: Liz, you cant use the same collector on wood and metal at all - not even
: once.
: The metal sparks will ignite the sawdust. Or the bags. Or the oil smoke will
: blind the bags. Metalworking needs a wet collector, or simply an outside
: vent.

Thanks, Don and everyone who replied. I have learned a lot about dust collection in the last few days! Your replies and links have helped a lot in sorting out the system(s) we need to meet our needs, budgets and building limitations. As you noted, we are not going to use the same system for the metalwork because of the dangers you mentioned. Fortunately, the metal waste is mostly shavings and less of a problem anyway than the sawdust. Because of several factors, we can't vent outside -- close neighbours, gas furnace and water heater, bringing in cold northern air to replace vented (expensively heated) indoor air, etc. We decided finally on a two bag system which is semi-portable (and also at a reasonable sale price at the nearest Busy Bee store). This should at least make an improvement in the workshop and to the washer and dryer that I use as auxiliary benches.