Only when the knife is really sharp will a clean cut result when mowing the lawn. How to sharpen the lawnmower blade of your rotary mower yourself.
Like any tool, a lawnmower needs care and maintenance. The centerpiece – the knife – needs special attention. A sharp, rapidly rotating lawnmower blade cuts the tips of the grass precisely and leaves an even cut. Frequent use and the inevitable running over of sticks or stones wear out the metal of the lawnmower blade and gradually dull the blades. The result: the grass is no longer cut properly, but hacked off brutally, which leaves severely frayed interfaces. They dry out, turn unsightly gray and form a gateway for diseases.
The cut pattern is therefore a good indicator of when the knives need a new sharpening. As a rule of thumb, it should be sharpened at least once per season – preferably before the start of the new season.
Have lawn mower blades sharpened by a professional
Those who do not trust themselves to sharpen the lawnmower knife can simply take the entire lawnmower to a specialist workshop for maintenance – the sharpening or replacement of the knife is usually included. Alternative: You let a sharpening professional run: Scissors and knife grinding shops, tool manufacturers and also DIY stores and garden centers offer sharpening for little money. In this case, you will have to do the installation and removal of the lawnmower blade yourself.
Sharpen lawnmower blades yourself
If, on the other hand, you have a little practice and have the appropriate tools, you can also take over the sharpening of the lawnmower blade yourself. The coarse knives of the mower, unlike kitchen knives, are not very sensitive and do not have to be sharpened to a razor sharpness. Straightening the cut surface and restoring the cutting angle are completely sufficient here. Compared to household knives, the steel of the lawn mower knife is rather soft so that it does not splinter when hitting a stone. Therefore, the knife can easily be sharpened by hand. Deep notches in the cutting edge that cause such minor accidents do not have to be completely removed. Attention: Self-sharpening usually also invalidates the manufacturer’s guarantee on the knives. In any case, this is only effective for a very short time on wear parts. However, if you have a new lawnmower, read the warranty conditions first before you do it yourself!
Preparation and removal of the knife
If you have decided to sharpen your lawnmower knife yourself, the first thing to do is to remove it from the mower. In older hand-operated sickle mowers, it is usually located directly on the crankshaft. Newer, well-equipped lawn mowers nowadays have a blade clutch. It decouples the knife from the drive and only creates the frictional connection when the corresponding lever on the handlebar is pulled and held. In any case, make sure that the device cannot accidentally start by turning the knife. For this reason, on petrol mowers, you must first remove the spark plug connector. Electric mowers must be disconnected from the power supply and, in the case of battery mowers, the battery must be removed. Then carefully lay the lawn mower on its side. Caution: Always store gasoline lawn mowers with the air filter facing up to prevent gasoline or oil from spilling in, and place a thick piece of corrugated cardboard underneath in the event that fuel does leak. Then release the cutter bar from the mower using protective gloves according to the manufacturer’s instructions. As a rule, the screws have a right-hand thread, which means that they are loosened counterclockwise.
But there are also manufacturers who use 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 soak in overnight, and a few careful blows with a hammer on the screw head – do not 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). A spanner slips easily, which can lead to injuries.
Tip: To prevent the crankshaft from turning when the screws are loosened, the end of the knife is best wedged onto the inner housing wall with a suitable hardwood wedge. Also make sure to remove and store any washers so that they can be put back in the correct order after sharpening the lawnmower blade.
The right tool for sharpening
To sharpen a lawnmower knife yourself, you definitely need a vice in which you can securely fix the cutter bar. This will prevent injuries and allow you to set the grinding angle well. For the sharpening itself, experts recommend hand files of different strengths, as they allow an exact cut. Especially inexperienced grinders should definitely use a hand file, as the grinding process is slow and controlled and errors can be repaired more easily. After the rough sanding, professionals also use a sanding file to work on the lawnmower blade, which enables more precise work. Finally, perfect sharpness is achieved with a whetstone.
Those who shy away from the strenuous manual labor or have an extremely worn knife in front of them can also use power tools. A wet grinding machine with clamping device and speed control is the first choice here. Alternatively, there are sanding finger attachments for various multitool devices, which, however, require a little practice. An angle grinder is not suitable for sharpening lawn mower blades. It cannot be guided precisely, removes a lot of material at once and heats the blade very strongly due to the high speeds. Temperatures that are too high cause the soft steel to “burn up”: it then turns black and loses its elasticity. We do not recommend dry, fast rotating whetstones because they also generate a lot of heat.
Sharpen lawnmower blades: that’s how it works
After removal, the lawnmower blade should first be roughly cleaned so that the cut surfaces can be seen clearly. Then clamp the cutter bar horizontally in the vice with the wings pointing upwards. Attention: Lawn mower blades may only be sharpened from above, the underside remains untreated. Keep the given angle as precisely as possible when sanding. Use a coarse file to remove old burrs and other damage and sharpen a new cutting edge in the knife sides. The cut edges are reworked with a finer file or a sanding file.
Make sure that the same amount of material is removed to the left and right of the cutter bar so that the cutter remains in balance. You can determine this by putting the knife bar with the center hole on a screwdriver or in the middle of a mandrel or a small elevation. If the cutter bar tilts to one side, a little more material has to be removed from it. If the lawnmower blade is no longer in balance after sharpening, an imbalance occurs during subsequent mowing due to the high speed, 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 in balance, the cut edges are honed away from the cutting edge with a whetstone. At the end of the process, rust stains can be removed from the knife with a wire brush. Reinstall the lawnmower blade in the mower so that the wings are up and the flat, unpolished side is down.