Large Wood Sphere

[Mike Johnson]

My next tool/mold order is an 18" diameter wood sphere.
I am considering making this item as a segmented piece to reduce the amount of wood chips.
I will also be making the item hollow to reduce the weight of the item.
These larger spheres I usually turn outboard and use a mating joint to marry the two halves together.
I will post an image when I get it completed.

 

[Pete Blair]

Looking forward to some photos. Please post some in progress ones!

 

[robo hippy]

Sounds like the proper way to do it. I would turn two hemispheres, then glue them together, then finish turn. I guess if you really wanted to, you could make a 'tongue and groove' type joint, but I would think flat joint would work. Not sure if the volume of air inside would need 'relief' for changes in barometric pressure or not. Maybe if you went from sea level to 20,000 feet...

robo hippy

 

[Owen Lowe]

Segmenting or stave-ing should also alleviate warping, cracking, and large-scale seasonal movement.

Is this a pattern for mold-making? (just curious)

 

[Mike Johnson]

I don't have the large lathe to use, but will assemble and turn in stages to keep things manageable.
As you increase items in size the challenges increase exponentially.
A sphere has so much surface area which creates the biggest challenge, more time sharpening tools,
and more time with the tools getting things turned to proper diameter and shape. Over shoot on your
shaping and cutting of the wood and you start over with that stack of segments. I should make a larger
tool to cut this size of sphere, but I am working outboard so it would be more complicated to fabricate a
tool that would work on the lathe turning a piece outboard. This is a one off piece so I will be doing this
by hand with conventional lathe turning tools. My shoulder and elbow is starting to hurt already.

 

[Mike Johnson]

Segmenting or stave-ing should also alleviate warping, cracking, and large-scale seasonal movement.

Is this a pattern for mold-making? (just curious)

Owen,
I make molds and tools for the Pyrotechnic industry, this mold will be used to make hemi shell casings for
large diameter fireworks ball shells.

 

[hockenbery]

That seems like a good plan. Turn two hemispherical bowls and glue them together.
Segmented rings would work but may not be necessary.

Depending on the application the rings could be cut from glued up strips of wood or sheet material like plywood, mdf.... only need to turn the outside. mdf is really dusty but it is really cheap!

Using cut circles
The largest diameter ring can be screwed to a plywood disc on a faceplate and glue all the rings together turn the outside unscrew and glue to the other half.

3" ring cut from PVC pipe is a quick gauge for roundness on a ball this size. The ring will sit flat on a sphere. Turn where the ring is being held up.

Al

 

[Mike Johnson]

Al,

I have made a number of these and some of these I make hollow so the different sized spheres
fit inside each other, takes up less space for the guys in that hobby. I have made several sets that
were all shipped out in one box to the customer, Saves quite a bit on shipping that way and they
like that the set of "tools" fit inside the largest sphere. Some of these guys take great pride in the tools
that they use and have them displayed in their shops on the wall. The segment rings cut down on the
amount of wood shavings that end up on the floor. I have made some larger solid wood spheres and you
end up with a pile of shavings on the floor when the item is complete. I usually start with a laminated square
billet and trim the corners off of the billet on the table saw and finish turning between centers. Even doing this
you end up with a lot of wood shavings to end up with the perfect sphere. I have a sphere jig for the smaller sizes, its the big ones that take time. When I get a bigger lathe I will build a larger sphere cutting jig to speed
the process if the demand exists. Not too many guys in that hobby/industry that make the big shells, usually a once or twice a year event for those size of shells.

 

[Bill Boehme]

I turned an oak sphere once that was about 8½" diameter and it was about as heavy as a bowling ball. A solid oak 18" sphere would weigh about 9½ times more.

If the wood doesn't matter, one of our club members turned a large hollowform from OSB which would be about as dimensionally stable as Baltic birch plywood and it was amazingly nice looking. It sold for somewhere around $600 at our Christmas banquet last night.

 

[Mike Johnson]

I had a large solid wood sphere come off the lathe one time, it was definitely an Indiana Jones moment.
That is another reason I like to make the large spheres hollow, all it takes is one large knot in the wood
or a heavier denser piece of wood on one side of the sphere and it can make turning the sphere a pain
in the butt trying to get it turned round when the blank is out of balance. A solid composite material takes
that issue out of the equation, however a wood grain material looks traditional which is what I am shooting for.
Using segmented rings allows me to check the weight and balance of each ring and place the heavy side where
it needs to go to balance the sphere as I am building the stacks and turning it on the lathe.

 

[Tom Albrecht]

Interesting... I'd like to see that 18" shell blow. What kind of lathe are you using? Is it set up like a machine lathe with a feed?

 

[Mike Johnson]

Tom,

I am using a conventional lathe and hand tools to make the large spheres, smaller ones I have a sphere
cutting tool that I use. One of my other hobbies involves working with a Pyrotechnic Group that builds
various "fireworks" items and puts on displays throughout the year. There is an annual convention held
each year in several locations across the country where about 5000 people in this guild (PGI) meet for a
week long training/building/display/competition. One of the long term goals for some of the guild members
is to build a 48" plus diameter shell to break the world record. The Japanese have a festival each year where
they shoot a 48" diameter shell each year, There are several video's on YouTube that show these shells. Pretty
impressive considering that they use a black powder lift charge to launch the shell a mile high before it goes off.

 

[Tom Albrecht]

Thanks Mike, got any you can share of some of your larger spheres?

 

[Mike Johnson]

Tom,

This is one of the hollow nested sets and some of the solid spheres I have made.

 

[Tom Albrecht]

Tom,

This is one of the hollow nested sets and some of the solid spheres I have made.

View attachment 21527

View attachment 21528

VERY COOL! Now, got any videos?

 

[Mike Johnson]

I have a full time job and a home based business and you also want me to make videos?
I have watched too many videos that had way too many critics comment on every imaginable
aspect of the content (safety, technique, process, materials, equipment, etc. etc. etc.)
I enjoy turning wood too much to open the door for trolls to swoop in with negative comments.
The method I use is old school, locate and mark 2 or 3 axis on the billet, cut the corners off, mount
the billet on one of the axis between centers and rough the ball to size and shape, I usually use
a cloth measuring tape to make circumference measurements when getting close. I also rotate
the sphere to different axis and trim and balance the sphere by watching the shadow of the sphere
on each axis. Getting the axis points located correctly is critical, sometimes I will locate a 2nd set
of axis by drilling one hole on the equator and then measure half way around the ball and drill another
hole while the ball is mounted between centers on the lathe. Sometimes it is hard to get all of the
axis points marked on the billet prior to turning because it is not perfectly square. So locating one
set of axis and then turning the billet into a smooth surface on the equator is the best way to attain
an accurate 2nd axis to rotate the billet and turn from. The 3rd axis points can also be located in this
same manner if required. The smaller spheres are quicker and easier using a sphere turning jig.

When turning a hollow ball I mount a layer of birch plywood to a large face plate and turn the billet
to the correct circumference and cut a mating edge on the (2) halves so they lock together. I then
glue a solid layer of wood or glued segment ring to the birch plywood ring and start turning it to the
proper radius inside and outside and work my way up the ball by gluing each layer let it dry and mount
it back on the lathe and trim each layer to size. I have a template that I use on the outside and inside of
the sphere to gauge my cuts with. The last layer I glue on has a tenon on the outside which allows me
to remove the half sphere from the face plate and mount it to a bowl chuck and finish turning the inside
round and sand it smooth. I also have a spur and center point driven onto the end of the tenon so when
I remove it from the bowl chuck I can put the (2) halves together and mount it between centers and trim
the tenons down to small diameter nubs that can be sanded smooth. This also allows me to finish trimming
and sanding the sphere on the outside while it is mounted between centers.