Sharpening your lawn mower

This month we are looking at sharpening and adjusting you mower ready for the season ahead.

Before you make the first cut of the new season, take a bit of time to prepare the mower blades. There is enough advice on other websites about general mower maintenance so we will not cover that here. The only thing I would say is that for a mower to cut grass cleanly, the engine must be well maintained so that it runs at full speed.

Be aware that the following factors will all reduce engine speed and therefore quality of cut:

  1. Air filter clogged
  2. Fuel system and filters clogged with deteriorated old fuel
  3. Poor adjustment of throttle lever (remember the cable can stretch over time)

Why should we worry about sharpening our lawnmower blades?

Well when I am called into look at lawns that are not thriving, apart from the popular drainage, rootzone or shade problems, the next most popular problems are associated to poor mower sharpening and adjustment.

If the blades are not sharp, they tend to tear the grass. If you tear the grass you cause three problems:

  1. The torn area turns brown as it heals so giving the lawn a brown tinge
  2. Because there is a large torn area, the grass has to put excessive effort in to repairing itself so dropping its health
  3. Because the plant is highly stressed trying to heal, it is more susceptible to attack by fungal diseases.

We have to consider the two types of lawmower differently

Rotary mowers (Flymo type)

The blades are less sensitive and easier for DIY sharpening.

  1. Remove the blade from the mower (following your manual for safety advice, especially the removal of the spark plug.)
  2. Clamp the blade in a vice and sharpen one end of the blade at a time
  3. Use a file to copy the angle of sharpening that is on the blade originally.

    4. There are a few aids for doing this, see these web pages:

    Blade Sharpeners
    Electric blade sharpener

  4. Once you are happy with the edge on your blade you must check the balance of the blade. Hammer a nail into a vertical wall and try to balance the blade by hanging the blade by the central hole. Take more metal off the heavier side.
  5. Replace your blade and fire up your mower and check for vibration. If there is too much vibration, go back to stage 4, repeat until mower is smooth in operation.

I have to say that when dealing with rotary mowers, you will find that the cost of a replacement blade is not excessive. Consider just renewing it annually, it will save a lot of time and sweat using a file. Checkout these suppliers:

Mower Magic
Parts Depot

Cylinder mowers

Although these are by far my favourite mower, (they give a much cleaner, crisp cut on the leaves when sharp and well adjusted) they are much more fickle to sharpen, adjust and maintain.

In the first instance I do not recommend even attempting to sharpen these yourself. To be done properly they must be sharpened on a machine.

There are 3 places where you might be able to get this done:

  1. Your lawnmower supplier
  2. Your local golf course
  3. Your local “saw doctor” (find this in your yellow pages under “Grinding and sharpening services”)

You should arrange to have this done at least once per year.

In between sharpening you must maintain adjustment and sharpness.

To maintain sharpness you should use a procedure called “backlapping”. This is where the blades are coated with backlapping paste and spun in reverse. If your mower does not have a reverse drive then slow turning with a spanner on the end of the rotor will work.

To check adjustment you should follow this procedure on a regular, monthly basis:

Set the mower where you can easily see and access the lower blade. Fold up a piece of printer paper lengthways. (not newspaper, it is too thin) then carefully place the end of the paper where the grass would go and carefully turn the rotor in the cutting direction. (remember that the blade belt or chain must be disconnected from the engine drive. The blade should cut the paper cleanly. Check both ends and the centre of the blade. If the blade is too stiff to turn then raise the rotor up equally on both sides. If the paper doesn’t get cut then lower the rotor a very small amount on both sides, equally. Adjust until the rotor spins freely but cuts paper at both ends and the middle.

If you have not had the blades sharpened professionally and you find it impossible to get a setting with results as described above, then that is a sure sign that a professional sharpen is required. When it comes back from sharpening, it should be set “on-cut” but you should check using the folded paper technique described above before attacking your lawn.

If you have an old mower and everything is seized up and never adjusted, trust me that it is worth getting it off to a mechanic to get everything loosened up and lubricated as running a cylinder mower requires fine adjustment continually. When we are cutting turf fields we often have to adjust the mowers half way through the day!

Now that you mower is sharpened and adjusted you can get ready for your first grass cut. When do you cut your grass in the Spring?

Cut as soon as there is evidence of growth, but wait until there is a good 3 days of warm weather forecast for after you mow. This allows the grass to heal before coming under cold stress.