A Basic Approach To Sharpening
Skip the discussion, just give me the bottom line.
Sharpening is (in theory) a
large and complex
topic. However, if you're just getting started
in woodworking with handtools, the vast
majority of your blades will be reasonably long,
rectangular and sharpened straight across. This
type of blade includes all bench planes, block planes,
rebate and plough planes, and "normal" chisels, which is
enough tools to do rather a lot.
The Desired Edge - Bevels and Angles
In tools used bevel up (block planes and chisels), the bevel
angle effects cutting angle and edge strength. Lower angles
cut more sweetly, but the edge is prone to chip or crumble.
In bevel down tools (bench planes) the bevel angle (is believed to...) only
effect edge strength - as long as it's lower than the bedding
angle, and thus provide "relief".
Many learned discussions have been held on
which exact edge angle is required for which task
in which material with which blade steel. If you want to
know about all that, find a non-basic
reference on sharpening.
Back in my basic world:
- Paring Chisels and Block (bevel up) planes
Edge bevel of 25°
- Chopping/Sturdy Chisels and Bench (bevel down) planes
Edge bevel of 30°
What Abrasive to use - is there a "magic stone" out there?
The short answer is "no". Any sharpening process
will require abrasives in a range of grits. The coarser
ones are used to remove damage or worn edges. The finer
ones are used to polish and refine the 2 surfaces (back and bevel)
that intersect to make the edge.
Since everybody wants to make an informed decision, here's
a thumbnail description of the major abrasive choices on the market.
To repeat, any of these can be used to generate
the sharp edge you desire.
- Oil Stones
The oldest variation. Man made stones cut fast,
and don't wear very quickly (at least good brands
like Norton don't). For a fine abrasive you'll need
a natural stone. The famous Arkansas oil stones
give a very fine edge, and barely wear at all.
Since they've been around a while, oilstones
have the advantage of being available second hand (and thus
cheap), and the process of using them leaves nice rust preventing
oil on your tools. Using them is (however) messy, and you run the risk
of getting dirty oil in places you don't want it, like on yourself,
or your woodwork.
- Wet 'n' Dry sandpaper
Used for the famous
system. Your abrasive is
is simply Silicon Carbide (SiC) sandpaper fixed to a reasonably
flat and hard surface.
This approach gives very low initial cost (one sheet of paper in each grade
and a bit of MDF or tile), but has minor problems with edge dubbing (where
the sandpaper won't sit quite flat), and you never stop buying
- Water Stones
The magic of the Orient. These work a bit like oil stones,
except that the lubricant (as you might expect) is water.
man made waterstones are available in
grades from coarse to very fine. Anybody intending
to buy natural waterstones (which are quite remarkably expensive)
won't be getting their sharpening information from this page.
A waterstone tends to cut more rapidly (for a given particle size)
than the other abrasives because they wear away quickly, continually
revealing fresh, sharp, abrasive particles. However
the same rapid wear results in dished stones, which need flattening.
The lubricant (water) is also prone to invoking rust if you're not
careful about drying your tools after sharpening.
- Diamond Stones
A recent entry to the market. Diamond sharpening stones
are lubricated with water, and cut rapidly. However,
they don't last forever (advertisements notwithstanding)
and are expensive. They are also not available in fine enough
grades for final honing. The coarse ones are the most useful.
- Ceramic Stones
Claimed to have the advantages of waterstones (fast cutting,
fine grits available) with oilstones (low wear rates) and
add the benefit of not needing a lubricant at all. I know
very little about them in practice (so if anyone wants
to tell me of their experiences feel free).
Techniques When Putting Tool to Abrasive
Having decided what angle to work at, the difficult
(but oh so important)
task is to maintain a constant angle over all your
sharpening strokes. The problem is that it is amazingly easy
for the hand holding the tool to vary in height
depending on how extended your arm is, which
in turn varies the angle at which the blade is presented
to the abrasive.
- Parallel Hand Motion
This is the only technique I've ever
seen in old handtool texts. They describe it in a casual
"it comes with a little practise" manner. It simply
requires the ability to move your hands in a perfectly horizontal
plane. This is (actually) extremely difficult.
- Balance the Blade on the Bevel
In this approach, the bevel is placed on the abrasive,
and a good deal of pressure placed on it with the lower hand.
As long as the twisting force generated by the upper hand is
low, the bevel contact will define the angle for the blade.
This approach works well with large bevels, which are
a consequence of thick blades or small bevel angles.
- Linear Body Pivot (from your feet)
This technique relies on the idea that a 4' vertical beam moving
though a 8" circumference arc follows a circle which is close to a straight
line. The beam is your body, and the pivot is your feet.
- Radial Body Pivot
Place the stone parallel to the edge of your work surface,
and present the blade so that the edge is parallel to the stone;
consequently the blade is perpendicular to the stone, as viewed
from above. Now swivel your body, so that the edge moves along
its own length on the stone. Since there is no tendancy
for the supported end of the blade to move in the vertical
dimension, this gives very accurate control of the bevel. However
this technique does make it difficult to distribute the wear
over the stone's surface. As an aside (in the context
of a basic sharpening page), I find it works well
for out-cannel gouges.
- Use a Jig
Jigs give guarenteed bevel angles.
The downside is the time of fitting
the jig, and the jigs tend to inhibit your natural motion. When using
any jig that relies on projection to control the bevel angle, it's
worth making a simple gauge; this gives exact consistency of bevel
angle between sucessive sharpenings. This is particulary important
with single bevel sharpening.
For the purposes of this page I recommend
the cheap side-clamping
jigs, which (here in the UK) are referred to as "Eclipse Style" after
their best known (and now sadly defunct) manufacturer. These jigs will allow you to sharpen
the blades this page is about; other jigs are better (though
more complex and/or expensive) for more unusual blades.
Single or Double Bevel?
- Double Bevel
The traditional approach (those old books again!) is to hand
sharpen (technique 1) using a "double bevel". This involves using
coarse abrasives, or even a grinder, to establish a bevel
at around 5° lower than your desired edge bevel. This is called
the primary or grinding bevel.
Final honing, using fine abrasives is carried at the desired bevel angle.
This is called the secondary or honing bevel.
The difference in the angles means that (at least to start with)
the fine abrasives are working only at the tip of the primary bevel,
on a small surface resulting in relatively rapid progress. As time
passes, the secondary bevel gets larger, until the coarse abrasive
is once more used to reestablish a (nearly...) full length primary
Double bevel sharpening can be achieved with any of
of the honing techniques listed except for (2).
The secondary bevel is deliberately small, and there's no way
you could balance a blade on it.
- Single Bevel
This is easy to describe - the whole bevel is at one angle. This is
essential for technique (2); I would claim that technique (1) is almost
impossible to use for this, since to repeatedly work on a large
single bevel, the tool must be presented to the abrasive very
accurately. Even when using a jig, you probably won't get the bevel
"spot on" without using the recommended projection gauge.
Putting It All Together - a system that works
There are many systems that work. You can
find out about them later. In the meantime...
I reckon a Norton combination stone, a natural fine stone,
a side clamping jig and some oil can be snarfed
up at a decent car boot sale for around a fiver.
Knock up a simple projection gauge, and use single
bevel sharpening at the angles I recommended earlier.
The first time you do this on an existing blade
you'll spend a while on the coarser ("medium") side
of the combination stone. This is a once-off activity.
To re-sharpen a tool, all you do is:
Wipe off the oil, and you're done.
- Fit the jig
Place the tool in the jig, offer the jig-and-tool
up to the gauge, and tighten the jig so the tool
projects to give the desired angle (30° if you're
working on a bench plane blade).
- Abrade away the rounded (blunt) tool edge
Spread a little oil on the "fine" side of the combination stone.
Apply downwards pressure to the tip of the tool to with one hand
(the left, if you're right handed). The other hand provides
most of the driving force. It is important not to "lift" the upper
hand - try to let the jig give you information about the correct angle, as opposed to relying
on the jig physically imposing the correct angle.
Now just move the tool backwards and forwards on the stone,
working over the whole surface to avoid wearing the stone
in any one spot. Continue this until a burr or catch can
be felt on the back of the blade. This is easily detected
by running either your finger tip or finger nail over the edge. As long
as you haven't let the blade get too blunt, 15-20 strokes
should be enough.
- Polish the bevel
Switch to a finer (natural) stone. You should only need 8-15
strokes to refine the surface of the bevel. If you have a nice
super-fine natural stone, switch over to that, and repeat.
- Polish off the Burr
Now just turn the tool over and place the back of the blade
dead-flat on the finest stone. Rub up and down the stone
3-5 times. This should remove the burr, or at least move it
to the other face (the bevel). If the burr is still there, simply
alternate a couple of strokes on the bevel, a couple of strokes
on the back, until it comes away. (note - not all jigs allow
you do this with the tool still in the jig, which is one
of the reasons I recommend the side slamping style).