Starting Comments
The description "Crescent Platters" denotes platters on which one or more
crescent-shaped decorative grooves are cut in the rim. The procedure involves
turning a conventional platter with a smooth upper surface on the rim. The axis
of the platter is then changed by inserting a shim between the face of one
chuck jaw and the base of the recessed chucking dovetail. This results in an
induced wobble to the upper surface of the rim so that a tool presented to that
face will cut only at the high point of the wobble. Deepening the cut will
cause the groove to widen and extend further around the arc of the rim creating
a crescent shape to the groove.
Design Considerations
Before beginning the platter several decisions must be made: How wide should
the foot be? What diameter should the chucking recess be? How deep should the
chucking recess be? How thick a shim should be used? How should the chucking
recess be shaped?
The first step in turning a platter is to mount the blank on the lathe with a
screw chuck or face plate attached to the upper face of the blank. The bottom
of the platter is then turned to create a chucking recess that later will allow
mounting on a scroll chuck to turn the upper surface of the platter. The
diameter of the chucking recess will depend on the size of the platter and the
size of chuck jaws available to the turner. A typical diameter of the foot on
which the platter will rest is about 50% of the finished diameter of the
platter. In this demo Dale used a 10" blank of Oregon myrtle (not maple as
stated in the demo) so he chose a foot diameter of 5". His chuck jaws close to
just under 4" O.D. so he chose a target diameter of 4" for the chucking recess.
The depth of the recess must be enough to allow the chuck jaws to maintain a
grip around the entire circumference of the recess after a shim is inserted
under one jaw. Dale was using a shim about 3/32" thick so he chose to make a
¼" deep recess. Since the shim is placed about ½ the distance to
the rim, the 3/32" offset at the shim will result in about 3/16 inch wobble at
the rim. The thickness of the shim can be chosen to give either larger wobble
(thicker shim) or smaller wobble (thinner shim) depending on the depth of
crescent cut the turner desires to make in the rim. The depth of cut will also
affect the length of the arc. It is usually wise to leave the interior of the
recess domed to allow jus a little extra thickness in the bottom to prevent
piercing through the bottom when shaping the interior of the platter later.
However, the surface immediately adjacent to the dove-tail wall of the recess
should be flat to allow good registration between the flat face of the chuck
jaws and the wood without effectively sacrificing some of the recess depth.
Turning the Platter
Mount the blank with a screw chuck or faceplate attached to the upper face of
the blank. Face off the exposed face of the blank with a ½" bowl gouge to
make it run true and to remove any saw marks. Mark the foot and recess
diameters with a pencil. Dale prefers to make some preliminary cuts with a
gouge to define those marks so they won’t disappear if the pencil lines are
eroded. Use a bowl gouge or spindle gouge to cut the chucking recess to the
desired depth and shape. A ½" skew can be used as a scraper to create the
desired dove-tail angle in the wall of the recess and to create the flat zone
at the base of the dovetail. Holding the skew flat against the tool rest but in
a negative rake position should give a pretty clean face to that flat zone. The
3/8" spindle gouge can be used to refine the shape and finish of the dome
in the center of the recess. Ensure that the high point at the center of the
dome is still lower than the edge of the foot on which the platter will
eventually rest. The spindle gouge can also be used to undercut the face of the
foot to ensure that the platter will rest only on the outside edge of the foot.
Make that undercut very slight to avoid sacrificing depth of the recess. Now
face off the upper face of the blank until it runs true so you know where the
true edge of the platter will be. You may also want to true up the
circumference of the blank so it runs true, but this can also be done while
shaping the bottom profile.
Use a bowl gouge to shape the profile of the bottom of the platter. Bear in
mind that the foot should be as high as the recess is deep to minimize risk of
piercing through the bottom when shaping the upper part of the platter. Also be
aware of the edge thickness of the rim. If it is made too thin you will be very
limited in the depth of cut you can make later when creating the decorative
crescent grooves. Dale chose to use a simple ogee profile for the bottom, but
more elaborate profiles including steps, grooves, or beads are possible. Once
the upper profile is turned there will be no convenient way to re-grip the
piece to work on the bottom face again. When the desired bottom profile is
established, sand to finish using a power sander with a two-inch sanding disk.
The idea here is to completely finish the bottom, especially the recess and
foot, prior to reversing the chucking to turn the upper face profile.
Remove the piece from the screw chuck and remount it on a scroll chuck using an
expansion grip inside the dove-tail recess in the base. Take care to mount the
piece squarely on the chuck. As with all re-chucking, you might introduce a
slight misalignment wobble due to very slight differences between the axis of
the chucking. Slight wobble can be removed with gentle shear-scraping cuts then
followed by re-sanding. Now turn the upper face of the rim to produce a smooth
surface. It could be slightly domed or slanted, but must not have beads. Dale
chose to create a flat rim to facilitate sanding later. Make clean shearing
cuts to avoid grain tear out. Sand this area to reasonable smoothness (150-180
grit) to create a uniform working surface and minimize open grain areas that
could become paint traps later. It will probably be helpful to remove some of
the waste wood from the interior of the platter adjacent to the rim to provide
some room for tooling and sanding, but don’t remove enough to cause flexibility
in the rim or create new wobble due to relieving internal stresses in the wood.
The piece is now ready for cutting the crescent grooves.
Cutting the Crescent Grooves
Remove the piece from the chuck. Rotate the spindle so the #1 jaw of the chuck
is at the top. Place a shim of the desired thickness (3/32” in this case)
between the flat face of the #3 jaw and the flat registration area inside the
chucking recess. As suggested by several in the audience, this is much easier
if the shim is taped to the jaw to hold its position. A piece of masking tape
across the face of the shim and into the interior of the jaw works nicely
forming a sort of hinge so the shim can drape down over the face of the jaw
without conflicting with the dovetail. A longer shim is better than a shorter
one since it makes the task of mounting the piece easier. Determine where on
the rim you want to make your grooved cuts. The cleanest cuts can probably be
made if the apex of the crescent is tangent to the grain direction.
Carefully mount the piece on the shimmed chuck so the apex of the crescent is
directly below the shim (i.e., at the center of jaw #3. Carefully apply
pressure to the piece to ensure it rests squarely against the shim with no
sideways canting. The vertical axis will be canted by the shim, but the
horizontal axis should be perpendicular to the lathe ways. Any sideways cant
will cause the apex of the crescent to be misplaced. Tighten the chuck by
gently but securely expanding the jaws outward to grip the dovetail walls of
the recess. You can test placement of the proposed crescent by turning the
lathe on and holding a pencil against the face of the platter rim. The pencil
will mark the high point of the wobble and show where the apex of the crescent
will occur. If the mark is not where you want it to be, rotate the spindle so
jaw #1 is again at the top, loosen slightly the expansion grip and carefully
rotate the piece on the jaws by the amount needed to correct the discrepancy.
Recheck the same way and repeat until the new pencil mark is where you want the
crescent to be.
The crescent groove can be cut several ways. 1) Use a skew held flat on the
tool rest and make scraping cuts into each side of the crescent. If the freshly
sharpened tool is held in a negative rake attitude it can provide reasonably
clean cuts. 2) Use a skew held vertically on the tool rest to shear successive
straight sided “V” cuts. 3) Use a 3/8" spindle gouge to make shallow cove
cuts from each side of the crescent. It is recommended that you experiment with
these techniques on a waste block to decide which you want to use. Dale found
that the skew cuts were impossible to sand and did not produce as smooth a cut
as he desired. The cove cuts were better and could be sanded more effectively.
It is also helpful to choose closed grain woods that take detail well. Maple is
obviously preferable over elm, for instance.
When making the actual cuts on the platter, stop frequently to inspect the
groove for cleanness of cut, depth and extent of arc. Continue the cuts until
the desired effect is achieved. If a series of cuts at different radial
distances on the same side of the rim are desired they can be made successively
without changing the shim position. However, if cuts are desired on the
opposite side of the rim the platter must be rotated 180° in the chuck
relative to the shim position.
When the crescent grooves are all completed the platter can be removed from the
lathe and the grooves carefully hand sanded to improve the quality of the
surface as necessary.
Finishing the Platter
The platter can now be remounted squarely on the scroll chuck with the shim
removed so the piece again spins true on the original axis. At this point you
can begin removing the remainder of the waste wood from the inside of the
platter with a bowl gouge. It is generally preferable to create an interior
profile that drops abruptly from the inside edge of the rim and then
establishes a curve that parallels the outside profile with an appropriate wall
thickness. In fact, if the abrupt drop from the rim is slightly undercut it
will create a shadow line that distinctly separates the rim from the interior
and is esthetically pleasing. In this demo Dale chose to further accentuate
that rim definition by cutting a narrow and shallow (1/16" x 1/16") square
groove in the edge of the rim with the ½" skew used as a negative rake
scraper.
The interior profile should be refined to create a smooth flowing curve to the
center of the platter taking care to monitor the depth of the profile to avoid
piercing the bottom. Dale used a 3/8" bowl gouge for these delicate finishing
cuts. He used a ½" bowl gouge with a traditional grind for the finish
cuts at the very center of the platter.
The finished interior can now be sanded to finish. Dale normally power sands to
320 grit and then hand sands to 400 and 600 grit if the wood chosen
accommodates that fineness.
The platter can now be sealed with a lacquer-based sanding sealer or straight
lacquer thinned for improved penetration. Dale uses the common ratio of 3 parts
lacquer (Deft) to one part thinner. The basic purpose of this sealing step is
to ensure the surfaces of the crescent grooves and the adjacent rim areas are
well sealed so the paint to be applied in the next step will not penetrate into
end grain or open grain pores. When the sealer is dry, the inside of the
crescent grooves can be painted with any pigmented paint desired using a fine
artist’s brush. Dale chose to use an inexpensive acrylic paint available in
craft supply stores in 2 oz. bottles. The paint should be applied fairly
liberally so it covers the edges of the crescent cuts completely. This will
unavoidably slop over onto the rim surface to some degree.
When the paint is thoroughly dry remount the platter squarely on the scroll
chuck. Hopefully the platter will spin true, but if not do not attempt to
re-turn any surfaces. Just live with the slight wobble. This is merely a
sanding step to remove the excess paint from the face of the rim with 2"
sanding discs. Turn the lathe speed down low enough that the sanding disc will
not bounce on the rim but will remain in smooth contact throughout the
rotation. Dale began sanding with 120 grit. If the sanding disc blinds with
paint and/or sealer you might want to drop back to 80 grit. It is also
possible to clean the disc with a stiff nylon or brass brush. Sand until all
the unwanted spill-over paint is removed, then progress through successively
finer grits to your finish grade. The end result should be sharp-edged crescent
grooves that stand out smartly from the adjacent rim area.
Clean the sanding dust out of the crescent grooves with a soft brush and/or
tack cloth. Reseal the freshly sanded area and let it dry. Finally, overspray
the entire platter with two or three coats of spray lacquer. A final tip: If
you are going to burn identification into the bottom of the piece that is best
done before finish is applied to that surface.
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