How to Sharpen a Knife From a Makita Lawn Mower
Despite the fact that the blades of a lawn mower are made of high-quality steel, they need to be sharpened from time to time. Fortunately, this does not require special skills and equipment, and today we will tell you how to sharpen the knife of the lawn mower ourselves, without compromising the quality of the metal.
You can cut off excess grass from the lawn even if the edges of the mower’s knife are obviously dull. However, the shoots will delaminate in the longitudinal direction, turning into a fringe. A few days after mowing, the grass will become stale and will hurt for a few more weeks. The cut should be smooth and neat, then it will drag out in just a few hours and the lawn will always be juicy green. This requires that the knife be sharpened almost perfectly.
Left: grass cut by a blunted mower blade. Right: sharp-cut grass
How to remove a knife from a lawn mower
Before proceeding with the partial dismantling of the mower, you need to protect yourself and her. If the device with an electric drive is enough just to disconnect from the network, then gasoline lawn mowers require preliminary training. First of all, you need to remove the cap from the spark plug. Next, fuel leaks should be excluded, that is, either drain the tank or place a sealed rubber membrane under the lid and turn off the fuel cock. Turn the mower only toward the handle, unless otherwise specified in the operating instructions. Before turning the mower upside down, be sure to squeeze the crankcase exhaust hose so as not to spill the oil.
Knives of various configurations are used in lawn mowers. In the simplest case, this is an ordinary two-bladed plate of complex shape, but the knife can also be composite. the upper blade is designed to mulch the mowed grass. Four-bladed knives are also found, which can be both composite and on a common base.
The knife can be mounted either with one bolt in the center through the shaped washer, or with two bolts with a fit on the center hole or pin. In any case, the thread on the bolts is right, respectively, you need to unscrew it counterclockwise. To block the shaft from rotation, substitute a block resting on the ground under the knife. It is advisable to remember or outline the position of the blade (s), so as not to confuse anything when reassembling. Loosen the bolts temporarily into the free holes.
Cleaning and editing
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 trim. The strip of the knife should be clean from all sides: not to a metallic luster, but without the smallest adhering fragments.
Often, when hard objects such as stones and wood chips get into the work area, the blade is very noticeably deformed. Most blades themselves are not straight, but the cutting edge and strip 10-15 cm from it should be examined for the presence of local tubercles and curvatures. This is done with a short straight ruler.
You can straighten the blade with an ordinary hammer, whose edges of the striker are slightly rounded. The knife is laid on an even massive support (anvil), then, with weak strokes, the metal is given the correct shape. You need to edit the knife, starting from the edges of the curved section, gradually moving towards the center. The curved body of the blades is a small problem, but it is very important that the cutting parts are straightened into one straight fishing line. You also need to check the throwing blades, which often bend over from the impact. If the blades on different shoulders of the knife are bent at different angles, this will worsen the air flow and the mowed grass will not be properly mulched.
Video: How to Sharpen a Knife From a Makita Lawn Mower
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 removing 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 several cutting planes will form on the mower.
Lawn mower knives have one-sided sharpening, the descent of which, as a rule, is turned up. To bring the descent at the right angle, you should use an electric sharpener with a stone from carborundum, and in the absence of such. a small 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 angle of sharpening are not critical, however, the more even the descents are brought out, the longer the knife will be. When deriving the shape of the blade, it always moves to the grain to avoid the formation of large burrs and gaps. Do not use an angle grinder for preliminary grinding. overheating can degrade the quality of the metal. However, it is possible to install a flap or disk disc under the emery paper on the angle grinder.
The purpose of sharpening itself is to eliminate small chips of the edge and to 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 there is too much effect on the cut stems. In addition, a rounded-shaped 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; more “delicate” stones do not make much sense. Soak the bar in soapy water for 10-15 minutes and periodically moisten while turning. The knife is also sharpened for grain with a small broach of the bar from the center of the sharpened area to the edge.
When sharpening, try to adhere to the same position of the stone in order to maintain a constant angle. As with the removal of descents, sharpening should be carried out with the condition that the same amount of metal be removed on opposite blades. This condition, however, need not be strictly observed when sharpening four-blade knives.
When the blades are sharpened, they need to be fixed. 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, due to which the thinnest edge of the blade can wrap. The emery paper rolled into the cylinder should be slightly pressed to the plane of descent and stretched in a smooth motion from the grain. It is also recommended to make a smooth shift from the center to the edges.
When removing descents and sharpening, all manipulations were carried out with only one side of the blade. In the process of dressing, the knife is also processed from the back. Here, the paper is pressed almost close to the body of the knife, but with a slight slope towards the edge. The effort of pressing the paper during editing is very small, but the movements need to be done quite a lot, trying to eliminate large scratches on the shiny descent that remained after sharpening.
For reference: You can also sharpen the lawnmower’s knife with a rivet, similar to a conventional manual scythe. True, this requires the presence of a special device. a headstock, as well as a specific skill. But then with this method of sharpening, it is not necessary to balance the knife.
Before installing the blade back, you need to make sure that its shoulders have at least about the same weight. Otherwise, vibration will occur during operation, which will not affect positively the spindle durability.
Balancing is carried out mainly for double-sided single and compound knives, four-blade is not so much prone to imbalance. To check the knife, you need to clamp a smooth bar in a vise in a horizontal position. A 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 in one direction.
The equation for the weight of the shoulders is carried out by grinding a small amount of metal from the back of one of the shoulders of the knife. It is not necessary to achieve ultra-precise balancing, however, the knife must be held at least for a few seconds in a strictly horizontal position. When the balancing is done, you can put the knife in place and finally update the look of your lawn.