How to sharpen a lawn mower blade
To achieve a perfect lawn you need a sharp blade - follow our step by step instructions below.
21.06.2023
When to sharpen your mower blade
If you have a green garden to be proud of, it’s disappointing when you notice that your mowing is leaving behind uneven and ragged grass. This is easily avoided though, if you regularly and proactively sharpen the lawn mower blade.
Inspect the blade regularly
Don’t wait until mowing results deteriorate: check regularly to see if you need to sharpen or replace the blade.
Replace a damaged blade
If you find a crack in your lawn mower blade, don’t try to sharpen it. You need to replace the blade entirely.
How often to sharpen the blade
Even if your regular inspections don’t find any problem, you should sharpen your mower blade or take it to a dealer for sharpening after 20-25 hours of use.
Should I sharpen my own blade or take it to a professional?
Sharpening and balancing the blade correctly requires a lot of practice. STIHL recommends having the blade sharpened and balanced by a STIHL specialist dealer.
Your STIHL dealer and other professionals have the right tools and expertise to sharpen your mower blade to a high standard, so this is the safest choice to ensure the job is done correctly. If you have the right tools to hand however, you can sharpen your own mower blade by following our instructions below.
Sharpening the blade yourself
Every time you sharpen a mower blade it also needs to be balanced to prevent unwanted vibrations and damage to the motor during use. To do this you need a lawn mower blade balancer – a good eye won’t do the job. Damaged caused by an unbalanced blade will not be covered under the machine's warranty.
Professional mower blade sharpening
If you prefer not to sharpen the blade yourself, your STIHL dealer will be happy to take care of it, and will usually sharpen the blade as part of a scheduled lawn mower service. Your dealer may also be happy for you to remove the blade from the mower yourself, and take it in for sharpening.
STIHL professional tip: Have your mower blade sharpened by a professional who has all the necessary tools. This task can be done by a STIHL dealer.
How to sharpen a lawn mower blade: step-by-step instructions
Materials:
- Vice
- Work gloves
- Spanner
- Monkey wrench/pipe wrench
- Workshop files
- Grinding stone or whetstone
- Balancer
Before you start to sharpen your mower blade, you must always first ensure that the tool is off and protected against unintentional activation. If your mower is electric, disconnect the cable from the socket and wind it around the power tool so you can see no one has reconnected it while you work. For a cordless lawn mower, remove the battery, and for a petrol-driven machine, disconnect the spark plug.
You need to access the blade on the underside of the mower to sharpen it, but take care as you do so: though electric mowers can simply be turned upside down, petrol mowers cannot. On a petrol mower the air filter must always be at the top, otherwise there is a risk of engine oil running into the filter and damaging the tool. To access the blade you can tilt a petrol mower to a maximum angle of 45°.
The bolts that hold the blade onto your mower are always right-hand threaded and must be turned anti-clockwise for removal. Important: wear gloves when loosening or removing bolts. With one hand, hold the blade in place, and use the other to remove the bolt. If it’s too hard to remove the bolt single-handedly, ask someone else to hold the blade or use a wooden block to wedge it in place.
Before you start to sharpen the mower blade you should clean any grass cuttings off it, ideally using a spatula or wire brush. Check the blade over at this point: if you see any cracks or notches, the blade must be replaced.
Once you have removed and cleaned the blade, clamp it in a vice in such a way that that the cutting edges can be easily filed with an upward motion to sharpen them. It’s a good idea to put some sort of cushioning between the vice jaws and the blade, such as scraps of old leather.
Take a workshop file and use it on the blade, working in one direction and following the curve of the edge. Keep doing this until the blade edge looks completely smooth, and be sure to maintain a sharpening angle of 30° as you file.
STIHL professional tip: A little chalk on your file produces even smoother filing results.
After filing, unclamp the blade and place it on your balancer. If the lawn mower blade stays level, it’s in balance and can be refitted; if the blade leans to one side, you should remove weight from the heavier side, repeating steps 6 and 7 until it is perfectly balanced.
Always remember: safety first
Before you look at the blade, make sure that your lawn mower is definitely switched off and cannot accidentally restart: remove the battery from a cordless lawn mower; pull out the spark plug connector on a petrol mower; or unplug the cable to an electric lawn mower. You should always wear gloves and safety glasses to sharpen lawn mower blades, as well as when removing and refitting the blade. Of course, the safest choice is to have the lawn mower blade professionally sharpened.
Summary: how to sharpen a lawn mower blade
- Keeping your lawn mower blade sharp is the key to a well-tended lawn.
- You can either sharpen your own lawn mower blade or have it professionally sharpened.
- Always ensure the lawn mower cannot start before sharpening.
- Always wear gloves and safety glasses to remove and fit the blade, and to sharpen it.
- The safest way to sharpen a lawn mower blade is to take it to a professional.
