Do-it-yourself correct sharpening of a lawn mower knife

Recommendation points

• How to remove a knife from a lawn mower
• Cleaning and straightening
• Taking out descents
• Sharpening the blades
• Editing edges
• Balancing check

Despite the fact that the blades of the lawn mower are made of high quality steel, from time to time they have to be refreshed. Fortunately, this does not require special skills and equipment, and today we will tell you how to sharpen a lawn mower knife yourself without deteriorating the quality of the metal..

It is possible to cut off excess grass from the lawn even if the edges of the mower blade are obviously dull. However, in this case, the shoots will delaminate in the longitudinal direction, turning into a fringe. A few days after mowing, the grass will become dry and sore for several weeks. The cut should be even and neat, then it will take just a few hours and the lawn will always be juicy green. This requires the knife to be almost perfectly sharpened..

Left: grass cut with a blunt lawn mower blade. Right: grass cut with a sharp blade

How to remove a knife from a lawn mower

Before starting to partially disassemble the lawn mower, you need to protect yourself and her. While an electrically powered device can be easily disconnected from the mains, petrol mowers require preliminary preparation. The first step is to remove the spark plug cap. Next, you should exclude fuel leaks, that is, either drain the tank, or put a sealed rubber membrane under the lid and turn off the fuel cock. Turn the mower only towards the side of the handle, unless otherwise explicitly indicated in the operating instructions. Remember to pinch the crankcase exhaust hose before turning the mower over to prevent oil spilling.

Lawn mowers use different blade configurations. In the simplest case, this is an ordinary two-bladed plate of a complex shape, however, the knife can also be composite – the upper blade is intended for mulching the cut grass. There are also four-bladed knives, which can be both composite and on a common base..

The knife can be fastened either with one bolt in the center through the shaped washer, or with two bolts with a fit on the centering hole or pin. In any case, the thread on the bolts is right-hand, respectively, you need to unscrew it counterclockwise. To block the shaft from rotating, place a block under the knife, resting on the ground. It is advisable to remember or outline the position of the blade (s) so as not to confuse anything during reassembly. Temporarily screw the removed bolts into the free holes.

Cleaning and straightening

When the knife is removed, it must first be cleaned of adhering dirt and hardened herbal juice. The easiest way to do this is to use a metal brush. The knife strip must be clean from all sides: not to a metallic sheen, but without the smallest adhering fragments.

Often, due to hard objects, such as stones and chips, entering the working area, the blade deforms very noticeably. Most blades are not straight by themselves, but the cutting edge and strip 10-15 cm away should be examined for localized bumps and curvatures. This is done using a short, straight ruler..

You can straighten the blade with an ordinary hammer, with the edges of the striker slightly rounded. The knife is laid on a flat massive support (anvil), then the correct shape is given to the metal with weak blows. You need to edit the knife, starting from the edges of the curved section, gradually moving towards the center. A curved blade body is a minor problem, but it is very important that the cutting parts are aligned in a straight line. You also need to check the kickback blades, which often straighten from impact. If on different arms of the knife the blades are bent at different angles, this will impair the air flow and the cut grass will not mulch properly..

Taking out descents

If the knife has not been sharpened for a long time, chipped chips are likely to appear on it. You need to get rid of them by bringing the edge of the blade at a uniform angle along the entire length of the cutting section. Please note that there may be several edges on each side, in which case the lawn mower will have several cutting planes..

Lawn mower blades are sharpened on one side, the slope of which is usually directed upwards. To bring the descent at the desired angle, you should use an electric sharpener with a carborundum stone, and in the absence of one, a fine file. It is very important to remove metal evenly from both blades. Therefore, keep the same pressure and at least approximately count the number of passes of the sharpener.

Small deviations of the sharpening angle are not critical, however, the smoother the slopes are brought out, the longer the knife’s resource will be. When shaping the blade, it always moves onto the grain to avoid the formation of large burrs and flakes. You should not use a grinder for preliminary sharpening – overheating can deteriorate the quality of the metal. However, it is possible to install a petal or disk disc under sandpaper on the angle grinder.

Sharpening the blades

The purpose of sharpening itself is to eliminate small edge chips and bring the descent into one straight plane. The presence of a “file” on the blade leads to a rapid decrease in sharpness, and with a rounded descent, the effect on the cut stems is too strong. In addition, the rounded blade becomes dirty much faster..

Sharpening the blade is best done with a flat emery bar with a grain size of about 400-600 grit, there is no particular point in using more “delicate” stones. Soak the block in lathered water for 10-15 minutes and wet it periodically while turning. The knife is also sharpened on grain with a small broach of the bar from the center of the sharpened area to the edge.

When sharpening, try to stick to the same stone position to maintain a constant angle. As with pulling out slopes, sharpening should be carried out with the condition that the same amount of metal is removed on opposite blades. However, this condition does not have to be strictly observed when sharpening four-blade knives..

Editing edges

When the blades are sharpened, they need to be straightened. This is done using a piece of moisture-resistant sandpaper with a grain size of about 600-800 grit. The paper should be moistened with water and rolled into a cylinder with a diameter of 100-150 mm.

Editing aims to remove the slightest burrs and flakes that can wrap around the thinnest edge of the blade. The sandpaper rolled into a cylinder should be lightly pressed against the descent plane and pulled out with a smooth movement from the grain. In this case, it is also recommended to make a smooth offset from the center to the edges..

When removing the triggers and sharpening, all manipulations were carried out with only one side of the blade. During the dressing process, the knife is also processed from the back. Here the paper is pressed almost close to the knife body, but with a slight tilt towards the edge. The paper clamping force when editing is very small, but a lot of movements need to be made, trying to eliminate large scratches on a brilliant escapement that remained after sharpening.

For reference:You can also sharpen a lawn mower knife with a rivet, by analogy with a conventional hand scythe. This, however, requires a special adaptation – grandmother, as well as a specific skill. But on the other hand, with this method of sharpening, you will not need to balance the knife..

Balancing check

Before attaching the blade back, you need to make sure that its shoulders are at least about the same weight. Otherwise, vibration will occur during operation, which will not have a positive effect on the durability of the spindle..

Balancing is carried out mainly for double-sided single and compound knives, four-bladed ones are not so strongly subject to imbalance. To check the knife, you need to clamp a smooth bar in a vice in a horizontal position. The blade is put on it through the hole in the center, after which attempts are made to install it strictly horizontally. If one of the shoulders weighs significantly more than the other, the knife will steadily turn to one side..

The shoulder weight equation is carried out by grinding a small amount of metal from the back of one of the knife arms. It is not necessary to achieve ultra-precise balancing, but the knife should be held in a strictly horizontal position for at least a few seconds. When balancing is done, you can put the knife back in place and finally refresh the look of your lawn..