A blunt chain does not cut more slowly, it cuts differently: fine dust instead of chips, you have to lean on it, the bar heats up, the clutch works. Same story with a mower blade, which tears the grass instead of severing it: the cut yellows within two days and the engine labours on every pass.
Winter is the right window to put all that straight, because the machines are standing idle and sharpening asks for time and light, not urgency. Three families of tools, three different logics, and one safety point almost nobody deals with: balancing a mower blade after sharpening.
A blunt tool costs more than a new consumable
The maths is quick. A chain costs £15 to £25, a mower blade £20 to £40, a bar worn out early because you forced a dead chain £25 to £60. A centrifugal clutch or a bent crankshaft and you are in a different bracket altogether.
There is a safety side too, and it is not theoretical. A chain that no longer cuts pushes you to force it, and therefore to put your body behind the machine: exactly the position to avoid in the event of kickback.
Chainsaw chain: the angle matters less than the depth gauge
Sharpening a cutter comes down to three parameters: the top plate angle, the diameter of the file, and the height of the depth gauge. Everyone talks about the first; the last is the one that makes the difference.
Most consumer chains are filed at around 30 degrees on the top plate. Some ripping chains or square-ground chains call for a very different angle, sometimes 10 degrees. That information is not in the chainsaw manual, it belongs to the chain: it is on the packaging and often in the reference stamped on it.
The depth gauge — that small raker in front of each cutter — sets the thickness of the chip. It does not wear: it is the teeth that recede as they wear, so the gauge becomes relatively too tall with every sharpening. The result is a chain that is perfectly sharp and no longer bites. You take it back down with a flat file and a gauge tool, generally every three to five sharpenings, and you round off its leading edge slightly so it does not catch the fibre.
Two very common mistakes:
- Filing the teeth to different lengths. The longest one takes the whole load, the cut wanders and the bar wears on the slant. Bring every cutter down to the shortest one, even if it means losing a bit of steel.
- Taking the depth gauge too low. The chain becomes aggressive, it bites too much, and the tendency to kick back rises noticeably. The gauge tool exists precisely so that you are not guessing.
For the detail of the movements and the faults you create without meaning to, our article on hand-filing mistakes on a chainsaw chain covers the subject in depth.
The file chart
The pitch of the chain is stamped on the drive links. It determines the file diameter, and it is read in inches. These values are industry standards, but the packaging of your chain remains the reference.
| Chain pitch | Round file diameter | Depth gauge, ballpark |
|---|
| 1/4 inch | 3.2 mm | 0.45 to 0.65 mm |
| 3/8 low profile (LP) | 4.0 mm | 0.50 to 0.65 mm |
| 0.325 | 4.8 mm | 0.55 to 0.65 mm |
| 3/8 standard | 5.2 mm | 0.60 to 0.75 mm |
| 0.404 | 5.5 mm | 0.70 to 0.80 mm |
A round file wears out: after fifteen to twenty chains it slides instead of biting, and you convince yourself the steel is hard. A file costs £5 to £9, the cheapest consumable in the workshop.
When a chain is finished
Three signals, independent of one another:
- The cutter length left. Most chains carry a wear witness mark on the tooth. Past that mark, or below roughly 4 mm of cutter, you replace it: the tooth no longer has enough material to carry the load.
- Play at the rivets. If the chain, held flat, bends sideways or if some links are loose, it is mechanically finished whatever the state of the teeth.
- A damaged drive link or a broken cutter. There is no recovering that, you replace it.
Check the bar rail groove as well: a distorted bar will ruin a new chain in a few hours. In practice, one bar sees out two to three chains.
Hedge trimmer blades: the bevel, the clearance, the alignment
A hedge trimmer does not cut like a saw, it shears: two blades cross and sever the branch between them. Two practical consequences.
First, you only sharpen the outer bevel of each tooth, with a flat file or a diamond file, respecting the original angle, generally between 30 and 45 degrees depending on the maker. The flat inner face, the one that rubs against the other blade, is never touched: it is what makes the shearing action work. Filing it means creating a clearance you will never be able to take up again.
Second, the clearance between the two blades matters as much as the edge. Too large, and it chews the shoots instead of cutting them, leaving crushed sections that go black. It is adjusted through the clamping screws, following the manufacturer's procedure: tighten, then back off a fraction of a turn so that the blades slide without a tight spot. Cold, with the battery removed or the spark plug cap disconnected.
Finally, degrease and protect: sap is acidic, it is sticky, and a sticky blade runs hot. Mild solvent, cloth, protective oil at the end of the session. The full ritual is described in our article on hedge trimmer blade care and sharpening.
Mower blade: 30 degrees, and above all no blue
The edge of a mower blade does not need to be a razor. Aim for about 30 degrees, keeping the original bevel, and deliberately leave a small flat of around half a millimetre on the edge. Too fine an edge collapses on the first stone and is gone in one mow.
The real risk is overheating. On an angle grinder or a bench grinder, steel turns blue at around 300 degrees and loses its temper locally. A softened area goes blunt within minutes of use and, more importantly, becomes a crack initiation point. Work in short passes, quench regularly in water, and if you are not comfortable with it, the file remains the safest method: slower, with no thermal risk.
Look at the back of the blade too, that curved lip known as the lift wing. It is what creates the suction that stands the grass up and fills the collector. When it is eroded, thinned or cracked, sharpening will not save the blade: it gets replaced.
Balancing a blade is not optional
This is the point that 90 % of home sharpening jobs ignore, and it is a genuine mechanical safety issue.
When you sharpen, you remove material. Never exactly the same amount from both sides. The blade becomes unbalanced, and it turns at 2,800 to 3,200 revolutions per minute, that is around 50 turns a second, with a tip speed in the order of 280 km/h.
Run the numbers with a deliberately modest assumption: 5 grams of imbalance sitting 22 centimetres from the axis, at 3,000 rpm. The resulting centrifugal force comes to around 110 newtons, the equivalent of 11 kilos pulling permanently on the shaft, and that force rotates fifty times a second. It never cancels out: it fatigues.
What it damages, in order:
- The bottom bearing of the engine, on mowers where the blade is bolted straight onto the end of the crankshaft — that is to say the vast majority of petrol push mowers. Play appears, then the oil seal leaks, then the oil goes.
- The crankshaft itself, which can end up bent over time. A repair that often exceeds the value of the machine.
- The bearings and the blade carrier on self-propelled and deck mowers.
- The user's hands and wrists, through vibration that tires you far faster than you would think.
Balance is checked with a cone balancer costing £8 to £18, or failing that a nail driven horizontally into a wall. Sitting on its centre, the blade should stay roughly horizontal. If one side drops, remove material from the back of that side, never from the cutting edge, and check again. A few passes are enough.
Two absolute rules to finish on: a bent or cracked blade is not straightened, it is replaced, because it can break while turning. And the blade bolt is retightened to the specified torque, often in the 40 to 70 Nm range on a push mower, exact value from the manual for your model.
Organising your sharpening session
Set aside half a day and work machine by machine, not tool by tool. Disconnect the spark plug cap or remove the battery before touching any blade, and work in cut-resistant gloves. The order that works: degreasing, checking the replacement criteria, sharpening, balancing for everything that rotates, oiling for protection, refitting to torque. Note on a label how many sharpenings each chain has had: it is the only way to know when it is reaching the end of its life.
If you do not know the blade bolt torque for your mower, the angle specified for your chain or the blade clearance adjustment procedure for your hedge trimmer, L'Atelier's assistant gives you the manufacturer's values from the brand, model and year, rather than leaving you to tighten by feel on a part that turns at 3,000 rpm.
