As was pointed out above, a filter unit doesnt do anything to keep the dust out of the air you breathe. To do that, you must either capture it at the source, wear a mask, or both.

If you purchase a dust collector, and hook it up to the tools, you will soon find it to be one of the most used tools in the shop, and you will wonder how you ever got by without it.

All that said, I'm assuming you have a box fan? Which is a propeller fan, not a certrifugal like is in a furnace. This will work, but not as well as a furnace fan. A prop fan will have significant changes in airflow with slight changes in the static pressure - filter pressure drop in this case. A furnace fan is better, and free or almost free from your friendly HVAC shop if youre willing to pull it from thier boneyard.

But it will still work. Filters are rated by pressure drop and efficiency at 500 FPM face velocity. As the air velocity drops, pressure drop goes down and filtration efficiency goes up - both desired effects. So add more filter area, the more the better. I have a rectangular box configuration, with a furnace fan blowing out one end. All the other three sides are solid filter area, almost 20 square feet in all. Doing this lets you use the cheap filters available at your local hardware store.

You'll want two sets of filters - one pad type, the lower efficiency. And directly behind it, a pleated type. Hardware stores have these in 1" pleats, 2" is better but either will do. This way the prefilter keeps the more expensive pleated filter clean longer. You can reuse them several times, by vacuuming them off. Also BUY THE FILTERS FIRST! Find a local supply, and a standard stock size for them. 20x20, or 16 x 20 probably. 24 x 24 is a great size but (as I found out) it's a commercial size, not residential and local hardware stores dont stock them.

Once you have the thing built, try to position it to keep the air in the shop stirring. This keeps the fine particles in suspension while the filters clean it. It will take several air chjanges in the room to clean it well, you want to size the volume (CFM) of the fan for about a 10-minute air change. For example, if the volume of your shop is 10000 cubic feet, you'd want a 1000 CFM fan. Dont bother to run the thing while you are working and producing dust - it's function is to clean the air AFTER the dust-makers are turned off. It may help a little while youre working, if youre standing in the clean air path. But generally it just stirs up the dust and brings more of it into the breathing space.

And finally, dont assume that the air is safe just because you cant see any particles: The most damaging particles are 50 microns and down, and you cannot see them anyway. You still need a mask, no matter what, if you are suffering from allergies.

: I had to replace the fan unit in my attic when my roof was redone. The
: plastic housing was cracked. Any suggestions for building some kinda
: filter system using the old fan. The fan is probably 14-16 inches in
: diameter..

: Would a simple box with the fan inside surrounded with filters do me any
: good?

: What kind of filters?