the lip flat or creating a deep hollow. Rotate the bit to sharpen the other lip with the same motion and pressure.
Compare the lips from the end of the bit. They should look alike, meet at the center, and have similar lengths. A difference of even a small amount can make the bit cut an oversized hole or wander. Remove only enough material to restore a clean edge. Repeat with short passes, alternating sides, rather than trying to finish one side at once.
Wood and metal: what changes?
| Use | What matters | Practical trade-off |
|---|---|---|
| Wood | A sharp, even point helps the bit start cleanly. Standard twist bits can work, but they tend to leave less tidy holes than brad-point bits. | For rough holes or pilot holes, a sharpened twist bit is fine. For clean, accurately located holes in wood, a brad-point bit is often the better purchase. |
| Metal | Matched lips and consistent relief are critical. Use cutting fluid where appropriate and let the bit cut without excessive pressure. | A carefully sharpened HSS bit can drill mild steel well. For frequent metal drilling, a quality HSS or cobalt set may be worth buying; cobalt bits cost more and still need correct speed and cooling. |
Do not sharpen a bit differently just because the next job is wood. Instead, choose the right bit and drilling technique. For wood where hole placement and a clean entry matter, compare brad-point wood drill bits. For regular work in steel, a cobalt bit set for metal can offer longer service than basic HSS, though it is not a cure for overheating.
Test the edge before using it
Clamp scrap material securely and drill a test hole at a sensible speed. In mild steel, a sharp bit should produce two fairly even chips and cut with firm, controlled pressure. If it squeals, skates, or makes a shiny rub mark, the point may be blunt, the lips uneven, or the drill speed too high. A center punch helps keep a metal bit from wandering.
In wood, check whether the bit starts where intended and produces a hole without excessive burning. Twist bits can pull themselves into wood, so hold the drill firmly and reduce pressure as the bit breaks through. A backing board helps limit tear-out. Use a drill rather than an impact driver for accurate drilling: impact action can damage the bit or make the hole less controlled.
Common mistakes and when to replace
The most common mistake is overheating the tip. Blue or straw-colored steel may have lost hardness, and grinding it again does not necessarily restore its performance. Another is sharpening one lip more than the other; the bit then cuts off-center. If you cannot match the lips after a few light attempts, a drill-bit guide or sharpener is a better option than removing more steel by eye.
Replace a bit with a bent shank, a badly chipped point, cracked steel, or flutes worn too far to clear chips. For a few household holes, replacing a dull inexpensive bit is often the sensible choice. If you regularly drill metal or rely on specific sizes, learning to touch up good HSS bits can save money and keep your drill working without forcing it.