
3
1.2 Principles of Sharpening
The reel type lawn mower cuts grass using the prin-
ciples of shears. It is necessary, therefore, to have two
sharp cutting edges making close enough contact to
cut the grass cleanly. This is the least harmful method
of mowing grass, because each blade of grass is sup-
ported by the bed knife while the reel blade shears it
off. This eliminates bleeding and brown tops which
occur when the grass is whipped off with rotary type
mowers.
On a ve bladed reel mower, the bed knife does ve
times the work of any one reel blade as all the reel
blades must shear against it. The bed knife, therefore,
is the master cutting element and although made of
heavier and harder steels, it is impossible to properly
sharpen a mower with dull reel blades without sharp-
ening the bed knife too.
Rake Angle
Rotation
Relief Angle
Land Area
The gure above shows the different features of the
reel mower blades.The reel blade ReliefAngle serves
reduces the contact area between the reel and the
bed knife. It is created by individually relief grinding
each reel blade. The Land Area indicates how much
of the reel blade is contacting the bed knife and is
created during spin grinding. The Rake Angle is set
by the slot in the spider and ensures that the cutting
edge is the rst part of the blade to touch the grass
blade. It can not be changed.
Front Face Angle
Top Face Angle
The bed knife Front FaceAngle is used to support the
blade as it is being cut. It does not need to be ground
regularly as it does not wear and change shape. Bed
knife life is increases, however, by grinding both the
top and front surfaces of the bed knife to eliminate
the rounded edge. The bed knife Top Face Relief
Angle provides reduced contact area between the
reel blade and the bed knife blade. It also creates
clearance for the cut grass and other materials. It is
the wear surface on the bed knife and must be ground
regularly to keep it straight and sharp.
For a mower to run easily and cut freely, it is important
that proper bevel or relief angle be ground on both
the bed knife cutting edge and the reel blades cut-
ting edges. This gives clearance or relief behind the
contacting edges and reduces drag and friction. Too
little relief angle would leave more metal in contact
causing the mower to run hard. Too much clearance
or angle would weaken the cutting edges and they
would nick easily and would not hold their edge.