Sharpening Woodturning Chisels
by Jon Siegel
This article first appeared in The Old Saw, the journal of the Guild of New Hampshire Woodworkers (www.gnhw.org) September 2005.
Part 1 – Tool Point Geometry
Woodturning chisels are subjected to more wear and tear than
any other kind of hand held chisels used in woodworking. Because
the wood is passing the chisel at 20 miles per hour for prolonged periods
of time, a turning chisel will traverse more wood in a few minutes than
any carving chisel could in years. Just look at the pile of chips
that accumulate around a wood lathe and you will be convinced of this fact. For
this reason, sharpening is a more important issue for turning than any
other branch of woodworking.
Sharpening is not handled well in most of the turning books I have read,
because the authors try to cover too much at once. I will divide
the subject into two distinct parts: Tool Point Geometry, which will
be covered here, and Sharpening, which will be in the next issue.
Tool Point Geometry describes the shape of the tool. To understand
this topic thoroughly, we need to explore why the tool is shaped the way
it is, and how the shape affects the way it behaves.
Sharpening is the routine maintenance of the perfect edge. This
operation assumes the correct geometry has previously been achieved, and
we aim to keep it that way as we sharpen the chisel hundreds or even thousands
of times.
Basics of Tool Point Geometry
An edge is formed at the intersection of two planes – these planes
may be flat, curved or a complex conical surface. The angle of intersection
is called the dihedral angle, usually referred to as the edge angle. It
is measured as the angle between two lines which are perpendicular to the
edge at their point of intersection and lie on the respective planes.
Looking at the chart (Fig. 1), you see two main groups of woodturning chisels — cutting
tools and scrapers. Cutting tools are divided into two main groups — gouges
and chisels. As we discuss these types, we will examine two aspects
of tool point geometry — the edge angle and the shape of the line
which forms the edge.
The best edge angle for woodturning chisels is 35 to 40 degrees (Fig. 2)
although turners differ greatly in working practices. I have seen
angles between 25 and 55 degrees used. Many people think that since
their cabinet chisels and carving chisels are ground at 25 to 30 degrees,
that this must be good for turning chisels also. However turning
chisels are subjected to much more brutal conditions than hand chisels. For
this reason, a more blunt angle is preferred because it produces an edge
which is more robust, stays sharp longer, and is less likely to overheat.
Also, unlike carving tools, turning chisels are usually used at a shear
angle. This means that the edge is not perpendicular to the direction
of travel. This produces an effective cutting angle which is much
less than the measured edge angle. To understand this concept, think
about a switch-back road which ascends up a mountain. The effective
angle of climb is reduced because the road does not go straight up the
mountain, but angles across the face of the slope. So when we use
a turning chisel at a shear angle, the actual cutting angle of the edge
is much less than the angle at which we ground the bevel on the chisel. For
these reasons, I do not think it is necessary to grind turning chisels
to a more acute angle than 35 to 40 degrees.
But decisions of tool point geometry should be based on how they affect
the behavior of the chisel in actual use. I have found that chisels
which are too acute are harder to control than those ground at about 40
degrees. Some top woodturners, teachers and writers, such as Mike
Darlow disagree with this and use more acute angles.
Flat Grind Versus Hollow Grind
I am absolutely convinced that wood turning chisels should not be hollow
ground — this does not apply to scrapers which can be either flat
or hollow. I learned this from trial and error, and decades of
experimentation. When I switched to flat grinding, woodturning
suddenly became much easier. The reason is because the flat grind
defines the bevel as a straight line — at least in the longitudinal
plane. This allows better control of the chisel. If you want your
chisel to go in a straight line, you need a straight bevel. If
the bevel is curved in a hollow shape (concave), then the chisel is always
trying to follow this curve, and it is a struggle to keep it going straight.
Nose Radius of Gouges
No factor of tool point geometry affects the behavior of a gouge more than
nose radius because a smaller radius produces a narrower chip and results
in a smaller cutting force. Figure 3 shows five different shapes possible
from the same chisel profile (or cross section). These differ
only in their nose radius. The first example is a “straight
across” grind (radius = infinity). This is the way
we grind a roughing gouge for spindle work; it has no nose and has sharp
outside corners. Until recently this is the way most English turners
ground their bowl gouges too. (See Frank Paine – The Practical
Woodturner, or Peter Child, – The Craftsman Woodturner) The
remaining four examples in the illustration show a decreasing nose radius
from about one inch down to 1/16 inch. 
Besides the straight across gouge already mentioned for roughing and working
into corners, each of these variations has certain strengths and weakness
for applications of various kinds. For example, the smaller nose
radius allows for easier piercing of the surface for initiating cuts (coves)
at very high vertical angles, while the wider nose chisels generally make
it easier to produce a smooth cut on flat surfaces or the bottom of coves.
During sharpening it is important to maintain the nose radius to the shape
you prefer. Note that there are two distinct types of gouges — those
forged from a flat piece of steel which has be bent into the “U” shape
common to all gouges, and those created by milling a groove into a piece
of round steel (Fig. 4). The milled gouge is the modern convention. Gouges
made from round steel present a cross section which is thicker in the middle
than at the edges. Because there is more material in the middle,
this part resists the grinding process more. As a result there is
a tendency for these chisels to become more pointed (toward a smaller nose
radius) as they are sharpened. You must be conscious of this, and
dwell on the grinder longer in the middle to counteract this tendency,
and thus maintain the correct nose radius.
Scrapers
Scrapers are used in bowl turning, chuck, and faceplate work, where the
grain of the wood is perpendicular to the axis. In this kind of work,
the grain is constantly changing from end grain, to cross grain, to long
grain, and a scraper is well suited to the task. Scrapers are almost
never used in spindle turning.
A scraper does not stay sharp very long even under the best conditions,
but this disadvantage is outweighed by its ability to handle variable grain
direction without lifting or tearing-out the grain.
A gouge (cutting tool) is the best choice for roughing because it has positive
rake, which removes wood more quickly with less friction, and the tool
stays sharp longer. (Figure) However there are many situations where scrapers
are necessary — smoothing and blending curves, fine details, and
awkward situations where a gouge or other cutting tool cannot be presented
at the correct angle.
A scraper is used in a very different manner than a cutting tool, because
the bevel of a scraper does not rub the work. Instead a scraper
is controlled by pressure only, and the handle is held higher than the
edge, giving the tool a negative rake. It is this negative rake and
the ability of the scraper to take microscopically fine shavings that reduces
tear-out and makes it the best finishing tool for difficult varying grain
situations.
Because the bevel of a scraper does not rub, the exact angle or shape of
the grind (flat or hollow) does not make much difference. Usually,
scrapers are ground at a very blunt angle, corresponding to an included
angle of 70 to 80 degrees. When the tool is used with a negative
rake angle of 10 degrees, then the resulting clearance angle is 20 to 30
degrees.
Gouges — Axial vs. Oblique Grind
There has been a revolution in the way most turners sharpen their bowl
gouges in the last 15 years. To some degree this has changed for
spindle gouges too. The new geometry is usually referred to as
side grind, and is achieved by swinging the handle from side to side
during grinding, instead of simply rotating it. What this produces
is a bevel surface which is not referenced to the axis of the tool, but
oblique to it. The obvious effect of this is that the gouge has
longer edges on the sides (or wings) than a conventional grind (Fig 7). 
These long side edges are useful in many bowl turning situations, especially
reaching into end grain boxes or goblets where the cut must be made in
a pulling motion (working toward the mouth).
The disadvantage of side grind is that the angle of the bevel changes whenever
you rotate the tool. This means that if you rotate the gouge during
the cut (to alter the shear angle), the gouge reacts by changing its direction. On
a conventionally ground gouge, the angle of the bevel is constant with
respect to the handle, and you can rotate the gouge any way you like during
the cut (to alter the shear angle) without changing its direction of cut. Also,
it is much easier to start a cut when you know that its angle is not dependent
on the rotation of the tool.
Skew Chisels
Skew chisels are part of a whole class of spindle turning chisels which
are ground on both sides. Most skews are made from rectangular
stock, and the corners are extremely sharp (Fig 8-A). The chisel cannot
successfully be used in this condition, because the corners dig into
the tool rest (not to mention your fingers). This will prevent
the tool from sliding along the tool rest smoothly, and eventually will
damage the surface of the tool rest.
Such a skew chisel can be put right by rounding over the corners (Fig 8-B).
A new type of skew is now available which has its short sides completely
rounded over; so the cross section takes the shape of a race track (Fig
8-C). These are called the “rolled edge” skews, and I highly
recommend them. Also, some of the new skew chisels are made from round
stock (Fig 8-D), and these are excellent in the small sizes. Avoid
the “oval skew” (Fig 8-E) with the long sides rounded.
The shape of the edge of the chisel can vary. It may be a straight
line which is square across (Fig 9-P) or slightly rounded (Fig 9-Q).
The standard skew (Fig 9-R) has a straight edge which is ground to an angle — usually
20 to 40 degrees. This is an extremely versatile tool which offers
the choice of using the heel or the toe in different situations. 
The edge may be a curve (Fig 9-S) which creates a narrower chip and behaves
more like a gouge since it has a nose radius. Note however that a skew
which is shaped to a curve does not have a distinct sharp point (the toe
of the skew) and this is a limitation.
The Myth of the Original Grind
Many beginners think that they should maintain the geometry that the tool
had when they bought it. While this may be true of the Ellsworth
signature gouge and a few others, in general this is a dangerous assumption. The
person who ground the tool at the factory probably never turned a piece
of wood in his life. He is just grinding it the quickest possible
way so it looks like it has been sharpened; and this is OK because it
gives you a head start in regrinding the tool the way you want it.
With all the chisels available and all the possible ways to grind them,
there are too many variations for anyone to master in one lifetime. So
I will leave you with this final thought — Take time to experiment. If
you do, you will soon realize that this article has barely scratched the
surface of this subject.
