Sharpen your lawnmower yourself: you have to pay attention


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Like any tool, a lawnmower needs to be cared for and maintained. Special attention needs the heart - the knife. A sharp, fast-rotating lawn mower blade precisely cuts off the leaf tips of the grasses and leaves a uniform cut. Frequent use and the inevitable overrun of sticks or stones use the metal of the lawnmower blade and make the blades gradually dull. The result: The grass is no longer neatly cut, but chopped off harshly, leaving heavily shredded interfaces. They dry up, become ugly gray and form a portal of entry for diseases. The cut is therefore a good indicator of when the knives need a new cut. As a rule of thumb, you should at least sharpen them once a season - preferably before the start of the new season.

Have the lawnmower knife sharpened by a professional

If you do not trust yourself to sharpen the lawnmower blade, you can simply put the entire lawnmower into a specialist workshop for maintenance - sharpening or replacement of the blade is usually included. Alternative: You can do a sanding pro: Scissors and knife sharpeners, tool manufacturers as well as DIY superstores and garden centers offer sharpening for little money. However, you must do the installation and removal of the lawnmower knife yourself.

Lawn mower service in the specialized trade

When lawn mower service in the specialized trade, the mower is maintained and maintained all around

Sharpen lawn mower blade yourself

However, if you have a little practice and have the appropriate tool, you can also take care of the sharpening of the lawnmower blade. The coarse blades of the mower are unlike kitchen knives, little sensitive and need not be sharpened razor sharp. A straightening of the cut surface and the restoration of the cutting angle are completely sufficient here. Compared to household knives, the steel of the lawnmower blade is rather soft, so it does not crack when impacting a stone. Therefore, the knife can be easily ground by hand. Deep notches in the cutting edge that cause such minor accidents do not have to be completely removed. Attention: With self-sharpening, the manufacturer's warranty on the knives usually also expires. On wearing parts but this is only very briefly effective anyway. Nevertheless, read the terms of guarantee for new lawn mowers before you even lend a hand!

Preparation and removal of the knife

If you have decided to sharpen your lawnmower blade yourself, you must first remove it from the mower. It sits in older hand-held sickle mowers usually directly on the crankshaft. Newer, well-equipped lawn mowers nowadays have a blade clutch. It decouples the knife from the drive and only makes the traction when pulling and holding the corresponding lever on the guide rail. In any case, make sure that the unit can not accidentally start by turning the knife. For gasoline mowers you must therefore first remove the spark plug. Electric mowers must be disconnected from the power supply and battery rechargeable batteries removed. Then carefully lay the lawnmower on its side. Caution: Always store petrol lawnmowers with the air filter facing up to prevent petrol or oil from entering, and place a thick piece of corrugated cardboard in case any fuel leaks. Then loosen the knife bar with protective gloves from the mower according to the manufacturer's instructions. In general, the screws have a right-hand thread, that is, they are loosened in the counterclockwise direction.

Remove lawn mower blade

The knife bar is held by one or two screws depending on the model

But there are also manufacturers who use screws with left-hand thread - so have a look in advance in the instructions for use. You can tighten stuck screws with some creep oil, which is best left to work overnight, and a few careful strokes with a hammer on the head of the screw - never hit too tightly, otherwise the crankshaft bearings or the blade coupling 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: So that the crankshaft does not rotate when loosening the screws, the knife end is best wedged with a matching hardwood wedge on the inner housing wall. Also, be sure to remove and store any washers so they can be reassembled in the correct order after sharpening the lawnmower blade.

The right tool for sharpening

To sharpen a lawnmower blade yourself, you definitely need a vise in which you can securely fix the knife bar. This helps to avoid injuries and allows you to set the grinding angle well. For sharpening, experts recommend hand files of different thicknesses, as they allow an exact finish. Especially inexperienced grinder should definitely resort to the hand file, since the grinding process is slow and controlled and errors can be repaired easily. Professionals also machine the lawn mower blade after coarse sanding with a sanding piece that makes more precise work possible. Perfect sharpness is finally achieved with a whetstone.

angle

The angle grinder is not a good choice for sharpening lawn mower blades

Those who shy away from the strenuous manual work or have an extremely worn knife in front of them, can also fall back on power tools. A wet grinder with clamping device and speed control is the first choice here. Alternatively, there are Schleiffinger essays for various multitool devices, however, require some practice. An angle grinder is unsuitable for sharpening lawnmower blades. It can not be guided exactly, carries off a lot of material at once and heats up the blade very much because of the high speeds. Too high temperatures cause the soft steel to "burn": it then turns black and loses its elasticity. We advise against dry, fast rotating grindstones, because they also cause a strong heat development.

Sharpen lawnmower knives: That's how it works

After removal, the lawnmower blade should first be roughly cleaned so that the cut surfaces are clearly visible. Then clamp the knife bar horizontally with the wings pointing upwards into the vise. Attention: Lawn mower knives may only be sharpened from above, the underside remains untreated. Keep the given angle as close as possible to the cut. Using a coarse file, remove old burrs and other damage and sharpen a new cut edge on the knife sides. The cut edges are reworked with a finer file or a sanding piece.

Sharpen lawn mower blade

With the hand file, the grinding angle can be well maintained

Make sure that the same amount of material is removed from the left and right of the knife bar, leaving the knife in balance. You can determine this by inserting the knife bar with center hole on a screwdriver or placing it in the middle of a mandrel or a small elevation. If the knife bar tilts to one side, a little more material must be removed on this side. If the lawn mower blade after sharpening is no longer in balance, occurs during later mowing due to the high speed an imbalance, which can lead to increased wear of the crankshaft bearings.
When both sides are sharp again and the knife is in balance, the cut edges are removed with a whetstone and the newly created ridge on the underside is carefully removed. At the end, the knife can be freed of rust stains with a wire brush. Reinstall the lawnmower blade into the mower with the wings facing up and the flat, unground side down.

Video Board: Lawn Mower Repair : How to Remove Lawn Mower Blades.

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