|
Straight edge, sheepsfoot & wharncliffe
Middle
overgrinding risk
Straight
edge looks deceptively easy to sharpen, but it is unforgiving to
inaccuracies in your grinding technique.
Knives
where straight edge extends over 3" before it curves upwards to the tip, tend
to concave in the middle when sharpened on a 2" rotary wheel.
Usually the concavity forms in the portion of the blade where the knife
jig is clamped.
Because the heel of the blade gets the least grinding time, undergrinding the heel section, and overgrinding the middle section of the blade is inevitable, if you keep the pressure even and the blade feed rate consistent.
This issue affects less the tip section of the blade: even though, similarly to the heel, the tip gets less grinding time, the heel section is the thickest of all, whereas the tip section is the thinnest thanks to the blade taper towards the tip.
Thickness behind the edge is the greatest at the heel and the smallest near the tip, and because of that the heel section always falls behind the tip section during grinding.
The
following approach helps to eliminate the unintended middle overgrinding.
Clamping
the jig in the middle of the straight stretch (not in the middle of the
knife) gives the best result.
Sharpening
direction: unilateral from heel to tip.
(Bilateral grinding, heel to tip and then tip to heel, as you might have
seen in some training videos, furthers concaving.)
Sharpening
movement is 2-step: one short pass from the bolster for 1 inch, followed
by 1 full pass from the bolster to the tip.
When doing
the 1st inch pass, press on the heel of the blade only.
Then start
again from the bolster, and do one full bolster-to-tip pass with pressure on the heel and tip segments.
Take care not to pull the tip past the middle of the
stone.
Flip the
blade, and do the same on the other side.
And so on
alternating sides, till the burr forms.
Inspect the
edge frequently for segments that may be falling behind, and where this
happens spend more time there.
With this
technique, a continuous burr forms almost simultaneously along the entire
length of the edge.
New Tormek
operators, especially those with bench stone experience, initially tend
to keep pressure even and consistent - that doesn't work with rotary
sharpeners. You have to exert more force on the bolster end of the knife,
because the heel section of the blade gets the least grinding time and is the thickest behind the edge.
Wherever
possible, avoid coarse wheels when sharpening straight blades.
Starting on
grit #400-600, you can grind bevels within reasonable time.
However, it
is not always possible. With seriously dull knives, hard alloy and tool
steels the grit #400 will take ridiculously long to grind bevels, especially at lower angles. When you
have to start on a coarse grit, alternate sides with
every pass, and do not raise the burr, but change to a finer wheel short of the edge apex.
Jig
locking collar
For this
type of knives, the movement to keep the blade on the wheel must be
strictly perpendicular to the grinding wheel surface, axial to its shaft.
Any axial
tilt, shifting other than vertical results in uneven bevel.
As a training aid, use a
12mm diameter shaft locking collar, locked at 12mm from the jig
adjustable stop (the flat black plastic part), to ensure the jig slides
on the Universal Support perpendicular to the wheel.
|
