Introduction: Faux Metal Tesla Turbine
This instructable is a continuation of a post from 2011 (https://www.instructables.com/id/Machining-a-Mini-Tesla-Turbine-from-Cardboard/). The Cardboard Tesla Turbine (CTT) was based on a Popular Mechanics article describing the construction of a steam-powered, metal turbine that was published nearly 50 years ago. A year later, I've added spray painting and detailing tips as well as some disappointing results when attempting to power the CTT using a cartridge of compressed CO2. I chose colors to create the appearance of precision machinery crafted from brass, copper and titanium alloy. The rotor discs were painted to resemble a carbon fiber composite of high tensile strength.
The original project was made from closely spaced, cardboard discs centered on a threaded steel rod. The rotor assembly was suspended in a bathroom tissue role by a supporting assembly. The aviation grade bearings were housed in retaining assemblies. When a stream of air from an electric air pump was injected into the manifold, the stream spiraled inward between the discs toward exhaust ports surrounding each disc's center. The stream exited the turbine along the axis of rotation. Peak rotor speed was measured at 2,220 RPM. Note that the following instructable assumes that you have a fully functioning CTT that is ready for painting and customizing.
Anything attached to the rotor shaft could fly off suddenly if not properly secured. In addition, the sudden decompression of CO2 as the gas escapes from the cartridge produces extreme cold. Use heavy gloves when handling the cartridge.
Step 1: Items You Will Need
300 Grit Sandpaper
Ceramcoat All Purpose Sealer (http://www.deltacreative.com/pcid/86/Tips-Technique.aspx) or something similar
Hobby Paint Brush
Silver Marker Pen
Spray Paints: Black, Blue, Brass, Clear Enamel, Copper & Silver
Spray Paint Safety Mask
Popsicle Sticks to Use as Rotor Shims (4) - 1.5 mm thick
25g Threaded CO2 Cartridges w/Regulator (http://www.innovationsaz.com)
38 cm plastic zip ties (2)
Plastic Propeller & Mounting Hardware
Step 2: Disassemble & Prep Components
Use a screwdriver to disassemble the turbine (unfortunately, you can't avoid this step). It's important to label all components so they can be reassembled properly.
Prep all cardboard surfaces w/a coat of Ceramcoat. Sand w/300 grit when dry. Remember not to saturate the cardboard! Ceramcoat is water based and can seriously warp (and ruin!) your turbine as it dries.
Step 3: Spray Painting
Here is the painting scheme for the turbine. Several light coats will produce a more professional appearance than one heavy coat that sags or drips.
Base - gun metal blue
Bearing Retainers, Manifold Straps & Support Assemblies - gloss metallic brass
Manifold & Plastic Spacers - gloss metallic copper
Rotors - high gloss black
Turbine Housing & End Seals - textured metallic silver
The rotors require special attention. According to the helpful folks at the Tesla Engine Builders Association website, turbines work better when the rotor discs are as smooth as possible (http://www.teslaengine.org/page/bi.html). Apply 3 coats of high gloss paint. Gently sand each coat w/300 or higher grit paper when dry. The paint manufacturer recommended a clear top coat to preserve the gloss finish; so I added 2 successive coats of clear enamel. I used a silver marker pen to ID proper orientation of each disc and facilitate rotor reassembly. BTW, one rotor was ruined when I applied too much Ceramcoat. I painted it before realizing the extent of the damage ...
Step 4: Reduce Rotor Gap
While the original CTT used 3 nylon flat washers between rotor discs, this upgrade reduces that number to just 2 washers. Discs are clamped in place by a flat washer and nylon hex nut. In a preliminary test using an electric pump for inflating beach toys, turbine speed increased from 1,980 to 2,220 RPM when the rotor gap was decreased from 5 to 3 mm.
Step 5: Rotor Clearance & Alignment
Mount turbine shell on base using ceramic insulators as stand-offs. Next, reassemble rotor and install in shell. Verify 1.5 mm clearance between disc edge and turbine shell by inserting popsicle sticks as shown. In addition, sticks ensure that rotor remains properly aligned when you reassemble the support and bearing retaining assemblies in the following step. A silver reference line is visible on disc.
Secure manifold to turbine housing w/zip ties (I held ties in place w/two plastic cable clamps bolted through the support bar as shown). Trim excess w/scissors. Make sure all components are thoroughly dry before moving to next step. If any components become paint stuck, it is nearly impossible to separate them (words of experience!).
Step 6: Reassemble Remaining Components
After the rotor is nestled in the shell, reassemble end seals, spacers, bearing housing and retaining plates (don't forget to install bearings). Refer to original instructable as needed for proper assembly sequence: http://(www.instructables.com/id/Machining-a-Mini-Tesla-Turbine-from-Cardboard/). Slide prop on rotor shaft and secure firmly w/flat washers, split rings and nylon hex nuts.
Step 7: Detailing
Use rub-on letter stencils to identify your turbine. I removed the turbine and manifold then placed a strip of paper tape along the base and vent slot to ensure an even line of lettering. Lettering Tip: Locate the letter in the middle of each line, apply letters in sequence going forwards and then backwards.
BTW, wording on the manifold reads: "CAUTION HIGH PRESSURE VENT." When your lettering is complete, apply a protective coat or two of clear enamel.
Step 8: Preliminary Test
Test your turbine by manually spinning rotor. Ensure that outside discs do not rub against retaining bolt heads which can ruin the gloss finish or physically damage discs at high RPM. Tighten clamping nuts on threaded rod to increase side clearance if needed.
Step 9: Results: CO2 Vs Air Pump
Spool up turbine by attaching the regulator to a CO2 cartridge and inserting nozzle in manifold. Stay clear of the vent slot as you open regulator pressure valve because dry ice may be ejected.
The maximum rotor speed of nearly 300 RPM when measured by a Digital Mini-Tach (http://www.towerhobbies.com) was disappointing. There just wasn't enough gas in the 25 g cartridge to drive the turbine. Compared to an electric air pump (PolyGroup, Model AP-01; no pressure specs) which spooled up the turbine to over 2,000 RPM in seconds, a CO2 powered CTT is unusual but unimpressive.
Step 10: Before & After Shots (w/Mods)
For the record here are before and after shots of the CTT w/a few minor modifications. The 12 hex nuts securing the bearing assemblies were replaced w/matching brass acorn nuts to improve the general appearance of the project. In addition, I slipped a cardboard tube over the end of the injection manifold (painted flat black). The prop was replaced by a more interesting hypnotic wheel pattern obtained from a Google search. This was glued to a discarded disc, sprayed w/clear acrylic paint and then mounted on the rotor axle w/brass acorn and hex nuts.