A project log for MultiBot CNC v2

A low cost 3D printed CNC that can be built with minimal tools yet is capable of great things.

David TuckerDavid Tucker 09/26/2021 at 04:070 Comments

So it was time to upgrade my smoke filter.  Previously I had just taped a 20"x20"x1" merv13 filter to a $20 box fan.  That was working ok but I wanted to see if I could make it perform better.  

From my last post I worked out what size baffles to make.  After spending a bit of time with the scissors I came up with baffles that just fit inside the frame so they could sit flush against the grill.  To attach the baffles i experimented with various glues and tape but ended up removing the grill and hot melt gluing them from the back.  You want the baffles on the outside of the grill so the suction pulls them tight against the grill. That makes it so you don't need a very secure mount since the air pressure does all the work.

To make it easier to maintain the filter I printed some small brackets that can be attached to the face of the fan to hold the filter in place.  Again the air is pulling the filter tight against the fan so no need for this to be overly complicated.

Since I had some cork laying about I went ahead and added it to the feet.  That seems to have stopped the fan from rattling around when it was running.

I also wanted to see how much air flow was lost with the filter in place.  Measuring near the center of the blade (not the center of the fan) I was getting 6 m/s of air without a filter and 4.3 m/s with the filter.  That is a loss of  about 20%.


I'm using a Lasko 20" box fan, the cheapest model I could find.  It looks to be rated for 1820 CFM and rated at about 50 watt's for the motor.  

They make a more powerful model for $30 that has a 100 watt motor and claims 2000 CFM.  For twice the power they are not getting much more air flow, I suspect the problem is my model is overstating its CFM.

And an even more powerful model for $77 that has a 130 watt motor and claims 3460 CFM. That is almost twice the CFM of my fan.