- Grass that looks ragged and whitish a day after you cut it, or gets torn up in clumps, is almost always a blunt blade tearing the lawn rather than slicing it, not a problem with the grass itself.
- Taking the blade off, filing the existing bevel back to a clean edge and getting it to hang level is a twenty-minute job with a file you can buy for under £12 (about $15), against roughly £25 to pay a shop.
- Sharpen it to a sturdy working edge rather than a razor, get the two ends hanging balanced, and the lawn cuts clean again, heals faster and shrugs off the disease a torn lawn invites.
First, make sure the blade is what is wrong
Walk out a day after you last cut and look closely at the tips of the grass. Clean-cut grass ends in a flat, green slice. Grass cut by a blunt blade ends in a frayed, torn tip that dries out and turns whitish or straw-brown, so the whole lawn takes on a pale, hazy cast a day or two after every mow. That haze is the clearest sign there is, and most people read it as the lawn struggling when the mower is the real culprit.
There are other tells. The blade tears grass out in small clumps instead of cutting it, and sometimes pulls up tufts with soil still attached. The cut looks uneven, higher in some strips than others. You find yourself going over the same stretch twice to get it level, the job takes longer than it used to, and a push mower feels like harder work than last season. Tip the mower up and you might see dents or a rolled-over edge on the blade itself. Any one of these points at the same fix.
It is worth doing for the lawn, not just the finish. A clean slice seals over quickly. A torn, shredded leaf loses water through the ragged wound and sits open to infection, and turf cut with a dull blade browns off at the tips and picks up lawn disease and fungus far more readily than turf cut clean. A sharp blade is lawn care, not tidiness. The same torn-tip problem shows up when the underside of the deck is choked with old clippings, so if your blade turns out to be fine, the next thing to check is whether you need to scrape the caked deck clean.
Getting the blade off without hurting yourself or the engine
Before anything else, pull the rubber cap off the spark plug and tape the lead back against the housing so it cannot flick back on. This is not a box-ticking safety line. When you lever on the blade to loosen the nut, you turn the engine over, and if the piston is sitting at the top of its compression stroke a firm nudge can roll it past the hump and fire the engine. A mower that kicks over with your hand near the blade is how people lose fingers. Disconnect the plug first, every time.
Now tip the mower on its side with the carburettor and air filter pointing up at the sky, not down at the ground. Tip it the wrong way and petrol runs out of the tank and oil floods into the cylinder and air filter, which hands you a smoking, hard-starting engine on top of the job you started with. With a petrol mower, drain the tank low first if you can.
Pull on a pair of thick gloves and goggles. Wedge a short length of two-by-four timber between the end of the blade and the mower housing so the blade cannot spin while you work on the nut. Give the nut or bolt a squirt of penetrating oil and leave it a minute. Then, before you loosen anything, dab a spot of bright paint or a scratch mark on the face of the blade that points down at the grass, so there is no guessing which way round it goes back. A blade refitted upside down does not cut at all, and it is one of the most common mistakes people make. Fit a socket on the fastener and turn it anticlockwise. If it will not shift, slide a length of steel pipe over the handle of the wrench for extra purchase and it will crack free. Lift the blade off.
File the edge back, but do not make it a razor
Clean the old grass and grime off the blade with a wire brush so you can see the cutting edge. You will find a bevel, a flat angled face, already ground along the edge on each end. Your whole job is to file that existing bevel back to clean metal at the same angle it was made, usually somewhere around thirty degrees. Do not invent a new angle and do not try to grind the edge square.
Clamp the blade in a bench vice so one end sticks out. A ten-inch flat bastard mill file, the plain hand file sold in any hardware shop or on the rack at B&Q, Screwfix or Home Depot for about £8 to £12 (roughly $10 to $15), is the right tool. Hold it flat against the bevel at the angle the bevel already sits, and stroke it along the edge in one direction, lifting it off on the back stroke rather than sawing it to and fro. A file only cuts on the push. Work along the edge, counting the strokes, and you will usually have a clean edge in thirty to fifty passes. Then do the other end the same number of times to keep both sides even.
Here is the part that catches people out. You are not after a razor edge. A mower blade cuts by sheer speed, the tip travelling at well over 160mph (260km/h), so it does not need to be knife-sharp, and a razor edge is actually worse. A fine, thin edge folds over, chips and goes blunt the first time it clips a stone or a buried root, whereas a sturdier edge about as sharp as a butter knife holds up for a whole season of hitting the odd bit of grit. Sharp enough to slice a sheet of paper is plenty. Chasing a shaving edge wastes your time and gives you an edge that lasts a fortnight.
Balance it, or the engine pays for it
Filing almost always takes a little more metal off one end than the other, and a blade that is heavier on one side is the step people skip and later regret. Once the mower is running, that blade spins thousands of times a minute, and a heavier end throws it off centre and sets up a hard vibration. That vibration hammers the crankshaft bearing and the oil seals at the bottom of the engine, works fasteners loose, and over a season it can crack the deck or wear the bearing enough to kill the engine. Correcting it takes two minutes.
Hang the blade by its centre hole on a nail knocked into the wall, or on a cheap cone balancer that sells for around £6 to £8 (about $10 to $15). If one end swings down, that end is heavier. File a little more metal off that end, keeping to the bevel, and hang it again. Repeat until the blade hangs dead level, sitting horizontal on the nail with neither end dropping. When it balances, it is ready to go back on.
Refit it, and how often to come back to this
Drop the blade back onto the shaft with your paint mark facing the grass again, and do the fastener up firmly. Tightness counts here: a blade left loose can shift on the shaft, upset the engine timing and make the mower hard to start, and rattle itself looser every time you cut. Pull the two-by-four out, take the tape off the spark plug lead and push the cap back on, and start the mower to check it runs smooth with no new vibration. The whole job runs under twenty minutes and costs under £20 (about $20) in tools you keep, against roughly £25 to £30 each time to hand it to a shop.
For an ordinary lawn cut weekly through the season, sharpening twice a year keeps the edge working: once at the start of the season and once around the midpoint. Put another way, a blade wants attention after roughly every 20 to 25 hours of cutting. If you cut a large plot, run over gritty or sandy ground, or clip tree roots and stones often, you will need to come back to it far more often, every few weeks in heavy going. Either way, end of season is a good moment to take the blade off, sharpen it, and store the mower with a clean edge ready for spring.
One blade will not take this forever. A file brings an edge back many times, but a blade that has been ground down thin, has a chunk missing from hitting something solid, or is cracked or bent needs replacing, not sharpening. A thinned or cracked blade spinning at full speed is a real hazard, and a new one is cheap next to the engine damage a failed blade can cause. Know when to stop filing and fit a fresh blade instead, and the mower cuts clean for years.
