How the Sharpening Angle Guide Works
Every cutting edge is a wedge. A thin wedge (more rake, more relief, a lower bevel) cuts easily but is weak; a thick wedge is strong but needs more force and makes more heat. Soft, gummy materials like aluminium and softwood want a keen wedge; hard, abrasive or grabby materials like stainless, cast iron and brass want a stronger or flatter one.
The guide stores well-established textbook starting values for each tool and material and shows them with a typical range. They are starting points, not rules: tool makers, steels and machines differ, so if an edge chips, make it a little steeper; if it rubs or burns, give it a little more clearance.
The Sums Behind It
1. Twist drills
each side of axis = point angle ÷ 2
lip relief = size guide ± material
Relief by size (general purpose): up to 1 mm 24°, to 2.45 mm 21°, to 3.1 mm 18°, to 6.4 mm 16°, to 8.75 mm 14°, to 12.7 mm 12°, to 19 mm 10°, larger 8°. Hard steels take about 3° less, aluminium and plastics about 3° more.
2. Lathe tool bits
helix = atan(pitch ÷ (π × diameter))
leading flank clearance ≈ helix + 3–5°
Rake and relief come from a table by material. The helix sum is only needed for threading tools, so the flank on the side the tool travels towards does not rub.
3. Chisels and plane irons
bevel length = thickness ÷ sin(bevel)
bevel-up cutting angle = bed + edge angle
On a bevel-down bench plane the frog (usually 45°) sets the cutting angle and the sharpening angle does not change it. Clearance under the iron is the bed angle minus the micro-bevel.
4. Knives
included angle = 2 × angle per side
spine lift ≈ blade width × sin(angle)
The spine-lift figure treats the blade as a flat plate laid on the stone, so it is a rough guide for freehand work; a guided sharpener sets the angle for you.
Worked Examples
- 10 mm drill for mild steel: 118° point, so each lip is 59° from the axis. The size guide gives 12° of lip relief at the outer corner, and the chisel edge should look about 120–135° when you sight down the point.
- Same drill for stainless: 135° point and 9° relief (3° less), ideally with a split point so it starts without a pilot hole.
- 3 mm thick bench chisel for oak: 25° primary bevel, which is 3 ÷ sin 25° = 7.1 mm long, then a 30° micro-bevel about 0.5–1 mm wide.
- Low-angle plane for end grain: 12° bed + 25° bevel = 37° cutting angle. Hone a 38° micro-bevel instead and the same plane cuts at 50° for difficult grain.
- 40 mm wide chef's knife: 17° per side, 34° included. Laid on the stone, the spine sits about 40 × sin 17° = 11.7 mm up.
- M12 × 1.75 thread: helix = atan(1.75 ÷ (π × 12)) = 2.7°, so grind the leading flank with about 6.7° of clearance.
Practical Tips
- Keep the steel cool. On a bench grinder, use light pressure and dip HSS in water often. Carbon-steel chisels and knives lose their temper quickly if they turn blue at the edge; finish them on stones or a slow wet grinder.
- Use a gauge. A cheap drill-point gauge or an angle template cut from card is far more consistent than eyeballing.
- Hone after grinding. A ground edge has a fine burr and scratches. A few strokes on a fine stone make it last much longer.
- Check speeds after sharpening. A keen tool still burns if the speed is wrong: the feed and speed calculator gives rpm for drills and lathe work, and the drill size chart lists letter, number and metric sizes.
- TIG tungstens have their own grind angles: see the TIG tungsten guide. For hardness figures such as HRC, use the hardness converter.
- Safety. Wear eye protection at the grinder, set the tool rest within about 2–3 mm of the wheel, and never grind on the side of a straight wheel that is not made for it.
Frequently Asked Questions
Use 118° for general work in mild steel, aluminium, wood and most other materials. Use 135° for stainless, tool steel and other hard or tough materials: the flatter point has stronger corners and, with a split point, starts without wandering. A 135° point is also harder to grind by hand, so most hobbyists keep 118° for everyday drills.
For general-purpose drills of about 6 to 20 mm, roughly 8 to 15° measured at the outer corner, with more on smaller drills and less on bigger ones. Hard materials want a little less, soft materials a little more. Too little relief and the drill rubs and will not cut; too much and the edge chips and the drill chatters.
The helix of a normal drill gives the lips a positive rake, and brass lets a positive rake pull the drill in, especially at break-through. Dub the lips by honing a narrow flat on each cutting edge, parallel to the drill axis, so the rake is about zero. Many people keep a separate set of drills dubbed for brass.
Grind a 25° primary bevel and hone a 30° micro-bevel. That suits most bench chisels and bevel-down plane irons. Paring chisels can go finer (about 20–25°), and mortise chisels steeper (30–35°) because they are levered and struck. If the edge chips in hard wood, raise the micro-bevel a few degrees.
European kitchen knives are usually about 15–20° per side (30–40° included), and Japanese knives about 10–15° per side because their harder steel can hold a thinner edge. Pocket and outdoor knives are commonly 20–25° per side for toughness. Sharpening guides are set by the angle per side.
Aluminium likes a lot of rake: typically around 20–35° back rake and 12–20° side rake on an HSS bit, with about 8–12° of relief and a polished top face. That stops the metal sticking to the edge. Brass is the opposite: grind it with zero rake or it will dig in.
Angles shown are common textbook starting points for hand-sharpened HSS, carbon-steel and stainless tools. They are not a maker's specification. Follow the tool maker's guidance where it differs, and adjust by results.