WARNING! WARNING! WARNING!
The safest position for ANY chuck, is to be as close to closed as possible. As you open them, they become less effective at holding the piece. This is due to the fact there is more flex, especially with cole jaws. When you add a loosely(Expanded to the extreme) set of jaws to a flexible sidewall, it makes for an accident waiting to happen.
There are two home made chucks that are very safe on larger pieces. I don't know the names, but one consists of a flat piece and a ring that screws to the flat piece and the bottom sticks out of the hole for cleaning up. The other one uses two pieces of plywood, MDF, etc, with opposing spirals to hold the work.
Hope this helps.
Bruce Shiverdecker - Retired Starving Artist ( No longer a Part timer at Woodcraft, Peoria, Il.)
"The great thing about turning is that all you have to do is remove what's not needed and you have something beautiful. Nature does the hard part!"