Air cleaner 3

April 2016

Having previously built this air cleaner and this one with a homemade pleated filter for my other shop, I still wanted a second one for my basement workshop.

A few years ago, I experimented with making a mount for an oscillating fan motor that mounted the motor by same post that it pivots on when part of the fan. I cut a thread into the post that sticks out of the motor, then used a nut to pull it against a wooden bracket that I shaped to fit around the motor.

On an oscillating fan, the blade is quite a distance from the motor. But I didn't want to make the box too deep, so I wanted to move the blade closer. Having recently taken apart another fan, I realized the fan's rotor is only attached to the shaft with a press fit, so by taking the rotor out and pounding on one end of the shaft, I could move it back. That was much simpler than what I did last time.

Then cutting off the part I didn't need from the other end with an angle grinder.

Unfortunately, I discovered there were some lines punched into the shaft where the rotor was, probably to give it a better grip. I polished these out on the buffing wheel as well as I could.

When putting one of these motors back together, even with fresh oil, it's often hard to turn. The self aligning bearings don't align that easily. But tapping the motor in a few places with a mallet is often enough to get the bearings to move into alignment.

Placing the fan motor and blade on top of the filter (with a spacer for distance) I determined the total thickness I needed was just 15 cm.

I cut the pieces out of a 75x75 cm piece of Baltic birch plywood, cutting one piece off the end before ripping the other three pieces. That way, the remaining piece of scrap is a more usable shape.

Then cutting 1/4" (6 mm) box joints on the ends of the pieces, using my box joint jig.

And gluing the box together.

I made the fit too tight. In fact, the filter is more of an "interference fit" in the box, it's 1-2 mm larger than the box opening.

So I tapered the edges of the box with a small hand plane and tapered into the corners with a carving knife.

Then gluing a strip of wood near the edges to fix how far into the box the filter can be pushed.

I noticed the sides of the box had a tendency to bow inwards slightly, which didn't help in getting the filter in. So while the glue dried, I braced the sides outwards. The pieces I glued in plus the box itself form a sort of composite member, so once the glue dries, it will retain some of that shape.

The motor mount I made years ago turned out to be slightly too short, so I made two pieces to glue to the ends of it to make it reach across the box.

After letting the glue dry, I measured where I need to place the motor, then tapped the screws with a hammer to mark where they need to go.

I then drilled pilot holes where the divots from tapping the screws were.

The buttons from the fan can fit in a slot about 1/2" wide. My first thought was to make this slot with my slot mortiser.

But instead I drilled a series of overlapping holes with a Forstner bit to hog out most of the slot. I love how you can do that with a Forstner bit.

I straightened the sides of the slot with a rasp.

The buttons fit nicely, but I didn't want them sticking out the side.

So I glued some blocks to the inside to move the button unit just far enough in to make the buttons flush with the outside.

One of the reasons I hadn't done anything with this fan after making the mounting bracket was that the motor didn't run all that well. I suspected the capacitor and, indeed, connecting a 0.47 µF capacitor in parallel to the motor's run capacitor made it run faster.

Luckily, I had another motor run capacitor kicking around. This one was a 3 µF capacitor, whereas the motor came with a 2 µF capacitor, but engineering-wise, these capacitors are kind of a kludge anyway, so I figured 3 µF is ok. It certainly ran much better with that one than with the one it came with.

I still needed a front panel around the fan blade to make sure all the air comes through the filter instead of just flowing in the front around the fan.

I set my beam compass to slightly wider than the fan's radius and marked a circle on the wood.

Then cut that out on the scroll saw.

Placing it in the fan box, with just a friction fit for now.

I had always assumed the best place to put the panel, or baffle, was upstream of the fan blade (slightly behind it), because centrifugal force would throw some of the air out from the fan sideways.

But checking the air flow with a piece of thread attached to a stick, I found the fan actually pulls in air from the side, especially if there is some air resistance from the filter behind. So putting the baffle just slightly downstream of the fan would make the air sucked in from the side also be air from the filter.

With the baffle where I put it, there was still some air pulled back near the edges of the fan.

You can also make an air cleaner just by strapping a filter to the back of a box fan, but box fans lack internal baffles to stop air back-flow, which means if the filter provides any resistance to air flow, much of the air blown out the front of the box fan is actually air that has bwwn sucked in from the front. So building a fan box like this is a more elegant solution.

Once I established how far back the front panel should go, I glued some pieces of wood around the edges, which the front panel then screws to.

The inside of the box, other than the motor, is empty.

I made two blocks of wood with hooks on the end. These are tied together with pieces of bicycle inner tube. One block gets screwed to the ceiling, the other to the top of the fan. This makes for a little bit of vibration isolation. That wasn't really necessary, but it was easy to add.

And here it is mounted. It's in the only spot that isn't in the way and doesn't add visual clutter. To the left of it is my old air cleaner. I added a switch near my table saw to turn both of them on at the same time. With both turned on, they can clear half the dust out of the air in my shop every three minutes.

This part of the shop is the designated chaos zone, which is why it rarely appears in any of my articles or videos.

You can see my other air cleaner, this "dresser" on which are my miter saw, my lathe, partly disassembled. On the wall is my shelf with drawers, on which are some older small projects like cube, impossible joint and screw jack. On the right on the shelf behind the lamp is my router pantograph, and lower down my screw advance box joint jig, and below that lots of firewood lumber.

Update 2019:
I mounted the air cleaner just 10 cm from the wall, figuring that was enough for it to pull air from the back, but I kept getting comments that it was too close to the wall. So in Nov 2019, I experimented with how close it can be to the wall before airflow suffers. I figured even 5 cm would be enough, but I was wrong. 3 cm is adequate and even at 0.9 cm from the wall, it still has more than half the airflow that it has when it's far from the wall!