This article was published in the October 2013 (#45) edition of Woodturning Design.
Sliding Eccentric Sphere Chuck as a 7 page pdf
A version of the Sphere chuck that can be mounted on the headstock spindle or converted to a sliding mount.
INTRODUCTION:
The Variable Eccentric Cup Chuck Chucks that I wrote about in the last issue of
WTD are great for adding lines and arcs to the sides of a sphere.
But if you want to hollow the sphere or add nearly
circular or oval features,
you need a chuck that can hold the sphere by one end.
This is that chuck.
The Sliding Eccentric Sphere chuck is
a jam chuck combined with either tape or vacuum to hold the sphere.
It is by no means the first sphere chuck that I tried.
I first used modified PVC pipe fittings held in a 4-jawed chuck, then
with a double eccentric ring set up for variable eccentricity.
I tried a solid PVC chuck that was mostly a larger version of the
Variable Eccentric Cup Chuck. And
several others I won't bore you with.
Most of the worked, but they all shared a flaw, in that it was hard to
tell the orientation of the maximum eccentricity, or apogee, and this is
critical to getting the intended results.
A crooked smile means something entirely other than a centered smile.
The Sliding Eccentric Sphere Chuck has two rebates at the base and is
held by a 4-jawed chuck with two #2 jaws removed, hereafter called the lathe
chuck. A stop pin serves to locate
the axial position. As the chuck
slides along one axis of the chuck, it's obvious where the apogee is.
It's always on the same place on the chuck so you can draw a big black
line there if you like.
You can make the Sliding Eccentric
Sphere Chuck out of any material you can turn (that is, or can be made airtight
if you're using vacuum). I have used
solid PVC, maple, and glued up mdf, as shown in the lead photo.
As with the cup chucks, there is a compromise between range and mounting
security. The steeper the walls of
the chuck, the smaller the range of spheres it will hold, but the more securely
it will hold them. A Cut on my left
hand convinced me that 25 degrees is too wide−20
degrees seems to be about right. The
chuck is lined with 2mm craft foam.
The foam helps form an airtight seal between the chuck and the sphere even with
minor irregularities.
The pictures will follow making a
sphere chuck out of Maple sized for 2-3/4 to a little over 3" spheres.
This size will come in handy for the article in the next issue.
Starting with a square maple block, rebates are cut on the base.
The block is mounted between 2 jaws and a zero pin added.
The chuck is hollowed, sealed with CA glue, and lined with foam.
Then a flexible hose is added to bridge between the vacuum fitting on the
headstock and the chuck. The chuck
can be used with blue tape instead of or in supplement to vacuum.
PREPARING THE CHUCK BLANK:
Begin by looking at Drawing A, which shows the cross-sectional dimensions
of the chuck. If you wish to make a
different size, you can probably get good results using a turning square about
the size of the biggest sphere you want to hold.
If you would rather plan more precisely, download
www.DavidReedSmith.com/articles/SlidingEccentricSphereChuck/PlanAChuck.pdf from
my website, which shows a sequence you can use to calculate the size required.
Drawing A:
The cross-sectional plan of the chuck.
Cut a chuck blank to 3x3x2.5".
You can use whatever grain orientation fits the stock you have, but
spindle orientation is easier to turn.
Layout the rebates at the base of the chuck.
If you are making a different sized chuck you will have to allow for to
your jaw size and the amount of eccentricity required.
For big chucks consider switching to #3 tower jaws.
For this chuck the rebates should be just under 1/2" deep to allow for
maximum jaw contact without bottoming out in the jaws and 3/4" in from the sides
as shown in Fig01.
Fig01:
The chuck blank with the rebates laid out.
If you don't have a table saw you can
cut the rebates on a bandsaw. A
large blade is helpful for straight cuts.
Cut up from the bottom and in from the sides to form the rebates as in
Fig02. If you do have a table saw
you can cut the rebates more accurately.
I usually don''t bother to mount a dado set, but make multiple passes
using a sled. Set the height of the
blade to just under 1/2", and set the stop block 3/4" from the left side of the
blade. Make the defining cut with
the blank against the stop block and then make a series of cuts until the rebate
is completed as in Fig03. Figure04
shows blanks rebated on the bandsaw (left) and table saw (right).
Fig02:
Cutting rebates on the bandsaw.
Fig03:
Cutting rebates on a table saw sled.
Fig04:
Blanks with completed rebates.
Draw corner to corner lines to locate
the center of the top of the chuck blank as in Fig05.
Remove opposing jaws from your chuck and mount the chuck on the lathe as
in Fig06. If your chuck has a pin on
one jaw to limit jaw opening, leave that jaw mounted.
Mount the blank between the two jaws, using your tail stock at the
intersection of the diagonal lines to help center it as in Fig07.
Fig05:
Locating the center of the blank.
Fig06:
The chuck with two #2 jaws mounted.
Fig07:
Mounting the blank centered with the aid of the tailstock.
Cut the head off of a 12D (or so)
common nail and chuck it in a drill, then spin the nail into the rebate against
the edge of a jaw as in Fig08 to serve as zero stop.
Unchuck the nail from the drill, remove the blank from the chuck and trim
the nail so it protrudes about 3/8" as in Fig09.
Fig08:
Installing the zero stop with a drill.
Fig09:
After trimming the zero stop to length.
TURNING THE CHUCK:
Remount the blank in your chuck being careful to have the zero stop
against the side of a jaw. It's
prudent to pick one jaw to use consistently and even write the jaw number on the
blank. Begin by turning the blank
round, except at the back, as in Fig10.
If you're using wood in spindle orientation you can use a spindle
roughing gouge. For other materials
and face grain orientation, use a bowl gouge.
Turn the face of the blank true and mark a diameter of 2-7/8" as in
Fig11.
Fig10:
After turning the blank round.
Fig11:
After facing the blank and marking the maximum diameter of the recess.
The recess should have walls at an
angle of 20 degrees to the sides.
Rather than trying to measure the angle it's easier to drill a hole to a
calculated depth and turn to a line that joins the rims of the chuck and the
hole. Mount a 1-3/4" drill in a
tailstock mounted drill chuck and drill a hole 1.55" in depth as in Fig12.
The result is shown in Fig13.
Then turn the walls of the recess to form a straight line between the rims as in
Fig14. Regardless of material, this
is probably easiest with a small bowl gouge.
You may wish to tweak the straightness of the recess walls with a side
cutting scraper.
Fig12:
Drilling a 1-3/4" hole 1.55" deep.
Fig13:
After drilling a hole to mark the extent of the recess.
Fig14:
After turning the recess.
Now mount an O drill in your
tailstock drill chuck and drill a hole for the vacuum line through the bottom of
the blank as in Fig15.
Fig15:
Drilling a hole for the vacuum tubing.
SEALING AND LINING:
Unless you've turned the chuck out of PVC you'll have to seal the
chuck--wood is not nearly as air tight as it looks.
If you're going to use the chuck as a jam/tape chuck you can skip this
step. Three (thick) coats of varnish
will do, but usually one coat of medium CA glue will work, and is a lot quicker.
Sure CA glue is relatively expensive, but when's the last time you
actually used all the CA glue in a bottle before it went bad?
Put a nitrile glove on your dominant hand (you could put some blue tape
around your index finger instead, or use a hand held square of waxed paper).
Apply glue to a finger as in Fig16 and spread the glue on the chuck,
prioritizing coverage rather than appearance.
You may wish to turn your dust collector on to clear fumes.
Turn the lathe spindle by hand and apply until the accessible parts of
the chuck are well covered as in Fig17 and then allow the CA glue to cure.
Fig16:
Applying CA glue to seal the chuck.
Fig17:
After applying sealant.
After the CA cures on its own time,
reverse the blank in the 4-jawed chuck and apply CA glue to the back of the
blank to complete sealing the vacuum chuck.
Fig18:
After applying CA glue to seal the back of the sphere chuck.
To help create a vacuum seal between
the sphere chuck and a sphere, apply 2mm craft foam to the inside walls of the
sphere chuck. Do this even if you
don't intend to use vacuum. The foam
allows even a slightly irregular sphere to jam into the sphere chuck and
increases the friction between chuck and sphere to help hold it in place.
The size foam required can be calculated from measurements of the maximum
and minimum diameters of the recess and the length of the side walls.
You can measure the maximum diameter and side wall directly with a dial
or electronic caliper. It's easiest
to measure the minimum diameter with a pair of inside calipers.
Or, if your turning has been accurate to the rim of the drilled hole, you
can use 1.75" Fig19 shows the set-up
for measuring the recess.
Fig19:
Tools for measuring the dimensions of the recess.
Once you have the measurements you
can calculate the foam size required.
Call the maximum diameter D1, the minimum diameter D2, and the side
length S all in decimal inches. The
formula yields the measurements for the foam pattern given as maximum radius R1,
the minimum radius R2, and the angle A.
The formula is R1=(D1*S)/(D1-D2), R2=R1-S,
and A=(180*D1)/R1. If you
would rather not do the math yourself, you can download a spreadsheet from my
web site:
www.DavidReedSmith.com/Articles/SlidingEccentricSphereChuck/FoamCalc.xls.
Following DiagramB, construct a
pattern for the foam on paper. I
usually do this quickly with a CAD program but a compass, protractor and ruler
will work perfectly well. After
laying out the pattern, cut outside the lines and staple the pattern to a sheet
of 2mm craft foam as in Fig20.
DiagramB:
Plan for drawing the foam pattern.
Fig20:
The pattern applied to the foam.
Cut out the foam piece along the
lines of the pattern, extending one end by about 1/4" for overlap.
The staples will keep the pattern from slipping but won't compromise the
vacuum seal as they only lead to the sealed walls of the chuck.
Remove the pattern after cutting out the foam and then apply spray
adhesive to one side of the foam as shown in Fig21.
After allowing the adhesive to dry following the instructions on the can,
install the foam inside the chuck.
If the foam is only pressed gently against the walls or itself you can
reposition it. Form a loop by
overlapping the ends and position the foam in the recess opposite the overlap.
Then gradually gently press the foam into place. You'll have an overlap.
Use a utility knife to cut through both layers of foam in the middle of
the overlap. Remove the cut off
pieces and then press the foam firmly into place.
Fig22 shows the chuck after installing the foam.
Fig21:
Applying spray adhesive to the foam.
Fig22:
After installing the foam:
VACUUM BRIDGE:
To bridge the gap between the fitting and the chuck, where alignment
changes when the chuck is eccentric, use a piece of 3/16" ID clear vinyl tubing.
Cut a piece of tubing 4" long.
As is the tubing won't fit in the fitting (the vacuum fitting on my lathe
is based on the hollow 3/8" NF all-thread commonly use for lamps) but the tubing
is heat formable. So head for the
kitchen when the coast is clear and heat up a container with about 2" of water
to boiling. Stick one end of the
tubing in the water as in Fig23 and give it a minute or so to soften.
Then turn on the cold water at the sink.
Grab the cold end of the tube with one hand and the hot end of the tube
with pliers and pull hard. The warm
part of the tube will stretch out and narrow, leaving a tapered transition which
will seal in the fitting nicely.
Maintaining the pull, head for the sink and run cold water on the stretched part
to set it. Fig24 showed the tubing
after stretching. Measure the
outside diameter of the stretched part to make sure it will fit in the
fitting--if, say, you allow the boiling water to cool carrying it down to your
photo light box, it may not stretch enough.
If all is well, cut off the deformed area where the pliers gripped the
tubing.
Fig23:
Softening the tubing end in boiling water.
Fig24:
After stretching the the tubing to narrow and taper it.
Insert the tapered end of the tubing
to make sure it fits as in Fig25.
You can put your finger on the end of the tubing and check the vacuum seal if
your vacuum system has a gauge. Now
insert the tapered end of the tubing into the hole in the bottom of the chuck
and pull it through from the bottom as in Fig26.
Mount your chuck on the lathe and test to see if the tubing protrudes the
right amount to get a vacuum seal with the chuck in place.
If the tubing is too short to seal, pull it through a bit more.
If it protrudes so far that you can't easily mount the chuck than pull it
all the way through, and reinsert the tubing.
Once you have the proper position you can mark or measure the excess
protruding into the chuck recess, pull the tubing out and cut it to length and
reinsert it, as excess tubing may interfere with smaller spheres seating in the
chuck.
Fig25:
Testing the fit of the tubing.
Fig26:
After installing the tubing in the sphere chuck.
USING THE SPHERE CHUCK:
When using the sphere chuck, first make sure the top sides of the dado
are against the top surfaces of the two 4-jawed chuck jaws.
If you want it to be axial, turn the lathe on
before mounting a sphere to check and see if it's running true--the stop
pin isn't fool-proof. Avoid
mounting a sphere that is too big.
It may hold briefly, but if the walls aren't tangential to the sphere it is much
more likely to shift position or come off entirely.
You can use any smaller size sphere that doesn't bottom out in the chuck,
however you may not be able get access to as much of the sphere as you like.
To mount a sphere simply push it into
the chuck and turn on the vacuum and check the gage to be sure you've got an
adequate seal. You may need to
encourage it to seat by smacking it with your hand.
If you do not have a vacuum set up
you can substitute blue masking tape.
Really. It's more secure than
you think. As the chuck is a jam
chuck, the tape is only helping keep the sphere jammed in−it's not providing the only holding
power. In addition, as we remove
tape a little at a time by peeling it off, we tend to underestimate the holding
power of tape when multiplied by a large area.
Simply push the ball into the chuck and wind some blue masking tape
around the joint between the chuck and ball as in Fig27.
Go around a couple times if you like.
Then press the tape into the ball where it bridges between chuck and
ball. This is also a good supplement
to using vacuum if you have switch delexia and tend to sometimes turn off the
vacuum before the lathe. The major
drawback to using tape is that it takes more time to reposition the ball.
If you were trying to turn dimples like a golf ball it would probably
drive you crazy. But it's a good way
to try the system out to see if you like it without buying a vacuum set up
first.
Fig27:
Using the Sphere Chuck as a jam/tape chuck.
You can also use blue tape enable a
good vacuum hold when mounting it with already turned features in the chuck, or
when the ball itself is too porous to maintain a vacuum.
Using wide blue masking tape, tape over the area of the ball to be inside
the chuck. Don't worry overly about
the inevitable wrinkles, as the foam will accommodate them if you press the tape
tightly against the ball. Fig28
shows a ball with incised lines on the left, and mounted in the vacuum chuck
after covering the incised lines with tape.
Fig28:
Mounting an incised ball with the aid of tape.
If you want to hollow a sphere, or
want to turn a circular feature, mount the chuck axially, with the zero stop
against the jaw that you used when making the chuck.
Fig29 shows mounting the chuck with the zero stop against the jaw on the
left as indicated by the arrow, and the sphere spinning axially on the right.
Fig29:
Mounting a sphere axially.
To mount the chuck eccentrically,
simply loose the lathe chuck jaws slightly and slide the zero pin the desired
amount away from the jaw. You still
want the tops of the dados firmly against the tops of the lathe chuck jaws.
Fig30 shows the chuck mounted about 1/2" eccentrically on the left, as
indicated by the arrow, and the resulting ghost image on the right.
You can use this kind of mounting to turn tapered arcs, where the depth
is shallow at the beginning and end and deeper in the middle such as the
eyebrows in Fig31. You can also turn
features that appear elliptical, such as the eyes in Fig31.
Fig30:
Mounting a sphere eccentrically.
Fig31:
An example of what can be done with eccentric mounting.
It is important to correctly orient
the sphere with respect to the maximum eccentricity.
For instance a face looks happy if the elliptical axis of the eyes angle
up, and sad if they angle down. It
would look strange if the orientations were random instead.
This is one of the best features of using a sliding eccentric sphere
chuck--the maximum eccentricity is easy to determine.
When trying to lay out a feature, pencil in both the center of rotation
and the axis of maximum eccentricity.
You can then use your tailstock center to line up the center, and point
the axis along the direction the sphere chuck was slid in the lathe chuck.
For instance, going back to the eyes in Fig31, the tailstock pointed to
where the cross was carved in the eyeball, and the maximum eccentricity was
about 30 degrees down relative to the face.
I suggest playing with some practice balls to get a feeling for what you
can do and how to do it.
You can also use the sphere chuck to
hollow a hemisphere, as in Fig32.
Mount the sphere chuck axially in the lathe chuck.
Taping over the hemisphere is good insurance if the wood is porous to
avoid losing vacuum when you're almost done.
Mount the hemisphere in the chuck trying to get the rim evenly protruding
past the rim of the chuck. You can
use the tailstock ram, with the tailstock center removed, to aid in alignment if
you like. Hollow the hemisphere with
a bowl gouge. Compared to hollowing
through a small hole this goes astonishingly quickly as you are cutting, not
scraping, and you don't have to constantly stop to remove chips.
Then just glue the hemispheres back together and disguise the glue line.
Fig33 shows a tree ornament (to be featured in the next issue of WTD)
made of Baltic Birch plywood. Two
pairs of three segments were glued together, flattened, and then the pairs
assembled with a temporary joint and turned to a sphere.
The temporary joint was split and both resulting hemispheres hollowed and
then glued back together for sanding and finishing.
Fig32:
After hollowing a hemisphere.
Fig33:
A Christmas ornament hollowed as two hemispheres.
You can make the equivalent of eccentric cup chucks using the same sliding technique as for the sliding eccentric sphere chuck. Start by making the same rebate with a zero stop, mount it in the zero position with 2 jaws of the 4-jawed chuck and turn the size recess you desire. For the tailstock end, thread a wood block to fit the threads on a Oneway pattern tailstock center and turn it to a flat pad. Add a couple of layers of 2mm craft foam to the end of the flat.
Sliding Cup Chuck.
Sliding Cup chuck at zero or axial position.
Sliding Cup Center in eccentric position. The pad doesn't need to be that big.
TOOLS:
Bandsaw or Table saw
4-jawed chuck with #2 jaws
portable drill
Drill chuck
1-3/4" drill bit
O drill bit
Calipers
Utility knife
Pliers
MATERIALS:
Maple block 3x3x2.5"
12D Common nail
Medium CA glue
Nitrile glove
2mm Craft Foam (look in the craft
section at Wal*Mart or any craft store)
3M #77 spray adhesive
3/16" ID clear vinyl tubing (plumbing
section any hardware store)
Original blue masking tape
AUTHOR:
David Reed Smith is a basement woodturner and tinkerer living in Hampstead,
Maryland. By the time you read this
he will be on Medicare and hence retired.
He welcomes comments and questions via email at
david@DavidReedSmith.com.
This article, along with around 60 others will be available on his web site:
www.DavidReedSmith.com