Ron Hock's Sharpening Notes
[I finally got around to writing my own instructions
for sharpening after using a reprint from Woodsmith
Magazine on our blade wrappers for over twelve years. I heartily
thank them for their generosity but I always wanted to use my own words
about techniques that work for me. These same notes now appear on our blade
wrappers.
I also recommend Leonard Lee's The Complete
Guide to Sharpening from Taunton Press, Brian Burns' Double Bevel
Sharpening from Palo Alto Publishing (available from Luthier's
Mercantile International), and The
Japan Woodworker's sharpening info about chisels and hochos. Be sure
to check out the "ScarySharp"
system for still more tips on generating the perfect edge. Go to Ralph
Brendler's Use
and Preperation of SCRAPERS for the "how to" on Scraper
Blades. You can't have too much information. -- Ron]
Though many woodworkers
find the sharpening process a pleasant pre-work meditation, most
of us would just as soon get it out of the way and get busy woodworking.
There is more to sharpening than I can cover here and I refer you to any
of the many excellent books on the subject. What I offer here, in extremely
condensed form, are some ideas and methods to help make the task less forbidding.
First, The Goal: A sharp edge only exists
where two planes (i.e., the back and the bevel of a plane iron or chisel,
or the two bevels of a knife) meet with zero radius. Of course,
“zero radius” is a theoretical ideal that eludes us as we move to the next,
more powerful microscope.
There will always be some radius to an edge but The Goal is to minimize
it. (Our fine-grained steel helps you here; the hardened particles in our
steel are very small, allowing a smaller radius to be sharpened.)
Next, Getting There: Any of the popular abrasive
devices can and will sharpen your blade.The choice is yours. The venerable
“Arkansas” oilstones are legendary and keep their shape and flatness with
little maintenance. These are a natural, quarried product that will last
a lifetime. Man-made waterstones were more recently introduced from Japan,
having a long history there as a natural stone. These stones sharpen more
quickly because they are softer and thus wear faster, exposing fresh, sharp
particles as they wear. However, their softness requires they be flattened
often to avoid their tendency to “dish,” which makes accurate blade flattening
and honing impossible.
Many woodworkers use a series
of sheets of wet-or-dry sandpaper as their abrasive medium. A piece of
glass serves as a flat base-plate and the sheets are simply switched as
the blade is honed through successively finer grits. The low start-up expense,
ease of use, and variety of grits (up to 2000-grit or finer from the auto
supply) make this a great way to get started. Then there are diamond stones
(great for coarser work), lapping plates (those who know them, swear by
them), ceramic stones, leather strops (excellent for final finishing),
and
an overwhelming selection of powered machines all designed to make this
task easier. Whew!
If you have a method that
you like, that works for you, stick with it, use it. The following
steps are mostly generic and you can follow along regardless of your abrasive
proclivity. If you're new here and “grit-less”, head to the store that
sells automotive paints and related supplies and buy a sheet or two each
of 180-grit (180X), 320X, 400X, 600X, 1200X, and 2000X. Some folks like
to use 3-M #77 spray adhesive to stick down the sandpaper sheets; they
sell it where you buy the sandpaper. Next, to the glass shop for a piece
of 1/4” glass about a foot square. A marble floor tile, or scrap piece
of monument or countertop granite, works well, too. Now go clear a spot
on a workbench for the glass or tile. With a new blade, start with
the 600X paper. If the back needs a lot of flattening, don't be afraid
to use a coarser grit to save time. When resharpening a blade, if the edge
is chipped or horribly dull you may need to start coarser: 320X or 180X
may be necessary.
Honing guides are
useful things. If you have one, now is a good time to use it. Most block
and bench plane blades are ground to 25° but some smart folks argue
that there need only be clearance under the heel of the bevel. In other
words, since the average bench plane blade is bedded at 45°, any bevel
angle less than that will provide the needed clearance. And a thicker bevel
is stronger so the edge should last longer. Bench plane and block plane
blades have traditionally been beveled to 25°. Our blades for the handmade
wooden planes were specified by James Krenov to have a 30° bevel. Chisels
get different bevel angles for different tasks: 25° or lower for paring,
30° or so for chopping. Experiment a bit with different angles to see
which one works best for the wood and your style of work. A honing guide
helps with all this by establishing an angle and sticking to it. It can
also shorten the whole process by letting you raise the blade a degree
or two so that you're only honing the very edge. The angle of the bevel
is determined by how far the blade sticks out of the honing guide.
At least one brand tells you right on it how far to extend the blade for
a 25° or 30° angle. If your honing guide doesn't tell you how far
to extend the blade, you'll have to experiment and measure to get what
you want.
No honing guide?
That's okay, but you'll have to exercise a bit more diligence and control
while honing the bevel. It is important that the bevel be maintained throughout
the sharpening process. If you rock the blade, the bevel will end up convex,
“roundish,” and the actual angle at the sharp edge will be greater than
you intended. Not the end of the world, but it makes apples-to-apples comparisons
between woods, steels, tools and bevel angles impossible. You can cut an
angle template from a piece of cardboard, or whatever, and use that to
check the angle as you go.
Start by “grinding” the bevel
until a burr forms on the back. It may not be very visible, and
will get smaller as you move to finer abrasives, but the burr will catch
your fingernail. If the edge radius is large (which is a fancy way of saying
“if the edge is really dull”), it may take a while before the burr will
appear but it must be there or you haven't done enough work. It's the burr
that tells us when the two planes have met (that zero radius thing, again.)
Now flip the blade over
to do the back. Flattening the back is as important as honing the bevel.
I
repeat: Flattening the back is as important as honing the bevel.
Think about it: in a bevel-down plane, like all bench planes, the back
of the blade is the cutting edge. So you have to make the back flat
to insure that the edge is straight, smooth and sharp - without waves,
valleys or “teeth.” Many woodworkers believe that the whole back, from
the edge to the slot, should be flattened and honed. Others figure that
a stripe about an eighth of an inch back from the edge is sufficient since
the chip breaker rarely exposes even that much. Your choice.
If you can leave the honing
guide on the blade, just hang it over the edge of the stone or plate. If
it's in the way, you'll have to measure and reset the blade extension from
the guide each time you change grits. Start with the coarse abrasive you've
been using and rub the back using even, firm, down-pressure and take even,
steady strokes keeping the blade flat on the surface. Do this until your
scratches are uniformly all over the area you want to hone. It's quite
common for a plane blade to have a slight “hollow” in the back and the
early honing will reveal an arc of fresh metal along the edge and sides.
You can expand this area as far as you want until the whole back is covered
with the coarse scratches. When you're down far enough, and the planar
surface of the back meets the planar surface of the bevel (zero-radius!)
you will raise a burr on the bevel side. You're there.
Change to a finer grit
and repeat the above process. Once the back has been ground flat with the
first grit, it gets much easier and goes much faster. It's a good idea
to angle the blade slightly while working on the back and to change the
angle with each successive grit. That way, you can readily see when you've
honed off all the scratches left by the previous grit; another clue that
it's time to change to a finer grit.
Check the blade to be sure that
it is staying square. If it's not, push a little harder on the high
corner while honing the bevel to bring it back square. Proceed through
the grits until you run out of them. After a few, the honed surfaces will
begin to act as mirrors; a sure sign of imminent sharpness.
For most efforts, the 2000X
paper is as fine as you need to go. But if you're doing the final planing
on a surface that you don't want to degrade by sanding, you may want to
go beyond the 2000X paper to a 6000X waterstone or a strop charged with
chromium oxide compound (“green oxide” or “knifemaker's green”). The 6000X
waterstone is a soft “stone” of cerium oxide that cuts fast but can be
tricky to use because the blade wants to stick to the fine surface. Slow
strokes, plenty of water and patience are required. The strop can be leather,
cardboard, or wood; a flat, fine textured surface that will take the crayon-like
super-fine abrasive is what you want. It's best to gently pull the blade
across the strop or you risk cutting into it. Be careful to keep the back
flat against the stone or strop and the bevel at the correct angle; you
don't want to round off the edge.
To test for sharpness,
you can always shave the hairs on your arm (or wherever). A sharp edge
will cut hairs with very little pressure. But if you're running low on
hair (or just hate that patchy look) there are other ways. A sharp edge
will catch on the flat of a fingernail or plastic pen barrel while a dull
edge will skid a bit. It's really that simple; try it a few times to feel
it but it takes only the lightest touch and if it skids, it's dull.
Also, you can see if
a blade is sharp. Closely examine the edge with good light and if the edge
reflects at all, it's dull. (Remember that zero-radius stuff? It's the
blade's edge radius that reflects light and if there is no radius -- The
Goal -- there will be no reflection.)
If you've just done a chisel,
block-plane blade or other breakerless blade, you're done! But if
you're working on a bench-plane iron, you're not done until you've polished
the breaker. Make sure the breaker, when tightened in position on
the blade, makes complete contact along its edge with no daylight showing;
no gaps at all where a shaving could catch. Now polish the breaker ramp-surface.
How much work is needed depends greatly on its condition, of course, and
how smooth is enough is a matter of experience and performance. Use the
same abrasives starting no coarser than you must. Breakers usually aren't
hardened so the work should progress quickly. Rock and slide the breaker
along the different grits until all coarse scratches are gone and the ramp
area looks and feels smooth. Now you're done.
Resharpen often and
lightly, no coarser than necessary, to insure good cutting performance
and save time in the long run. Goodluck, have fun, and... “Ommmm”
Ron Hock
HOCK TOOLS
www.hocktools.com
Ron Hock ©2002