PROJECTS

CNC Air Raid Siren

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It’s loud, annoying, and fun. Cut the parts on a CNC router, then motorize them with a cheapo bench grinder!

Written by Dan Spangler

Time Required:

A Weekend

Cost:

$60–$75

DAN SPANGLER

is the fabricator for MAKE Labs and our resident retro technology connoisseur.

Learn more about CNC:

ImageSKILL BUILDER

CNC PANEL JOINERY makezine.com/cnc-panel-joinery

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Learn a bag of tricks to design and CNC cut clever joints in plywood, acrylic, and other sheet stock.

AIR RAID SIRENS FASCINATE 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. Nowadays 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 saw DIY sirens online and instantly thought of our ShopBot CNC router as an elegant solution — it could cut a perfectly balanced rotor every time. So I designed this siren for CNC cutters. Here’s how I made it, and how you can too.

1. Motor. Fractional-horsepower AC motors cost $100 or more — so I used the MAKE Labs’ crummy bench grinder. Get one at Harbor Freight ($45 brand new). It even looks like a WWII siren.

2. Cutting. I cut the plywood to 24"×18" to fit our ShopBot Desktop (Maker Shed item #DSSBDP, maker-shed.com). I fit all the parts for two rotor-stator assemblies on one sheet of ¼" ply and two sheets of ¾", and cutting went off without a hitch. Adjust them to fit the cutter of your choice.

3. Assembly. Building the rotors is a matter of glue, dowels, and gentle persuasion with a rubber mallet. Testing them was scary — would they hold together or explode catastrophically? — but they spooled up to full speed and blew a surprising amount of air with almost no vibration. Sweet!

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Next I mounted the stators and adjusted them to eliminate any rubbing. It was finally time to see if this thing was going to work. As the motor picked up speed, a faint wail began to emanate from the device, which quickly got louder and louder. 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! Image

Hear it wail, download the CAD files for CNC cutting, and get step-by-step build instructions at makezine.com/projects/cnc-air-raid-siren.

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