This is a fan that friction fits onto the collect of a Dremel/Craftsmen 395 rotary tool. It blows air over the bit to clear the dust from the cutting area so you can see better.
I remixed the original design as I didn't like the exposed fan blades, so I added a shroud around them. I was also interested in coming up with an optimal blade design to move the most air. I designed and tested 23 different ideas then measured and charted their output. The top 3 are available for download.
There are 3 different fan styles, two that are 25mm in diameter, one that is 20mm. I added the smaller one as I found the larger fans sometimes obscured the bit too much doing fine work. There's also a version without keying for rotary tools that don't have any.
Designed for 10.6mm diameter collet. If yours is larger/smaller, just scale the part up down. It requires a friction fit to stay attached.