Lawnmower
Blade Sharpening
The
blades on rotary lawn mowers lose their edge fairly quickly. This is because there is a compromise taken in
the design of the machine that decides that the steel from which the blade is
made needs to be fairly soft to minimise chipping due to stones hitting the
blade. A blade that holds its edge would
soon have lumps knocked out of it by the stones.
Consequently,
the blade needs sharpening fairly often in order to maintain a reasonably clean
cut. A blunt blade will still cut due to
the high speed of rotation, but it will cut damp grass much more cleanly if it
is sharp. Also, an electric mower will
rotate at less rpm than a petrol mower and will therefore depend more on having
a sharp blade.
The
problem with home sharpening is that it is necessary to remove the same amount
of material from each end of the blade or the balance of the blade will be
affected leading to vibration. You can
get it nearly right on the first couple of sharpening, but eventually, the
blade will go out of balance and shake up the mower unnecessarily. It might
even lead to the blade becoming dangerously loose.
To
minimise this problem you can either take the blade to a mower shop and get it
sharpened in a jig that keeps it balanced by virtue of removing the same amount
of material from each end, replace the blade with a new one, or have a go at
balancing it yourself.
Assuming
that you are not technically inept.- in which case you would never manage to
get the blade off anyway - and have access to at least a suitable spanner and a
decent file, you will probably be able sharpen the blade yourself and thereby
extend its useful life.
The
first problem is to get the blade off.
When I was younger it was commonplace to fix the blade on with a nut
having a left hand thread, but all the modern machines I have encountered use a
standard right handed fixing. Just check the manual to make sure before trying
to undo the nut in case you are just tightening it further.
VERY
IMPORTANT: REMOVE THE LEAD FROM THE SPARK PLUG BEFORE YOU START. Otherwise you
might spend the rest of your life making unintentional rude gestures.
Be
careful how you tip the machine around or the oil will run out or end up in the
cylinder giving you a problem either cleaning up or getting the mower
restarted. Most Briggs and Stratton
engines have the oil filler at the pushing end and will tolerate raising the
pushing handle a bit. It would be best
to stand the mower on bricks or wood blocks to raise it off the ground, then
lie beside it to access the blade. Some repairers tighten the filler cap and
lie the machine on the carburettor side
I
use a motorist’s socket set to undo the fixing but a good ring spanner or open
ended one would do the job. The socket
set gives you the option of increasing the torque with a length of pipe over
the handle. The only thing stopping the
blade rotating is the engine brake, so you will probably need to wedge the
blade. Mower manufacturers are not nice
enough to provide a way of locking the blade, so I use a length of 50mmx50mm
wood between the blade and the deck.
However, I make a point of ALWAYS buying a mower with a metal deck. If yours is plastic, you’ll have to find a
way of whacking the spanner with a hammer to use the engine’s inertia to loosen
the blade. Of course, you may be lucky and the blade fixing might not be too
tight.
The
blade when removed may be found to be a simple plate, have a slight twist on
each end like a propeller, or have a bent over flap on the trailing edge.
The
simple plate is straightforward and should be reversible to effectively get
cutting life out of both pairs of edges.
If you are careful, you can easily keep this type in balance by treating
each half the same.
The
propeller shape gives the same reversibility and also provides a fan action to
suck the grass upright before chopping it off.
This sucking action can also be designed to assist the chopped grass
into the catcher box.
The
type with bent over flaps may have some esoteric use not apparent to me but,
the flaps should increase the air flow.
However, unless the flap bits have a special use in cutting, the blade
is not reversible.
Since
the blade is not made from particularly hard steel, a good quality file is the
best tool to do sharpening at home, unless the blade is badly chipped and worn,
in which case a small angle grinder of bench grinder can be used to restore the
original shape.. Sharpen by draw filing retaining
as much as possible the same form as the original and trying to remove the same
amount from each edge.
You
need a special device to properly balance the blade dynamically, but first
order balance can be achieved statically.
(Whatever the balance device adverts say.) To statically balance the
blade, find a chrome tool with nearly the same diameter as the fixing hole, or
at least get a bolt near the right size.
Insert the tool or bolt into the blade and ideally fit this assembly
into a vise with the blade vertical to allow it to spin freely.
Note
which end of the blade tends to point down and work out where you can remove
some metal near the end of this. Don’t reduce the length, remove metal from
trailing edge or reshape to be the same as the other end and re-sharpen. Approach a final state of balance slowly so
as not to remove too much metal. The closer to the end you remove metal the
more effective it will be. If a fair
amount of metal needs removing you will need a grinder to do this within a
reasonable time frame.
When
the blade doesn’t tend to favour any particular side, refit it into the mower
and do up the fixing bolt fairly tightly. Remember that the normal direction of
rotation is clockwise from above the mower and therefore anticlockwise below in
order to work out which way to fit the blade with the cutting edge leading.
Replace
the spark plug and let the mower stand awhile for the oil to all run back into
the sump. The vibration will hopefully
be less, which is less stressful and should lengthen the mower’s life
This
may get you an extra couple of sharpenings out of the blade before you need to
replace it.
