Laura, As a steel form or mandrel, I would suggest looking for
machined pipe couplings . These are available in sizes to 6" or
perhaps 8" they are well machined and are available in schedule 40,
80, 120 ( pressure ratings) so as to offer a selection of diameters.
There are also cut off sections of pipe available at pipe
fabricating & pipe welding shops. These sections of pipe are strong
and uniform . They can be sanded and polished to present a smooth
surface. A more accurate way to measure the diameter of a round
object is to measure the circumference . This is Diameter times 3.14
A good place to find what diameters are available is to go to the
library and check out a book on pipe welding and or fitting. There
are references in the back of the books .
Wear blue jeans , good shoes, carry a tape measure and get on good
terms with the guys . These guys work with steel as a potter works
with clay. You might find odd items which will make new and
innovative tools . Ask to look in the "bone yard"or scrap pile.
ROBB - Retired Old Baby Boomer