The Orange Screamer

As soon as I saw Kiteman's laser cut POCKET SIREN I thought it would be good as a 3D printed project too, so here we go with the ORANGE SCREAMER. I designed my own version from scratch using the excellent and *free* DesignSpark mechanical, and printed it on my Wanhao Duplicator 4 (Makerbot clone) 3D printer.

Step 1: Tools and Materials

A vital requirement for this project is a 3D printer, or a friend with a 3D printer, or someone who knows someone with a 3D printer and is willing to run parts off. In addition you will need cyanoacrylate adhesive (superglue); preferably the gel sort, a light lubricant (WD40 is ideal, or a light vegetable oil or graphite spray), fine sandpaper or emery cloth and a couple of matchsticks or similar to apply the lubricant and glue.

Step 2: Design and Printing

The design went through 3 iterations :-

V1 had a parallel blade profile and worked, but wasn't very loud and I couldn't get enough air through it without my eyes bulging and my ears popping.

In V2 I increased the size of the air inlet to get more flow and on Kiteman's advice changed the turbine blade profile to match the vent holes. These changes made a big improvement to the volume and I could blow without going bug-eyed.

The final version was a general tidying-up exercise. I slimmed down the top and base thicknesses to 2mm and changed a couple of dimensions to reduce the need to sand after printing.

I printed this in PLA. I imagine ABS would work, but I think the brittleness of PLA is the better choice for this. I used 40% fill and printed with 0.2mm layers, 2 shells and raftless onto blue painters' tape. I've found a plate temperature of 50C stops any warping of small, flat parts like this. The 3 parts took just under 30 minutes to print.

Once printed, leave to cool especially if using a heated plate. This will reduce any distortion of the parts as you remove them .

(You could use a colour other than orange, but it wouldn't be an orange screamer then, would it? ;¬)

Step 3: Source Files

*** If you previously downloaded the file and had trouble printing the rotor, re-download and use 'rotor8_fixed.stl'. ***

Rather than have a long list of images here, I've wrapped all the source files up in a ZIP file. The files in there are for the body, rotor and lid in RSDOC format (the native Designspark format) and STL format. I've also included files in SketchUp (V8) format, a THING file for use directly on a MakerBot and a file with just one rotor blade in case anyone wants to play around with the geometry. (The SKP files are an export from Designspark and I haven't tested how well they import into SketchUp.)

Step 4: Fettling and Assembly

After printing, the three parts should just clip together and the rotor turn. However, we want to reduce friction as much as possible to get maximum revs so I suggest smoothing down both sides of the rotor slightly and the underside of the lid. Also check for any stray bits of plastic at the ends of the blades and give the spindle and inside of the rotor hole a slight sanding just to remove any roughness left over from printing. The rotor should spin freely when you flick it without the lid on, and you should get a good 'wheeeeee' when you hold it together and blow.

To assemble, place the rotor on the spindle and then dab a few drops of lubricant into the axle and onto the rotor and base plate. Using a different matchstick, carefully spread glue along the top face of the base. I found it's easiest to keep a finger on the air inlet side while you're doing this so you don't end up with it stuck on the end of your finger.

Very carefully put the top plate onto the base making sure the two posts line up first, then turn over, press down onto a flat surface and hold for a minute or so until the glue goes off.

Once it's stuck, make sure you wipe any excess glue off the outside else you may end up with it permanently attached to your lips. Now BLOW and you should get a loud WHEEEEEEEEEEEEEEEEEEEEE which will very rapidly annoy family, friends, neighbours and pets.

Step 5: How Does It Work?

The Screamer operates on the same principle as a siren, in that the air you're blowing in is channelled to one side of the rotor (turbine) causing it to spin. There is enough space between the rotor and case to allow flow underneath the rotor, and this high pressure air would normally exhaust through the slots at the top. However, because the rotor is spinning, we get an alternating build-up of pressure when the holes are covered (1st picture) then release of pressure when the holes are open (2nd picture) causing the air to be released as a series of pressure pulses at audio frequencies (sound). As there are 8 blades and 8 holes, these pulses are in phase and so give a maximum amplitude.

I did a trial using 8 holes and 7 blades so the pulses were out of phase and I still got the WEEEEE effect, but it was nowhere as good as 8 and 8.

Step 6: The Proof of the Pudding . . .

Here is the video of the Orange Screamer in action. Unfortunately, the level limiting and frequency response of the camera don't really give a true impression of how loud and piercing it actually is. The video was taken about 1.5 metres from the camera. The sound is not earsplittingly loud, but certainly loud enough to be uncomfortable and would be heard over a large area.