Sharpening, Maintenance and Tool Care
Edge angles and honing, grinding without losing temper, preventing rust, lubrication and calibration schedules.
Edge angles and why sharp is safe
A cutting edge is a wedge. The smaller the wedge angle, the less force it takes to push it through the material and the cleaner the cut; the larger the angle, the stronger the tip. A dull tool needs more force and slips: it is more dangerous than a sharp one.
| Tool | Bevel angle | Use |
|---|---|---|
| Knife and razor | 12 to 20° per side | Soft materials; fine but fragile |
| Paring chisel | 20 to 25° | Softwood, light paring |
| Bench chisel | 25 to 30° | General work; a strong edge for mallet work |
| Plane iron | 25° primary, 30° secondary | Hand planes |
| Axe and heavy chisel | 30 to 35° | Rough, impact use |
| Drill bit point | 118° included (135° for hard steel) | Twist drill (the lip relief angle matters more) |
Worked example: how thick is the edge?
- The thickness of a chisel’s edge 0.1 mm behind the tip is 0.1 × tan(angle): 36.4 µm at 20°, 46.6 µm at 25° and 57.7 µm at 30°.
- A 30° bevel is 1.24 times thicker at the same depth than a 25° bevel, so it needs about that much more force and is stronger; the extra strength is the reason hard woods and metal use the larger angle.
- A hollow grind on a 200 mm wheel (radius 100 mm) over a 10 mm bevel leaves a hollow of depth L² ÷ 8R = 10² ÷ (8 × 100) = 0.125 mm: the two ends of the bevel support the edge on the stone and it sharpens quickly.
Sharpening: stones, hollows and micro-bevels
| Stone or abrasive | Particle size | Use |
|---|---|---|
| Coarse (220 to 400 grit) | 60 to 40 µm | Fixing a damaged edge, setting the bevel |
| Medium (800 to 1 000) | 20 to 14 µm | The main sharpening step |
| Fine (4 000 to 6 000) | 3 to 2 µm | Polishing the edge |
| Strop with compound | 1 µm and finer | Removing the wire edge |
- Hold the angle with a guide or by feel: keep the same bevel, light pressure, with the whole bevel flat on the stone.
- Raise a wire edge (a thin burr) on the back; work through the grits until it is the same width along the edge, then remove it on the finer stones and a strop.
- Flatten the back of a chisel or plane iron once, then only polish it: the back and the bevel meet at the edge.
- Flatten the stone regularly: a dished stone rounds the edge.
- Test: the edge should slice end grain cleanly, shave a hair or shave paper; if it reflects light at the edge, it is rounded or damaged.
Worked example: a micro-bevel saves steel
A plane iron ground at 25° with a 30° micro-bevel (a very small second bevel at the tip) needs only the last 0.3 mm of steel sharpened on the stone each time, instead of the whole 10 mm bevel. The area to remove is about 0.300 × 0.300 ÷ 2 ≈ 0.045 mm² per mm of edge instead of 50 mm²: roughly a thousand times less steel, a minute’s work. The primary bevel is reground only when the micro-bevel has grown to the full bevel.
Grinding without ruining the temper
Hardened steel loses its hardness when heated above about 200 °C: the edge turns soft and will not hold an edge. A dry bench grinder can do it in a second. The oxide colors show the temperature reached.
| Color | Temperature | Result |
|---|---|---|
| Pale straw | 230 °C | Lost some hardness at the edge |
| Dark straw / brown | 255 °C | Softer: the edge will roll |
| Purple | 275 °C | Soft |
| Blue | 300 °C | Ruined for an edge; regrind |
Worked example: how quickly the tip overheats
- A tool pressed on a wheel at 31.4 m/s with 15 N and friction coefficient 0.6 dissipates 0.6 × 15 × 31.4 = 283 W at the contact.
- Take a sliver at the edge of mass 0.2 g and specific heat 490 J/(kg·K): if all that heat went into it, it would heat at 2 884 K/s and reach 300 °C in 0.10 s. Real losses to the rest of the tool slow it, but the edge is a thin wedge: it gets hot long before the handle does.
- Light pressure, a slow wheel (a slow-speed or wet grinder), frequent dips in water, and a soft, coarse wheel that cuts cool all help. Stop when the steel starts to color.
- Remove 0.005 mm at a time at the edge, not the whole bevel, and finish on the stones.
Preventing rust
Iron rusts when water condenses on it (see the dew point in Planning the Workshop: Space, Layout and Workflow). Keep the tools dry, coated or sealed.
Worked example: a toolbox and silica gel
- The air at 20 °C and 60 % RH holds 10.4 g of water per m³ (saturation 17.3 g/m³). A 0.05 m³ closed toolbox holds only 0.52 g of water vapor.
- A sachet of 20 g of silica gel absorbs about 20 % of its weight (typical) = 4 g: 8 times the water in the box. The limit is leaks: a box that exchanges its air every day brings in 0.52 g each day and the gel is full in about 8 days: dry it in an oven every few months.
- The dew point of the 60 % air is 12.0 °C; at 80 % it is 16.4 °C. Tools colder than that collect water.
- An oil film of 5 µm over a hand saw blade (0.02 m² of surface) is only 0.10 ml of oil: a rag with a few drops is enough. Wipe tools with it after use.
- Wipe off sweat and salt (fingerprints rust steel in a day) and dry tools after use.
- Wax cast-iron tables and saw blades with paste wax; it also lets the wood slide.
- VCI paper or sachets (volatile corrosion inhibitor) in drawers protect tools in sealed boxes.
- Rust removal: a rust eraser, fine abrasive pad and oil; for heavy rust a chelating or citric bath, then oil at once.
- Keep the shop humidity down (a dehumidifier or heat); see Planning the Workshop: Space, Layout and Workflow.
Lubrication
| Lubricant | Use | How often |
|---|---|---|
| Light machine oil (ISO VG 32) | Hand tools, hinges, light slides | Weekly in use |
| Way oil / medium (VG 68) | Machine slides and screws | Before each session |
| Grease (NLGI 2) | Bearings and gears in cases | Per hour or season |
| Dry PTFE lubricant | Wooden slides, saw tables | As needed; dust does not stick |
| Paste wax | Cast iron tables, saw blades | Monthly |
- Less is better on dusty slides: oil and sawdust make a paste; use dry lubricant or wax.
- Clean before lubricating; add new oil only after wiping off the old, gritty film.
- Screw threads on vises and clamps need a little grease so that the clamping force matches the effort (see Workbench and Shop Setup).
- Cutting fluid for drilling and tapping steel; see Drilling, Tapping and Fasteners.
Calibration and checks
Worked example: calibrating, or not?
Sending four instruments (two calipers, a micrometer, a square) for calibration once a year at $40 each costs $160. A drifted caliper that leads to 6 bad parts at $25 each costs $150: the check pays for itself in a shop that makes parts to a tolerance. For hobby work, a monthly check against a gauge block is enough (see Measuring Tools and Accuracy).
- Check squares, straight edges and rules against a reference every few months.
- Check machine alignment: drill press table square to the spindle, bandsaw fence parallel to the blade, a saw fence at 90° to the blade.
- Record the checks in a log so that trends show.
A maintenance schedule
| Task | Time each | How often | Hours a year |
|---|---|---|---|
| Wipe and oil bare steel | 5 min | 52 a year | 4.3 h |
| Clean and wax the cast iron tables | 10 min | 12 a year | 2.0 h |
| Sharpen chisels and plane irons | 15 min | 24 a year | 6.0 h |
| Check cords, plugs and guards | 10 min | 12 a year | 2.0 h |
| Clean the dust collector filter | 20 min | 12 a year | 4.0 h |
| Check and calibrate measuring tools | 20 min | 2 a year | 0.7 h |
| Total | 19.0 h a year |
About 19 hours a year of care keeps a small shop’s tools in working order: less than the time lost working with dull, rusty or out-of-true tools.
| Tool | What to check | Interval |
|---|---|---|
| Cordless tools | Battery contacts, chuck, vents free of dust; store batteries half charged | Each use / month |
| Drill press | Chuck run-out, belt, table square, quill spring | Monthly |
| Bandsaw | Blade tension and tracking, guides, tires, dust | Each use / monthly |
| Grinder | Wheel ring test, tool rest gap (2 mm), guards | Before each use |
| Hand tools | Edges, handles loose or split, mushroomed heads | Each use |
| Cords and plugs | Cuts, bent pins, heat | Monthly |
| Dust collector | Filter, bag, hose blockages | Monthly |
Storing tools
- Dry and off the floor, in drawers or cabinets; not leaning against a damp wall.
- Protect the edges: guards, a roll or a slot for each chisel; never in a drawer where they touch each other.
- Measuring tools in cases, with the jaws open a little, not under other tools.
- Batteries and chargers in a cool, dry place away from flammable material.
- Keep spare parts (blades, belts, fuses, wheels) beside the tool that needs them.
Troubleshooting
| Problem | Likely cause | Try |
|---|---|---|
| Edge dulls fast | Bevel too thin, overheated in grinding, hard material | Raise the angle 3 to 5°; regrind cool; use a stronger steel |
| Chisel chips at the edge | Angle too small, soft spot, hitting a nail | Increase the angle; regrind past the damage |
| Wire edge will not come off | Steel too soft or the stone too coarse | Finer stone, strop; lighter strokes on the last pass |
| Rust spots appear | Humidity, no film | Dry, abrade and oil; dehumidify the shop |
| Slides are sticky | Dust and old oil | Clean; dry lubricant or wax |
| Measuring tool disagrees with another | Drift or damage | Check against a gauge block; replace the worse one |
Related
- Shop Tools: the overview of this guide.
- Planning the Workshop: Space, Layout and Workflow: humidity, rust and the dew point.
- Measuring Tools and Accuracy: checking measuring tools.
- Hand Tools, Marking Out and Cutting: using sharp tools.
FAQ
What angle should I sharpen a chisel at?
About 25° for general work (20° for paring softwood, 30° for hard or heavy work), with a small micro-bevel for quick touch-ups.
How do I stop my tools rusting?
Keep humidity below about 60 %, wipe tools dry after use, put on a thin film of oil or wax and store them in a drawer, with a desiccant or VCI paper in a sealed box.
Why does my grinder turn the edge blue?
The tip got above about 300 °C and lost its temper. Use a light touch, dip often or use a slow wheel, and finish on the stones.
How often should I calibrate my tools?
Check against a gauge block every few months and after a knock; send for calibration yearly if the work needs traceability.
References
- Oberg et al., Machinery’s Handbook (grinding, tool angles, tempering colors).
- Cosman, B., and others, edge-geometry and sharpening guides for woodworking; Leonard Lee, The Complete Guide to Sharpening, Taunton Press.
- Magnus formula for dew point (Alduchov and Eskridge, 1996).
- Tool and gauge makers’ care and calibration guides (Starrett, Mitutoyo).
Last reviewed: October 2026. Angles, times and quantities are typical: adjust for your tools and materials.