The main reason I made it adjustable is that every shaft has different dia. and tapers, so if you want to have the same amount of shaft sticking out of the chuck, you have to have either an adjustable bearing support, or a large variety of different collets. A bearing support is better than just a v-block, you don't want friction generated heat to damage the finish etc. Rubber wheels will have to be adjusted to accommodate each shaft dia.
Dave