Truing the shell form rim requires drawing concentric circles from the central post and then testing with a marked form made of heavy corrugated fiber board, grinding and filling as needed. The use of a wire instead of string yields a more dependable distance from the center post.
pgtruspace's blog 
Truing the shell form rim requires drawing concentric circles from the central post and then testing with a marked form made of heavy corrugated fiber board, grinding and filling as needed. The use of a wire instead of string yields a more dependable distance from the center post.