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The quality and ease of technique of woodturning can be enhanced if we
understand more of the nature and characteristics of the wood we turn. Mike
listed several things we should know about wood before undertaking a turning:
1) How the tree grows; 2) The wood's qualities; 3) The shape we want to turn;
4) How we want to use it. These points are not independent, but are
largely interactive.
The growth process
The basic architecture of the tree is established quite early in the growth
process as the shooter branches to form limbs.
All trees begin as a single sprout, or "shooter." See darker center of tree
in first illustration: first growth.
Most of the life of a tree is spent in adding bulk as successive concentric
annual rings form. See successive lines added around shooter in first
illustration; see also cross section in third illustration. The original
shooter becomes the pith, the darker center of the tree.
The physiology of the tree consists primarily of vertical tube-like fibers that
ressemble tiny straws and are called vessels, and bundles of horizontal fibers
called rays that grow from the pith radially toward the bark.
The vertical vessels carry the fluids and nutrients from the root system upward
into the tree. Some believe that the horizontal rays provide a means of
conveying oxygen into the tree. Both the vessels and the rays contribute to the
grain patterns and characteristics of the wood and can be quite variable from
one species to another.
In general, more dense woods have very narrow, tight vessels whereas less dense
woods have more open or wider vessels. This causes dense woods to be less
susceptible to grain tear-out.
Anatomy of a tree
The tree forms four zones in its cross section.
The zone surrounding the pith is the heartwood.
The zone surrounding the heartwood is the sapwood.
The outermost zone is the bark and the thin zone between the bark and the
sapwood is the cambium layer.
The heartwood is essentially the aged, dead wood of the tree's early years,
originally the shooter, and usually differs in color to varying degrees from
the surrounding sapwood.
The cambium layer is an active growth zone that contains much moisture. During
the spring and summer it is moisture-laden and swollen. During the winter it
is dormant. Hence, trees harvested in the winter will have a thinner and firmer
cambium layer than those harvested in the spring or summer and will be much
more likely to hold their bark tightly.
Shrinkage and warping
As wood dries the vessels (like straws) shrink in diameter. Hence wood will
shrink in the radial dimension faster and more than in the axial dimension.
See fourth-seventh illustrations of various orientations of bowls:
- figure 4 incorporates a natural edge or bark
- figures 5 and 6 demonstrate considerable warping of a walnut bowl
turned green and allowed to dry—it is nearly 2" wider along one
diameter than the other
- figure 7 shows a bowl with a calabash bottom incorporating heartwood,
which is heavier and causes the bowl to sit at an angleq
Quarter-sawn wood warps much more uniformly than plane-sawn wood when drying.
See second illustration of blanks all cut "quarter-sawn" from a billet cut
from a trunk.
Figuring
If a tree grows so that the vertical vessels remain straight, the wood produced
will be straight grained. Nevertheless, many things can cause the vessels to
become crooked or almost tangled resulting in interesting grain figuring such
as fiddle-back or quilted patterns. These figures are most likely to occur in
areas where the wood has grown under stress from the weight of the supported
tree, from wind movement or some other adversity. Obvious areas of higher
stress are at the base of the tree where the greatest weight is borne, areas
below major limbs where both weight stress and movement are likely and in
crotches where two limbs grow from. See first illustration—detailing
areas of stree on a tree.
Fiddle-back figuring is best revealed in quarter sawn wood.
The orientation of a blank within a log will affect how the natural growth-ring
patterns appear in a turned object. If a bowl is chosen oriented with the top
of the bowl near the pith center and centered on the pith, the annual rings
will form an "H" pattern in the interior of the bowl. However, if the bowl is
oriented with the bottom near the pith center and centered on the pith, the
annual rings will form concentric ovals in the interior of the bowl. Other
variations will result if the piece is not evenly balanced on the pith center
of if the blank is quarter sawn. See second illustration.
Obviously, the attractiveness of a turned piece will be greatly influenced by
choosing an orientation that best compliments the shape of the piece.
Burls
Burls form on the outside of a tree for a variety of reasons. However, they
generally represent some abnormality the tree is trying to reject. They
typically have confused grain structure that can create very attractive
figuring. While burls are often regarded as being without grain direction, they
do in fact grow concentrically as does the rest of the tree. This often results
in a preferred direction of cutting to produce clean cuts without tear-out.
Techniques: avoiding tear-out
It is almost always true that cutting toward supported fibers will generate the
cleanest cut. This is because increasingly long fibers, "standing up higher" in
a sense behind the ones you're cutting, support these and are in turn supported by the ones behind them, etc. and resist tear-out.
Other factors that influence cleanness of cut are tool sharpness, lathe speed,
and shearing angle of the cutting edge.
Mike demonstrated several types of cuts to contrast the quality of cut. He
generally classified them as scraping, shearing or peeling, and bevel-riding
cuts. Each can be useful in the appropriate circumstance, but generally the
bevel-riding cut will provide the cleanest cut and should be used whenever
possible.
A traditionally ground gouge will often give a cleaner cut
than a fingernail ground gouge since it presents a more acute shearing angle
where the cutting edge meets the direction of rotation of the turning wood.
However, the traditionally ground tool must be used very carefully to avoid
catches with the corner of the edge and it is more difficult to control through
a curved cut. Grinding off the heel of the bevel to shorten the length of the
bevel is often helpful to help get continuous curves on a concave profile. A
smaller diameter gouge may be used to increase control as well.
Turning a square-edge bowl
Mike started this demonstration with an ambrosia maple blank about
6"×6"×2". The tools he used included
- ¼ inch bowl gouge
- 3/8 inch bowl gouge with fingernail grind
- 3/8 inch bowl gouge with traditional grind
For a bowl of this type it is important that the blank be perfectly square and
that the center be carefully located in order to maintain symmetrical corners.
Mike began by drilling a carefully centered hole in the top of the blank and
mounted it on the lathe with a screw chuck. The tail stock was engaged with a
small cup center as a safety precaution. With the 3/8 inch bowl gouge, light
shearing (pull) cuts were used to remove wood from the outer edge (corners) of
the bottom of the bowl.
The corners represent hit-and-miss turning since the tool is contacting wood
during only part of the rotation and air the rest of the time. A high lathe
speed is helpful to minimize the hit and miss effect. It is important to take
light cuts to avoid tear out at the unsupported edges in this area. It is wise
to stop the lathe frequently and check for developing problem areas. While
there is still plenty of wood and rigidity you have the opportunity to test
different cutting methods to achieve the cleanest cut possible.
It can be helpful to put a sheet of white paper down on the lathe bed when
turning darker woods or a sheet of black paper when turning lighter woods. This
is helpful because it's very hard to see the corners of the bowl as it's
turning.
As the desired profile for the bottom of the bowl develops, form a tenon on the
bottom to suit the chuck that will be used in the next mounting. Once the tenon
is formed, remove the tail stock and refine the bottom profile using
bevel-riding cuts. The final cuts should be made with the lathe turning at a
high but safe speed using the sharp ¼ inch bowl gouge to achieve the
cleanest cut possible.
Remove the piece from the screw chuck and remount it in a four-jaw chuck using
the tenon formed on the bottom of the piece. (Engage and maintain the tailstock
as long as practical for safety.)
Begin shaping the upper surface of the bowl with the 3/8 inch bowl gouge
cutting from the outside toward the center so you are cutting supported fibers.
With light cuts continue cutting from the corners well into the solid-wood zone
of the bowl until the desired wall thickness is achieved.
Once the corner zone is finished, begin removing wood from the interior of the
bowl. With the tailstock still in place this wood removal is best done with a
series of shearing cuts parallel to the lathe axis so you are cutting face
grain rather than end grain.
When the bulk of the wood has been removed, use clean bevel riding cuts to
smooth the inner profile of the bowl. When the interior profile is smooth use
the traditionally ground bowl gouge to achieve uniform wall thickness from the
previously completed corner zone into the bottom of the bowl.
Now remove the tailstock, cut away the center stub where the cup center had
been and finish tooling the bottom with the traditionally ground bowl gouge.
Power-sand the interior surface with a two inch sanding disk through the
progressive grits. Always start sanding one grit coarser that you think you
need. Sand the corner zone with the lathe stopped and working from the center
toward the edges.
Remove the piece and reverse chuck it using a friction drive.
Mike mounted a short cylinder in the four-jaw chuck and turned its edges round
with a concave center so it would contact the inner surface of the bowl only on
its edges, not in its center. He then moistened a piece of paper towel, placed
it between the friction drive and the bowl interior and applied pressure with
the tailstock cup center. Moistening the towel increases the friction between
the bowl and the wooden friction pad. The moisture will raise the grain of the
bowl interior somewhat, but that is easily sanded out by hand later. Mike then
used shear scraping cuts to blend the dove-tail tenon into the base of the bowl
to form a gentle foot. He used bevel cuts to undercut the base of the tenon
slightly.
Power-sand the bottom profile. The corner areas should be sanded with the lathe
stopped and working from center toward the edges with a 2" sanding disk through
the successive grades.
Remove the completed bowl from the lathe. The remaining nub under the cup
center can be removed with a Dremel tool or sanding disk.
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