How To Sharpen A Knife From A Bosch Lawn Mower

How to sharpen a lawn mower knife?

How to sharpen a lawn mower knife yourself? Sharpening of knives for lawn mowers is quite within the power of most owners of these devices necessary in a summer cottage.

  • Gloves
  • 19mm open-end or socket wrench
  • Large and small files
  • Brush

Lay down or (if large) gently tilt the mower on its side. Put on gloves, grab a brush, and carefully clean around the knife to remove any grass clippings and debris. If the knife is dull, it must be removed. Unscrew the bolt that secures the knife with a socket or open-end wrench. The bolt is located in the middle of the knife. The knife is a metal bar about 50cm long and about 2.5mm thick.

How to sharpen a lawn mower knife? It would be nice to clamp it in a vice and file it. In reality, often only an abrasive wheel or a bar for sharpening knives is at hand. Note that the cutting surface of the knife is hardened, so your impact should be minimal. It is necessary to process with a bar or a small file only the really damaged edge of the surface. If the knife is bent, place it on a massive, obviously flat surface (a piece of railroad track, for example) and straighten it with a hammer. Remove large burrs with a file, then work the blunt fragments with a bar starting from the center of the knife and moving towards the edge. The movement of the bar should be “away from you”. Have you ever seen how a scythe is sharpened? This is approximately how you should act.

When finished, put the knife back in place and tighten with the bolt. Excessive efforts at the same time to nothing, subsequently you will have to unscrew it more than once. Lawn mower knives are usually sharpened in early spring before the first mowing. In the future, if you are cutting grass on an area of ‚Äč‚Äčabout 2 or 3 acres (200-300 sq. M.) During the season, you do not need to do anything with a knife.

sharpen, knife, bosch, lawn

You can also use the currently widely used electric grinders, but be careful when using them! With the help of such a machine, you can very easily grind off too much metal and thereby spoil the knife irrevocably. Practice first with kitchen knives, old scissors, or steel strips to get a feel for the force you need to apply to the part while sharpening. A knife that is heated during sharpening must be periodically lowered into water, this will harden it a little, in addition, a wet grinding wheel removes excess heat, preventing the workpiece from being overheated.

The ways

First of all, it is recommended to remove the knife from the mower.

It is unsafe to lift the machine itself onto a car overpass, drive it into a ditch or hang the mower without removing the knife. It may start when you accidentally press or touch the start button, pull on the starter cord, or the device is not fully functional.

  • The manual way to sharpen the blades of a knife is to use a sharpening stone and sandpaper without any motors or machines. Aligning a serrated and broken blade with a hammer or sledgehammer is carried out on an anvil, rail or black bar.
  • The mechanized method, although it is supplemented by “blacksmithing” processing, requires sharpening on an angle grinder, sawing machine or drill. The last two options require a special attachment on which the grinding disc is put on.

The mower blade sharpens evenly and along the entire length of (each) blade. To prevent the ball bearings of the motor or gearbox from breaking, centering the knife is best done by a turner on the machine. But even without a lathe, the result will be quite decent.

How to sharpen a lawn mower knife?

  • Required inventory
  • The ways
  • Angle selection
  • Phased execution
  • Do I need to sharpen a knife after purchase?

It is also possible to mow mechanical slanting grass on the site with a blunt knife. But rough cutting of the grass with a mower will spoil the appearance of the lawn: it will delaminate in the longitudinal direction. The resulting fringe will be an “callous” look for several weeks. To ensure that the lawn is completely level and completely level, it is recommended to check the lawn mower blade at least once a month. and, if necessary, sharpen it.

Phased execution

There is no need to look for a master to sharpen a lawn mower knife correctly. If you know how to handle manual and mechanical tools, then even without having the skills to sharpen knives and blades, but doing everything strictly according to the instructions, immediately learn how to sharpen cutters for mowers on your own.

Follow some steps to remove the knife.

  • Position the mower so that the bolt holding the blade is easily accessible to you.
  • To prevent the shaft from spinning, place a block of wood under the knife.
  • Try unscrewing the nut with a 19 mm wrench. Nut size may vary. On many mower models, the knife is fixed by means of a central bolt, and held by means of a spring washer with a nut.
  • If the nut does not turn away so as not to rip off the edges of the bolt or wrench, saturate the threaded joint with oil or grease. Leave it like this for a few hours.
  • If there is not enough force on the wrench lever, hit with a hammer in the direction of unscrewing the nut. counterclockwise. In the vast majority of cases, the nut can be removed without problems.

The last two steps will save you the adapter that secures the motor or gear shaft to the knife. The spindle mower will not work without it. If the cutter still cannot be removed with your own hands, then contact the workshop.

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In case of minor damage, the cutter is sharpened without removing.

Angle selection

The sharpening angle is just as important as keeping the center of the knife balanced. Ideally, it is 30 degrees. Too small an angle will turn the knife into a kind of razor blade, magnified tenfold and dull at the very first cutting of weeds or trimming bushes.

Too big. would rather hit and break plants than cut them.

Taking out descents

Blade slopes. the border zone between the base of the working part and the edge of the tip. As well as for kitchen knives that perform completely different functions, descents to the edge have their own normative spread of indicators.

Cutting out smooth and precise descents on the knife is half the success when restoring the original point.

On the mower knives, the main slope is used. Too “steep” descent (30-60 degrees). as well as too “gentle” (less than 30 degrees) will lead to rapid blunting of the point, a rapid loss of its cutting properties. On the cross-section of the knife, the start of the cutting edge should not be crooked. Smooth slope, in contrast to rounded, is much more resistant to dirt and easier to clean.

Sharpening the blades

Mower blades are sharpened with:

  • chisels, files and sandpaper;
  • angle grinder (using sanding discs);
  • with an electric drill or any motor with a special attachment on which a round whetstone is attached;
  • on an emery machine.

Make sure there are no dents, notches or dents on the knife blades. For high-quality sharpening, you will most likely need a vise. they will firmly fix the knife in a position ideal for quick sharpening. Small notches and gouges can be removed with a chisel or file or a coarse sharpener.

precise sharpening is carried out with a fine-grained sharpener or emery of zero or first grit.

Sharpening performance will increase when you use a drill with an adapter, on the chuck of which a sharpening stone or emery wheel is attached. Now plug in and run your drill or sharpener. Start sharpening the serrated edge of your knife.

When sharpening, you must adhere to certain rules.

  • Do not apply excessive force to the product, smoothly move the emery wheel.
  • Avoid friction overheating when sharpening.
  • Dampen the knife periodically with soapy water.
  • Keep the sharpening angle constant to avoid the sawing effect on the cutter. The roughness of the point, its “pockmarked” line will lead to the formation of the smallest notches in the first minutes of work.

You can attach a drill or sharpener, and move the knife along the edge of an emery or sharpening wheel.

Cleaning and straightening

After removing the knife, clean it of dried grass residues and dirt. There is no need to clean it up to a metallic sheen. But there should be nothing extraneous on the blades themselves.

Bosch Rotak 320 blade sharpening

If the knife is serrated and bent, it must be straightened.

Straightening is carried out using a hammer and an iron bar or a piece of rail that serves as an anvil. When, when leveling the knife, new breaks and cracks appear, it is useless to restore such a point. If, after leveling, the cracks are small, grind off the cracked strip of steel until the knife blade is completely flat.

On a professional machine

This option is more suitable for hairdressers, whose working tool requires regular. at least 4 times a year. and high-precision sharpening. At the same time, only a special machine can provide high quality of this process, which significantly reduces the time for this procedure.

The algorithm of actions in this case looks like this:

  • remove the knives from the trimmer;
  • cover the grinding disc, free from dust and dirt, with a uniform layer of abrasive paste;
  • we fix the knife on the magnetic holder with the cutting edge down;
  • we start the machine at a certain speed (indicated in the instructions);
  • we bring the knife to the circle and begin the sharpening process, with smooth movements of the blade, the teeth of which should be directed against the movement of the grinding disc, from the edge to the central part.

The main disadvantage of this method is the high cost of a professional machine and consumables.

A bar for sharpening knives

The clipper’s blades are sharpened on the block used to adjust the blades of kitchen tools. The process is similar to restoring the cutting edge with sandpaper. Bar requirement:

  • fine grain;
  • thicker than blade size.

For home restoration of parts, you should take a wide block. The knife should lie completely on its surface.

On a lathe

The preparation of the lathe begins with rearranging the chuck jaws. They should be installed in the opposite direction in order to clamp a disc instead of a part. an abrasive wheel. Treatment:

  • Turn on the machine, check the end face for runout. Adjust spindle rotation for 1000-1200 revolutions.
  • Bring the knife fixed in the magnetic holder to the edge of the circle.
  • Move the sharpened part to the center and back.
  • Check the sharpness of the blade.

The disadvantage of sharpening on a lathe is the increased risk of work. The hand can be caught in the chuck or damaged by the cams.

Rotary lawn mower blade sharpening

Rotary devices include:

Gasoline lawn mowers; Electronic lawn mowers. wire and battery.

In their equipment, mainly two-bladed knives are used, less often a knife with 4 blades. The sharpening technique for all types is the same, and does not have any differences.

To do the work you need:

Work gloves; A spanner or end wrench of the corresponding size; A set of ratfil, an abrasive bar, or an electronic sharpening tool.

Before you start sharpening the mower knife, you need to perform several preparatory operations:

Prepare the mower to remove the knife. In particular, this step should be taken into account by the owners of gasoline models of lawn mowers. It may be necessary to drain fuel, oil;

Reasons for removing the knife

Common reasons for removing or replacing a cutting element are as follows:

  • The knife is dull;
  • The element is bent and deformed;
  • Chips and cracks are visible on the knife;
  • The blade has worn off and thinned.
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When the knife is dull, bent or deformed, the lawn looks unkempt after mowing. Competent sharpening will correct the situation.

Professionals advise to carry out the procedure once a year. It is better to do this in the spring, before starting work. If the cutting elements become blunt faster, then sharpen them more often.

Knife sharpener

If chips, cracks are visible on the blades, or it has sharply worn off and become thin, then replacement is necessary in this case. There is no point in sharpening such a tool. In addition, the blade can break at any time and fly out from under the machine body, injuring people or animals that are nearby. Such a device can also damage the engine. It is allowed to change an element to a similar model. The characteristics of the knife are indicated in the instructions.


An independent process of partial disassembly of the vegetation shearing unit begins with preparatory work. It is necessary to observe safety measures and work with gloves. It is necessary to prepare a key in order to unscrew the mount, as well as a small plate that will help fix the knife. The electrical appliance is disconnected from the network. At gasoline, the fuel is drained and the spark plug is turned off to exclude the possibility of a sudden contact. Regardless of the type of machine, the owner must unscrew the grass catcher.

Important! Before turning the mower on its side or upside down, inspect it to ensure that there is no gasoline or oil leakage. It is best to drain the oil in the crankcase.

Otherwise, if a leak occurs, it will be problematic to start the unit.

You can also not turn the mower, but simply place it on a frame or two chairs that simulate a lift. After that, the person approaches the device from below.

Preparing to remove the knife

The mower can be equipped with:

  • A single two-bladed knife of different shapes;
  • Two knives;
  • Blades with four blades;
  • Composite cutting structure.

In the first case, one knife is used in the form of a small strip. Its length will be the same as that indicated in the characteristics of the device under the heading width of cut. Usually it is 30-50 cm.

In the second case, two knives are used, which are attached at an angle of 180 degrees relative to each other.

On a note! Another cutting system option is the four blade blades. They are connected to each other by a common base or not.

In the latter case, we are talking about several knives located at different heights. Such a nozzle in the form of an upper blade is equipped with modern models that have an additional function of mulching the lawn. Plants from the grass tank fall onto the upper blades. Sharp edges carefully crumble the greenery, which after the procedure covers the ground of the lawn and turns into mulch.

How to remove a knife from a lawn mower

How to remove a knife from a lawn mower

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 must 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 side. Here the paper is pressed almost close to the body of the knife, 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 the brilliant escapement that remained after sharpening.

For reference: you can also sharpen a lawn mower knife with a rivet, by analogy with a conventional manual 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.

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 keep the same position of the stone 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-bladed knives.

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 out the edge of the blade at a uniform angle along the entire length of the cutting area. Please note that there may be several edges on each side, in which case the 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.

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Small deviations of the sharpening angle are not critical, however, the smoother the slopes are brought out, the longer the knife resource will be. When shaping the blade, it always moves onto the grain to avoid the formation of large burrs and flakes. Do not use an angle grinder for preliminary sharpening. overheating can degrade the quality of the metal. However, it is possible to mount a flap disc or a sanding pad on an angle grinder.

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 with 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 issue, but it is very important that the cutting parts are straightened into one straight line. You also need to check the kickback blades, which often straighten from impact. If the blades on different knife arms are bent at different angles, this will impair the air flow and the cut grass will not mulch properly.

How to remove a knife from a lawn mower

Before proceeding with the partial disassembly of the lawn mower, you need to protect yourself and her. While an electrically powered device can simply be 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 stated in the operating instructions. Be sure to pinch the crankcase exhaust hose before turning the mower over to avoid spilling oil.

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 designed 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 that is resting on the ground. It is advisable to remember or outline the position of the blade (s), so as not to confuse anything when reassembling. Temporarily tighten the unscrewed bolts into the free holes.

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

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.

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

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

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, however, 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.