is quite simple to make.
Especially if one gets lucky and finds a squirrel cage blower from a
hot air furnace like I did. It has a 3 speeds and uses 20X20X1
inch filters, but I found out my original plan to use two
MicroAllergen 1000 rated filters on each caused the fan to work
at high speed all the time. Then I tried one MicroAllergen
1000 filters on each side plus a cheaper one on each side. Same
thing. I found out that just one filter per side allowed me to
take advantage of the multi-speed aspect. I have it mounted
on a cart with wheels so it is easily moved round the studio. I
have seen these hanging from the ceiling. In the second picture
you can see that the top is hinged to easily remove filters.
When grinding blades I roll it up to the
side and it works really well. I have have seen simular fans for
$100 plus. This was build from scrap wood, fan free, filters $9.00 each.