The sphere
used on the top on the vandegraff can be constructed from two stainless
steel food bowls.
The bowls I used are 270mm dia x 75min deep from the outer most rim. (see
figure y1) The bowls do not need to be fixed together with any
screws or fixing glue. The best method is to stickytape the rims together
with a complete circle of insulation tape around the entire rim. You will
need to remove the top bowl at periodic intervals during construction, so
don't use up all the insulation tape at this stage of the construction.
Use temporary strips of 50mm long insulation tape on 4 locations around
the rim, which will hold the top bowl into position while doing tests an
adjustments... Do not scratch or dent the bowls in any way. The more shiny
and clean the bowls are the better they are at holding the high voltage
charge on the surface. We want to hold as much charge as we can on the
surface. Any sharp points or scratches will give a jump off point for the
HV (high voltage). The bowls used in the construction of
this vandegraff unit are not the preferred items to use. The rims of the
bowls provide a narrow edge from where the high voltage can jump off. The
voltage will begin to breakdown (jump off) when the voltage starts to
exceeds above 50kV. If you can obtain a perfectly round sphere that would
be your best option.
Most people are
unable to obtain a perfect sphere so two stainless steel bowls will work
well enough. You may like to have a search for a metal constructed world
globe or atlas. The same world globes you use to study with at school.
Currently most of the spheres used in these world globes
are now made from plastic, so they cannot be used in our VG project. If
you happen to chance on an old globe made of metal you might be lucky
enough in that the sphere may have a thread that joins the two hemispheres
together. This would be an ideal purchase. Look in old antique or 2nd hand
stores an markets for these items. If you happen to obtain a metal
fabricated world globe you will need to remove the print or paint from the
outside. You also need to polish the globe to a shiny smooth finish to get
maximum storage of HV potential from the Vandegraff unit. There are
companies around that manufacture the perfect threaded spheres for
vandegraff machines. You may be lucky to find a company that doesn't
charge an arm or leg for such a purchase. For your first test bench
prototype two stainless steel bowls will work perfectly. I am sure you
will have loads of fun with this two bowl VG design in any case...
Cutting Out The Bowl Centre Tube Hole
Marking & Cutting
You will need to cut a hole in
the bottom of one of the stainless bowls. This hole is where the 80mm(o/d)
PVC main centre tube an one of the flanges will be fixed. Use a steel
scriber an the outer most 80mm(o/d) circumference of one of the PVC
flanges as a template. Centre the flange on the bottom of the bowl an
scribe around the outside of the circumference of the flange. The final
hole size should be around 85 mm dia.(see figure 6) This will leave 2.5mm
either side of the 80mm PVC pipe to be inserted. You will not see the
extra gap as the top 80mm flange will be formed into shape to fit the both
the main centre PVC pipe and the hole. You can use any method to cut out
the hole. I would use a metal shop lathe or a metal hole saw. You may use
which ever method you feel will work. Make sure you file a clean
smooth burr free edge on the final hole cut. This will keep any HV loss to
a minimum. Be sure that the outer 85mm(o/d) straight section of the PVC
flange can be inserted into the hole with ease. Do not be concerned about
the final alignment or the rest position of the top flange at this stage.
You will use this hole size when heating and forming the flange into the
curved shape. See below.
Forming The Flange Curved Shape
Because the bottom of the stainless steel bowls have a
curved dome bottom you will need to form only one of the PVC
flanges into the curved shape of the bowl. (See top flange in figure 6.)
You will need to use a heat gun or flame to heat the PVC flange until the
plastic is soft an pliable. Only heat the outer most 135mm rim of the PVC
as we don't want to push the 85mm pipe diameter out of shape.(see figure
7)
Use caution that you do not
over heat the material to where it starts to burn the outer PVC layer.
When you feel the flange 135mm rim is soft enough to form, quickly place
the heated flange into the bottom bowl with the hole cut out. Using the
second stainless steel bowl; place the top bowl inside the bottom bowl
then press an hold the two bowls together firmly for around a minute. This
will form the heated flange into the perfect curved shape of the bowl. Be
sure you placed the heated flange from the top down into the bottom bowl
and not from the bottom up direction, else you can not sandwich it between
the bowls or be able to form the heated flange to shape. You must insert
the heated flange (85mm section first) into the
bottom bowl with the hole previously cut out else this will not work. So
cut the hole in the bottom bowl first. Hint : Use an offcut waste piece of
80mm PVC pipe to hold the flange while heating the 135mm rim. This will
also prevent the 85mm section from deforming during the heating process.
Fixing The Curved Flange To The Bottom Bowl
Once the flange has cooled, fix the flange to the bottom of the bowl
with silicon/glue and 4 pop rivets. Be sure the flange is centre most
around the hole.Use the same 80mm PVC pipe waste offcut over the flange to
get the flange centred over the hole in the bowl. The flange is placed on
the bottom curve of the bowl from the bottom up.(see figure 3) Do not
place the flange down through the hole as you did when forming the curved
shape. The PVC flange must cover over any sharp metal edges that where
left when cutting the hole in the bottom bowl. The flange will insulate
any bad metal edges so that HV loss is reduced. After you have drilled and
pop riveted the flanged onto the bottom bowl you can cover the pop rivets
with a thin seal of silicon or insulation tape. This will help limit high
voltage jetting out from the pop rivets.
Remember:avoid all sharp edges and points on the
sphere construction. Smooth an shinny is the primary objective.
Alternative
The sphere
used on the top on the vandegraff can be constructed from two stainless
steel food bowls. 
used in these world globes
are now made from plastic, so they cannot be used in our VG project. If
you happen to chance on an old globe made of metal you might be lucky
enough in that the sphere may have a thread that joins the two hemispheres
together. This would be an ideal purchase. Look in old antique or 2nd hand
stores an markets for these items. If you happen to obtain a metal
fabricated world globe you will need to remove the print or paint from the
outside. You also need to polish the globe to a shiny smooth finish to get
maximum storage of HV potential from the Vandegraff unit. There are
companies around that manufacture the perfect threaded spheres for
vandegraff machines. You may be lucky to find a company that doesn't
charge an arm or leg for such a purchase. For your first test bench
prototype two stainless steel bowls will work perfectly. I am sure you
will have loads of fun with this two bowl VG design in any case...
will need to cut a hole in
the bottom of one of the stainless bowls. This hole is where the 80mm(o/d)
PVC main centre tube an one of the flanges will be fixed. Use a steel
scriber an the outer most 80mm(o/d) circumference of one of the PVC
flanges as a template. Centre the flange on the bottom of the bowl an
scribe around the outside of the circumference of the flange. The final
hole size should be around 85 mm dia.(see figure 6) This will leave 2.5mm
either side of the 80mm PVC pipe to be inserted. You will not see the
extra gap as the top 80mm flange will be formed into shape to fit the both
the main centre PVC pipe and the hole. You can use any method to cut out
the hole. I would use a metal shop lathe or a metal hole saw. You may use
which ever method you feel will work. 
Make sure you file a clean
smooth burr free edge on the final hole cut. This will keep any HV loss to
a minimum. Be sure that the outer 85mm(o/d) straight section of the PVC
flange can be inserted into the hole with ease. Do not be concerned about
the final alignment or the rest position of the top flange at this stage.
You will use this hole size when heating and forming the flange into the
curved shape. See below.
Use caution that you do not
over heat the material to where it starts to burn the outer PVC layer.
When you feel the flange 135mm rim is soft enough to form, quickly place
the heated flange into the bottom bowl with the hole cut out. Using the
second stainless steel bowl; place the top bowl inside the bottom bowl
then press an hold the two bowls together firmly for around a minute. This
will form the heated flange into the perfect curved shape of the bowl. Be
sure you placed the heated flange from the top down into the bottom bowl
and not from the bottom up direction, else you can not sandwich it between
the bowls or be able to form the heated flange to shape. You must insert
the heated flange 
(85mm section first) into the
bottom bowl with the hole previously cut out else this will not work. So
cut the hole in the bottom bowl first. Hint : Use an offcut waste piece of
80mm PVC pipe to hold the flange while heating the 135mm rim. This will
also prevent the 85mm section from deforming during the heating process.