Furnace fans are not designed to move debri, only air.
You need fans like are found in a dust collector.

You could try breaking off every other fin on the fan.
But still most likely it will clog up.

You mite could filter the air before it gets sucked thru the fan,
but this won't be efficient, and the filter will clog.

Dust collector fans work like the squirrel cage furnace fans,
but they only have 4 or 6 straight blades on them and
this makes them self cleaning.

Dave Gingery sells a book about building one of these fans.
I have the book and it doesn't look complicated.
Balancing the rotor is the hard part, and he tells how to do that.
You can get his book thru the Linday Technical Books site.

Pooh Bear

How to Design & Build Centrifugal Fans by Dave Gingery

Here is a link that might be useful: Linday Technical Books

Thanks for the info
I'm still checking other sorces/resorces
Roscoe