Friday, April 24, 2020

Attic Fan Screen

Another Covid project that I tackled was making a screen to cover my attic vent fan. The fan has a screen on the outside, so that bugs couldn't get into the attic from there. The issue we were having was stink bugs flying into the fan from inside and dying.

After a few weeks of not going up there, we'd find a small pile of stink bug bodies all summer long! So I set out to make a screen to stop them from flying into the fan.

The one design consideration that I had to work with was the BX wire feeding the motor. I wanted to try to get the screen as tight fitting around the fan as I could, so I was going to have to plan around the wire.

I can't draw worth a damn, but this is the rough plan that I came up with. The right side of the frame would have the notch for the wire cut into it. I figured that making all 4 pieces of the frame the same size would make it easier.

I decided to use a couple of scraps of wood that I had lying around the shop.

Here's an interesting tool. So when you use a table saw, you really want to keep your fingers safely away from the blade. Especially when you're cutting thin pieces. So you need to use some kind of push block or stick. I splurged and bought a Grr-ripper from Microjig. It's an adjustable push block that allows you to set it up for any kind of cut you need. In this case, I adjusted the foot to support the left side, and the rubber feet were adjusted to grab both the piece that I wanted to keep and the off-cut.

Being a maker is not usually about what tools you have, but in some cases having a better or more specialized tool can really help your accuracy. When I set up my shop, I looked extensively into compound miter saws, and settled on the Dewalt DW779. I got it on sale, and was able to add the most important feature of the more expensive model (DW780) for less than the cost of the difference between the two.

Rather than having a laser for alignment of your cut, the DW780 has an LED light that shines down over the blade projecting the shadow of the blade onto the workpiece. This allows for a much more accurate alignment of your blade, and it automatically compensated for different blade thicknesses because the shadow will change automatically based on the blade you're using. It's super handy, and much more accurate than a laser to me.

Setting up to make the notch on the one side, I used a forstner bit in my drill press to drill a hole that slightly overhangs the edge of the piece.

I then cleaned up the hole with a hand saw and sandpaper, and it was ready for assembly.

Screwing wooden projects together comes with special considerations. If you're screwing into especially hard woods, near the edge of your workpiece, you risk splitting the wood. The trick to making sure you don't split the wood is to pre-drill your screw holes. 

So it's a 3 step process. The 1st step, after lining up your two workpieces and locating where your screws will be, is to drill through both of your workpieces with your smallest drill bit. This provides clearance for the shank of the screw, while still allowing the threads to bite into the wood. This prevents splitting of the workpiece being screwed into. The 2nd step is to drill the workpiece through with the larger of the drill bits. This provides clearance for the screw to pass through but not have the threads grab. This prevents the splitting of the workpiece being screwed. The last step is to use a countersink bit to provide clearance for the cone shaped head of the screw. If you drill your holes in this fashion, you'll have nicely countersunk screw heads while not splitting the wood. If you over tighten the screw, though, the cone shaped head could split that piece too.

If you screw up like I did and use a bit that was too small to pre-drill for the threads, you could split the wood too. I stepped up my smallest bit to the next size up in my drill index, and that worked fine for the rest of the project.

And this is what it looks like if you do it right, more or less. I also used wood glue on the joints.

I laid the completed frame onto the screen material, and stretched and stapled it on opposing sides to keep it even.

I pinched and folded the corners over to make a nicely closed joint.

I cut and screwed 2 ears to screw the cover to the roof plywood (using screws short enough to not penetrate out the other side of the sheathing). I set it up such that I can take it down if it gets full of bugs or anything else from outside of the attic.

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