During the wildfires of 2020 I saw many instructions for how to quickly duct-tape air filters to a box fan, to remove smoke from household air. Years before that I was interested in buying a woodworking shop air filter that would remove fine sawdust that otherwise floats in the air for days or weeks.
This post describes my experience making a air-filter-box-fan design, using 3D printed clips instead of duct tape to hold the filter to the fan.
CAUTION: I have no idea how safe it is to clamp an air filter to a box fan. Such a filter may reduce the air flow to the fan’s motor, which might result in overheating and fire. Always supervise the running fan. You’re responsible for your own safety.
I began with a nominally 20-inch (about 510 mm) box fan, and unscrewed the front cover. I then measured the fan and chose an air filter (20″ x 20″ x 1″) slightly smaller than the outside of the fan.
![Box fan, with the front guard removed](https://bluepapertech.com/wp-content/uploads/2023/04/20220305_114937BSMALL.jpg)
Centering the air filter on the front of the fan, I designed a simple 3D printed clip, posted on Cults3D, to hold the air filter to the fan.
![3D printed clip, one of eight to hold the air filter to the fan](https://bluepapertech.com/wp-content/uploads/2023/04/20220305_170400B.jpg)
My plan was to print eight of these clips, and attach two per side of the fan, using sheet metal screws (8 x 1/2″).
Drilling the pilot holes for the screws proved a little challenging. I started by choosing a drill bit about the same diameter as the inside of the screw.
![Choosing the right size drill bit](https://bluepapertech.com/wp-content/uploads/2023/04/20220305_120310BSMALL.jpg)
I then marked the spot to drill by positioning the clip and marking the hole with a felt tip marker.
![Marking the spot to drill](https://bluepapertech.com/wp-content/uploads/2023/04/20220305_115434BSMALL.jpg)
I then tried to drill a pilot hole, but found the drill wandered all over the sheet metal.
![Trying to drill a hole; the drill bit wandered](https://bluepapertech.com/wp-content/uploads/2023/04/20220305_115722BSMALL.jpg)
I should have used a center punch to dent the metal, but I don’t have one. Instead I drilled a hole through a block of wood, then used the block of wood to keep the drill bit in place. It’s a bit of a McGyver job, but worked for me.
I found drilling right on top of the mark is very important. I accidentally drilled one hole too far backward of the mark, and the clip almost didn’t fit.
Although sheet metal screws are self-tapping, I found they needed more help than a screwdriver offered. So I used a socket wrench, which let me hold the screw upright while tightening. I put the screw in once to tap the hole, then removed it and screwed the clip on.
![Using a socket wrench to tap the sheet metal hole](https://bluepapertech.com/wp-content/uploads/2023/04/20220305_120548BSMALL.jpg)
I positioned the top two clips so the screws wouldn’t interfere with the power switch – I didn’t want any electrical problems.
Before screwing in the final two clips, I slipped the air filter onto the front of the fan, making sure that the air flow direction arrow pointed outward.
![The filter in place, with the air flow arrow pointing outward](https://bluepapertech.com/wp-content/uploads/2023/04/20220305_123816BSMALL.jpg)
To replace the filter once it’s dirty, I plan to remove two clips from one side, slip the old filter out and the new filter in, then replace the screws. As long as I don’t tighten the screws too much they should last through many filter replacements.
Finally I placed four felt pads on the fan’s bottom face, to prevent the screws from scratching the counter or floor.
![Placing felt pads on the fan bottom](https://bluepapertech.com/wp-content/uploads/2023/04/20220305_125529BSMALL.jpg)
![The finished bottom, with four felt pads](https://bluepapertech.com/wp-content/uploads/2023/04/20220305_125821BSMALL.jpg)
I’m really happy with how the project turned out!
![The finished fan](https://bluepapertech.com/wp-content/uploads/2023/04/20220305_123545BSMALL.jpg)