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Air_Raid_Marakids1H1A7694

Air raid sirens have always fascinated me, especially the ones from World War II. The infamous wail indicated danger but also sounded the all-clear, inspiring both fear and relief. They’re also just awesomely loud. Nowaday there are electronic sirens, but most civil-defense sirens are still the mechanical kind — basically blowers designed to make as much noise as possible. I designed one that you can cut out on a CNC router and assemble in an afternoon!

Online I found a guy who’d made a DIY siren, but when I saw how much effort he had to put into cutting, balancing, and aligning all the fins and spacers on the rotor, I instantly thought of our ShopBot CNC router as an elegant solution — it could cut a perfectly balanced rotor every time. I just needed a motor.

Turns out that fractional horsepower AC motors cost $100 or more — so I decided to use the lab’s crummy old Harbor Freight bench grinder, because it actually looks like some old WWII sirens I’ve seen, and it costs only $45 brand new.

Now I could start on the CAD work. I started by measuring all the relevant points on the bench grinder with the wheel covers and grinding wheels removed: screw holes, shaft diameter, and washer thickness. I wanted to make the rotors (or “choppers”) easy to assemble, concentrically accurate, and capable of withstanding considerable centrifugal force. So I decided on sandwiching several layers of plywood, held together by wood glue and ¼” dowels. The dowels helped to line everything up accurately while also acting as a direct brace against the centrifugal load.

The stators were simple; I only had to make sure the rotor blades completely covered the openings in the stator. It’s traditional to use two simultaneous tones, in a 5:6 frequency ratio; so I went with 5 ports in one rotor and 6 in the other. I threw all the parts together in Autodesk Inventor — and to my delight it actually looked like a vintage WWII siren.

Next came the CNC cutting. Setting up the CAM was relatively simple: all the cuts were simple profiles, with a few partial-depth “pocketing” operations. The plywood had to be cut down to 24″×18″ to fit on our ShopBot Desktop (http://www.makershed.com/ShopBot_Desktop_Package_p/dssbdp.htm). I managed to fit all the parts for two rotor-stator assemblies on one sheet of ¼” plywood and two sheets of ¾”, and cutting went off without a hitch.

Assembly required patience and gentle persuasion with a rubber mallet. Testing the rotors was the scariest for me — I had no real idea if the thing was going to hold together or explode catastrophically under the centrifugal stress — but they spooled up to full speed and blew a surprising amount of air with almost no detectible vibrations. Sweet!

Next I mounted the stators, and discovered that the screw holes on the grinder were not concentric with the motor’s shaft, so there was some rubbing happening between the rotor and stators. I widened the screw holes on the stator so I could adjust for the misalignment, and sure enough I got the rotor to spin freely within the stator with only a 1/8″ gap between the two.

It was finally time to see if this thing was going to work. So again standing off to the side I switched on the motor. At first there was no noise at all as the motor slowly picked up speed. All of a sudden a faint wail began to emanate from the device, which quickly got louder and louder as the motor continued to pick up speed. By the time the motor got up to full speed the siren sounded so real and loud I had half a mind to duck and cover under my desk — it works!

Steps

Step #1: Prepare the CNC files.

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CNC Air Raid Siren
  • Download the files at this link; they're in DXF, BAK, and DWG formats, so you can choose your favorite format to work with.
  • When you open the files in your CAD software, make sure they're scaled correctly before cutting. The outer diameters of the stators should be 7-1/2" and the outer diameters of the rotors should be 6-3/8".
  • The Stator Base B part (on both the 5 tone and the 6 tone) has a 3/8" recessed pocket cut that starts at the 6-1/2" diameter circle and ends at the the inner 2-1/2" circle which is cut all the way through.
  • The Rotor Base part (on both the 5 tone and the 6 tone) has a 1/2" recessed area that starts at the outside diameter 6-3/8" and goes to the inner 2-1/4" circle. The very center 1/2" circle goes all the way through.

Step #2: Cut the parts.

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CNC Air Raid Siren
  • Arrange the files to fit on the plywood according to the dimensions of your CNC router. For example, our ShopBot desktop accommodates 24"x18" stock, so we fit all the parts on one sheet of 1/4" and two sheets of 3/4" plywood.
  • Cut the stator base plates (2), stator top rings (2), and rotor top rings (2) out of 1/4" plywood.
  • For the 5-tone side, cut the stator slats (10), rotor blades (10), stator base plate (1), and rotor base (1) out of 3/4" plywood. For the 6-tone side, cut the stator slats (12), rotor blades (12), stator base plate (1), and rotor base (1) out of 3/4" plywood.

Step #3: Assemble the rotors and stators.

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CNC Air Raid SirenCNC Air Raid SirenCNC Air Raid Siren
  • Sandwich the rotor parts as shown here in red, applying wood glue between the parts. While glue is still wet, use the rubber mallet to gently hammer in 2" dowels to pin the rotor together. When it's all tight and flush, set it aside to dry.
  • Sandwich the stator parts as shown here in yellow, again applying wood glue between the parts and using 2-3/4" dowels to pin the rotor together. When it's all tight and flush, set it aside to dry.
  • (Optional) If you're painting your siren, now's a good time to paint the parts before final assembly.

Step #4: Mount rotors and stators on grinder.

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CNC Air Raid Siren
  • Dismantle the grinder by removing the wheels and covers. Use the grinder's original mounting hardware to mount the stators to the grinder body as shown. Make sure they're concentric with the shaft (you can adjust them slightly later).
  • Use the washers and nuts from the original grinder wheels to mount the rotors on the shafts. Tighten everything securely.

Step #5: Test!

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CNC Air Raid Siren
  • Spin the rotors by hand to check for concentric accuracy -- if they rub the stators, adjust the stator mounting position until the rotor spins freely with about 1/8" of clearance all around.
  • Once you're satisfied the rotors are running freely, power up the grinder motor.

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