In order for the lawnmower to work properly and to get the grass clippings you want, the blades need to be sharp. In this guide you will find out how to sharpen the blade on the lawn mower and what else there is to know about the subject.
How often do you sharpen the lawn mower blade?
Only when the blade on the lawn mower is really sharp will the device cut the grass tips off precisely. The result is an even cutting pattern.
However, regular use, small sticks and stones will dull the knife over time. As a result, the lawn mower blade no longer cuts the grass properly. Instead, it chops off the stalks. The interfaces fray badly. This will dry them out and discolor them.
It is best to sharpen the knife at least once a year. The beginning of the season is ideal for this.
Let a professional sharpen your lawn mower blade
Anyone who does not dare to sharpen the lawn mower blade themselves can simply take the entire lawn mower to a specialist workshop for maintenance – sharpening or replacing the blade is usually included. Alternative: Let a sharpening professional do it: scissor and knife sharpening shops, tool manufacturers and also hardware stores and garden centers offer sharpening for little money. In this case, however, you have to do the installation and removal of the lawnmower blade yourself.
How to sharpen a lawnmower blade yourself?
If you have a little practice and the right tools, you can sharpen the lawn mower blade yourself. Unlike kitchen knives, the coarse blades of the mower are less sensitive and do not need to be sharpened to a razor edge. Straightening the cutting surface and restoring the cutting angle are completely sufficient here. Compared to household knives, the steel of the lawnmower blade is rather soft so that it does not splinter when it hits a stone. Therefore, the knife can be easily sharpened by hand. Deep nicks in the cutting edge that lead to such minor accidents do not have to be completely removed.
Attention: If you sharpen your knife yourself, the manufacturer’s guarantee on the knife will usually also expire. However, this is only effective for a very short time on wearing parts. However, for new lawn mowers, first read through the warranty conditions before you do anything yourself!
Preparation and removal of the knife
If you have decided to sharpen your lawn mower blade yourself, the first step is to remove it from the mower. In older hand-guided rotary mowers, it is usually located directly on the crankshaft. Newer, well-equipped lawn mowers these days have a blade clutch. It decouples the knife from the drive and only establishes the power connection when you pull and hold the corresponding lever on the handlebar. In any case, make sure that the device cannot accidentally start by turning the blade. In the case of petrol mowers, you must therefore first remove the spark plug connector. Electric mowers must be disconnected from the mains and the battery must be removed from cordless mowers. Then carefully lay the lawnmower on its side.
Caution: Always store petrol lawn mowers with the air filter on top to prevent petrol or oil from spilling in, and place a thick piece of corrugated cardboard underneath in case fuel does spill. Then, wearing protective gloves, detach the cutter bar from the mower according to the manufacturer’s instructions. As a rule, the screws have a right-hand thread, which means that they are loosened counterclockwise.
Remove lawn mower blade
However, there are also manufacturers who install screws with a left-hand thread – so take a look at the instructions for use beforehand. Stuck screws can be loosened with a little penetrating oil, which is best left to work overnight, and a few careful taps with a hammer on the screw head – never hit too hard, otherwise the crankshaft bearings or the blade clutch could be damaged. Tip: It is best to use a ring spanner or a ratchet with a suitable socket to loosen the retaining screw(s). An open-end wrench slips off easily, which can lead to injuries.
Tip: To ensure that the crankshaft does not turn when the screws are loosened, the end of the knife is best wedged with a suitable hardwood wedge on the inner housing wall. Also, be sure to remove and store any washers so that they can be replaced in the correct order after the lawnmower blade has been sharpened.
What tool do you need for sharpening?
In order to sharpen a lawn mower blade yourself, you definitely need a vice in which you can securely fix the blade bar. In this way you avoid injuries and can easily adjust the grinding angle. For the sharpening itself, experts recommend hand files of various strengths, as they enable an exact cut to be made. Inexperienced grinders in particular should definitely use a hand file, as the grinding process is slow and controlled here and errors can be repaired more easily. Professionals also use a grinding file after the coarse grinding of the lawnmower blade, which makes more precise work possible. Finally, perfect sharpness can be achieved with a whetstone.
If you shy away from strenuous manual work or have an extremely worn knife in front of you, you can also use power tools. A wet grinding machine with a clamping device and speed control is the first choice here. Alternatively, there are sanding finger attachments for various multi-tool devices, but they do require some practice. An angle grinder is unsuitable for sharpening lawnmower blades. It cannot be guided exactly, removes a lot of material at once and heats up the blade due to the high speeds. Temperatures that are too high cause the soft steel to “burn”: it then turns black and loses its elasticity. We advise against using dry, fast-rotating sharpening stones because they also generate a lot of heat.
Sharpening lawn mower blades: step by step
After removal, the lawn mower blade should first be roughly cleaned so that the cut surfaces are clearly visible. Then clamp the cutterbar horizontally in the vise with the wings facing up. Caution: Lawn mower blades may only be sharpened from above, the underside remains untreated. When sharpening, keep to the specified angle as precisely as possible. Now use a coarse file to remove old burrs and other damage and sharpen a new cutting edge into the sides of the knife. The cut edges are reworked with a finer file or a grinding file.
When sharpening the lawnmower blade with a hand file, you should only remove material with the movement away from the blade. Otherwise worn steel collects on the cutting edge and a so-called burr is formed
Make sure that the same amount of material is removed to the left and right of the cutter bar so that the cutter remains balanced. You can determine this by placing the cutter bar with the center hole on a screwdriver or by placing it in the middle of a mandrel or a small elevation. If the cutter bar inclines to one side, a little more material has to be removed on this side. If the lawn mower blade is no longer balanced after sharpening, the high speed will cause an imbalance when mowing later, which can lead to increased wear on the crankshaft bearings. In addition, the device vibrates very strongly.
When both sides are sharp again and the knife is balanced, the raw edges are honed away from the cutting edge with a sharpening stone. At the end, the knife can be cleaned of rust stains with a wire brush. Reinstall the lawn mower blade into the mower with the blades facing up and the flat, unsharpened side facing down.