Is Your Drill Bit Actually Dull?
Before sharpening, check the symptoms. Common signs of a dull bit are:
- It needs a lot of pressure to cut.
- It smokes, squeals or gets hot quickly.
- It produces powdery dust or small chips instead of continuous curls.
- The hole is oversize or rough.
- The tip looks rounded or shiny when you look at it under good light.
Check the tool too. A drill running slowly or at the wrong speed can mimic a dull bit.
Which Bits Can You Sharpen by Hand?
Hand sharpening is practical for ordinary twist bits made from HSS. Some other bits can be sharpened, but they need different methods or tools:
- Brad point, spade and Forstner bits have different geometry. They can often be touched up with small files or stones, but the twist-bit technique here does not apply directly.
- Masonry and SDS bits have carbide tips that need diamond tools; many people replace them instead.
- Coated bits lose their coating on the ground surfaces. They can still cut, but the coating benefit is lost there.
- Very small bits are hard to sharpen by hand and are often cheaper to replace.
What You Need
- A bench grinder with a suitable wheel, or a flat diamond plate (a fine grit around 1,000 is often suggested), or a fine file for light work
- Safety glasses, and hearing and dust protection as appropriate
- A cup of water to cool the bit
- A marker to colour the tip, which helps you see where you are grinding
- Good lighting and, if possible, a new bit of the same size to compare
The Geometry You Are Recreating
A twist bit has two cutting lips meeting at a point. Common general-purpose bits have a point angle of about 118 degrees, and tougher or harder steels are often drilled with a flatter point of around 135 degrees, often with a split point. Check the bit maker’s data for your material. Behind each cutting lip, the surface is ground slightly lower. This is the relief, or clearance, angle. Without it, the lip rubs rather than cuts.
You are aiming for three things:
- Both lips the same length.
- Both lips at the same angle from the centre.
- Clearance behind each lip.
Step-by-Step: Using a Bench Grinder
- Wear eye protection and set the tool rest close to the wheel. Do not wear loose gloves or clothing near a spinning wheel.
- Colour the tip with a marker.
- Hold the bit at the angle of the cutting lip, with the lip level and parallel to the wheel face. Many people use the angle of a new bit as a visual guide.
- Touch the lip to the wheel with light pressure, then roll the bit slightly, as you pull it back, to create the relief. Remove only a small amount at a time.
- Cool the bit in water regularly. Overheating can soften the steel and ruin the edge, and blue discolouration is a sign the metal has been overheated.
- Rotate the bit half a turn and sharpen the second lip the same way. Count your passes so both sides match.
- Check the result. The two lips should be symmetrical and the centre chisel edge should sit in the middle.
Sharpening With a Diamond Plate or File
A diamond plate lets you work more slowly with less risk of overheating. Hold the bit at the same angle and drag it across the plate with a small rolling motion. Work evenly on both lips.
A file can touch up a lightly dulled bit, but it is slow on hardened steel and it is hard to keep the angles consistent. It is better for rescuing a bit in a pinch than as a first-choice method. Files can be too soft for hardened bits, so this may not work on all of them.
Test the Bit
Drill a hole in scrap of the intended material. Look at the chips: two equal curls coming off both lips is a good sign. If one lip does all the work, the lips are unequal or at different angles. The hole may also be oversize or the bit may wander.
Jigs and Sharpeners
If hand sharpening is hit and miss, a drill bit sharpening jig, a dedicated sharpener or a drill-powered sharpening attachment can give more consistent results. They suit people who sharpen often. You can also make a simple guide, for example a block with a drilled angle, though accuracy depends on how well it is made.
When to Replace a Bit Instead
- The tip has chipped or snapped.
- The flutes are bent or cracked.
- The bit has been overheated to the point of discolouration.
- It is a cheap small bit.
Care Tips to Make Bits Last
- Use the right speed. Slower speeds are generally recommended for larger bits and harder materials.
- Use cutting oil on steel, when appropriate.
- Let the bit do the cutting without forcing it.
- Store bits so they do not knock together.
To choose the right replacement, see types of drill bits or wood drill bit types.
FAQ
What tool would you use to sharpen a drill bit?
Most commonly a bench grinder, a diamond plate, or a dedicated drill bit sharpener. A file can handle light touch-ups.
Can I sharpen a drill bit with a file?
Sometimes, on softer bits or for light work. It is slow and hard to keep accurate, especially on hardened bits.
How do I change bits on a drill?
Open the chuck by turning the sleeve counterclockwise (viewed from the front), insert the shank, and tighten. For keyed chucks, tighten at each key hole. See drill shank and chuck types for more.