Pressure Relief Valve for Cyclone Dust Extractor (Parametric)

Paired with a cheap ebay cyclone dust extractor and a 30 litre bucket, my little Kaercher WD2 vacuum handles the sawdust and chips from my power tools handily. But I found that when I was using it for cleaning up, if I was careless about letting the nozzle get blocked, the suction would instantly crush the bucket. The solution is this little three-piece pressure relief valve.

Drill a hole through the lid of the bucket using a hole saw (the STLs are sized for a 32mm hole saw, but the scad file is parametric so you can resize it to fit your hole saw). Clean the edge up with a sharp knife, then push the part pressurerelief_outer.stl through the hole - should be a nice loose fit. Mark the screwholes (on the flanges either side) on the bucket, then remove the part, drill the holes (I used a 4mm bit). Now assemble pressurerelief_outer.stl (outside of bucket lid) and pressurerelief_inner.stl (inside of bucket lid) and bolt together. I used short M4 bolts, but you can resize using the scad file if you want to use something bigger. I slathered pressurerelief_inner.stl with silicone sealant but this probably isn't necessary. 

Next, I glued a disc of 3mm neoprene to the top surface of pressurerelief_closer.stl. It may work just fine without this, but I wanted a nice seal with minimal whistling. I drilled through the neoprene to clear the central hole. Now, run a hex-bolt head M4x40 machine screw (or edit the scad file to use a larger size) up through the bottom of pressurerelief_closer.stl, so that the hex head is capture in the socket on the underside of pressurerelief_closer.stl. This part will be inside the bucket. Next, run the end of the bolt vertically up through the centre parts pressurerelief_inner.stl and pressurerelief_outer.stl. On the outside, drop an M4 washer over the bolt, then a spring, then another washer, and then screw a nut on. A 5.5x17mm compression spring I had on hand worked fine, but almost any small spring might do - say the kind you get with a Parker ballpoint refill? By tightening down the nut on the spring, you increase the pressure required before the valve will open; loosening means it opens sooner. Adjust to suit your vacuum and bucket.

I ground the threads off the bolt where it passes through the plastic parts to help it move more freely. This may not be necessary - there's a bit of clearance in the shaft. If it's ‘sticking’, either grind the threads or maybe paint them smooth with nail varnish.

Printing is straightfoward in the orientation of the STLs - no supports required. Infill shouldn't be too big a deal, but you probably want 3-5 shells/walls for it to work well and last. I printed in PLA and it works fine.

Here's a wee video of the thing in action: