33
General Sharpening Procedures
While I am all for a method that reduces sharpening
time, hollow grinding often does so at the expense of
a strong edge. The undercutting of the bevel weakens
the edge. Equally bad, if you hollow-grind at an angle
that will give you the necessary edge strength, you have
to hone at a higher angle than is desirable for the best
cutting action.
The problem is not
nearly as acute if an
8-in. grinding wheel is
used, but most shops
are equipped with
6-in. wheels, which,
in time, become 5-in.
wheels or even smaller.
The undercutting is a
major problem with
small wheels.
There is really nothing wrong with hollow grinding,
but it should be limited to about half the bevel – the
half farthest from the cutting edge. As an example,
if you want to put a 25° bevel on a chisel, you can
remove 2° or 3° more from the back half of the bevel
to reduce your honing time substantially. For strength,
you have only to worry about the front half of the bevel
(with the exception of laminated chisels, which will be
dealt with later). What you do with the other half of
the bevel will affect rigidity, but it will not materially
affect edge strength.
Removing 2° or 3° saves time in sharpening on bench
stones because you have to deal with only the fi rst half of
the bevel. As this lengthens with repeated sharpenings,
you can periodically relieve the back part of the bevel
on a grinding wheel or belt sander.
Micro-bevels:
The best way to avoid a weak edge
and still reduce your sharpening time is to use micro-
bevels. A micro-bevel is a narrow secondary bevel at
an angle slightly higher than the primary bevel. As an
example, if you want to put a 25° bevel on a chisel,
you would shape the basic bevel at 24° on a coarse
stone and then hone on a fi ne stone at 25°. You need
only a very narrow bevel on the fi ne stone, because the
effi cacy of the cutting action is determined only by the
bevel angle and the edge keenness for any given chip
or shaving thickness. Such a narrow bevel might only
be
1
/
64
in. wide.
The sole purpose of a micro-bevel is to get the fi nest
possible edge in the shortest period of time. There is
absolutely nothing wrong with grinding and honing
the entire bevel at 25° if you wish to. The time it takes
to do this, though, is substantial, and the performance
difference is usually not detectable.
When to Resharpen?
There are several ways to tell when you need to resharpen
a chisel. One is when you notice a difference in use. If
the wood seems to be getting harder, check the chisel.
Another is to look at the edge under a good light. A
sharp edge refl ects no light. But the most usual indicator
is when you see scratches on your fi nished work. You
know immediately that you have a damaged edge.
Fixing nicked chisels:
If you have seriously nicked a
chisel, you may have to remove a fair amount of steel
to eliminate the nick. You can do this with a coarse
bench stone, but you should be prepared to spend a fair
amount of time at it. Even aggressive bench stones take
quite a while to work past a nick. It is faster to use a wet
grinder, a dry grinder or a belt sander. In the case of the
last two, you have to be very careful not to burn the tool
or draw the temper.
If you are using a belt sander or a dry wheel, set your
bevel angle either with the platform of the tool rest or by
setting the projection of your chisel from the edge of the
rest. In either case, use a simple grinding jig, which you
can make or buy. With a belt sander, use only a coarse
belt (80x is fi ne). With a dry grinder, use a soft-bond,
open-structure wheel (such as an A80H-8V). With
either, use a light touch and check tool temperature
often. Both cut quickly but can overheat an edge.
For a very deep nick, the best approach is to mark a
line across the chisel using a square and marker pen
and grind back to the line. I fi rst grind the edge square,
right back to the mark, then re-form the bevel. This lets
you work a bit faster with less danger of burning the
edge, since you get good heat dissipation from the blunt
edge to the main body of the tool. Heat dissipates more
slowly from a thin section.
Do not try to grind material off too quickly. Check blade
temperature frequently against the back of your hand,
and when the blade starts to get warm cool it down by
laying it on a slab of metal, which will act like a heat
sink. This is better than dunking the tool in water (see
the sidebar at the bottom of the next page). When you
have formed the bevel, revert to your bench stones for
the fi nal steps.
Hollow Grinding
Wheel
Bevel reflects
curvature of
wheel.
Tool rest
Relieved portion
of bevel
25°
Summary of Contents for Mk.II
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