Introduction: Machining a Mini Tesla Turbine From Cardboard
This instructable describes a cardboard version of an all-metal project that was featured in an issue of Popular Mechanics from 1965 (http://www.tfcbooks.com/teslafaq/q&a_031.htm). The turbine consists of a rotor suspended concentrically in a cylindrical shell. The rotor is fabricated from 16 shaft-mounted discs, each with a contoured edge to reduce turbulence. The rotor spins on low friction bearings.
An injection manifold directs a stream of air through a lengthwise slot in the turbine shell. The stream flows between the disc spaces in a spiral path and eventually exits through exhaust ports located around the center of each disc. This coil of moving air "drags" the discs in a circle causing the rotor to turn.
You can build this project using cardboard sheets obtained from end covers of discarded 3-ring binders, hand towel and bathroom tissue rolls, a high speed electric drill used as a lathe, basic hardware, white glue and simple hand tools.
Caution: Anything attached to the rotor axle must be securely held in place. With a sufficient air pressure, the turbine rotor can spool up to several thousand RPM in just a few seconds!
Step 1: Turbine Shell
The shell or housing of the turbine is made from one of those jumbo, bathroom tissue rolls used in public restrooms of chain restrauants and big box retail outlets. Make sure the edges are not dinged! As an alternative, you can use a cardboard mailing tube with the same dimensions (3-1/2" L x 3-1/2" ID x 1/16" thick).
Step 2: Cut Rotor Blanks
Of course, this cardboard conversion did require some mods of the original all-metal design. One change was the use of thicker material for the rotor discs -- 1/8" thick bookbinder's board vs. 1/32" thick stainless steel. I bought a stack of used binders with loose/broken rings at a yard sale for my supply of cardboard.
Remove cloth or plastic covering from binders. Cut twenty, 3-1/2" diameter rotor blanks from cardboard. Actually, only 16 blanks are needed, but extras are useful as spares. BTW, I used scissors for the prototype; but then switched to an adjustable circle cutter for the subsequent version to save time. Drill a 1/4" shaft hole through the center of each blank.
Step 3: Prepare Blanks for Grinding
Insert a threaded, 8" x 1/4" diameter rod through the shaft hole of each blank; this serves as the rotor axle. I temporarily secured the assembly with two 1/4" I.D. diameter flanged thread inserts just because they were the first items I grabbed from my parts box :>); but a steel nut and backing washer will work just as well.
Step 4: Grind Blanks to Specs
This step was a serious test of patience. Because my electric drill was too underpowered to grind the blanks properly, I replaced the tired 6V NiCad with a wired connection to a variable DC power supply. At 12 V and a 2.5 amp draw at least there was sufficient torque to begin the next step. Note: only attempt this workaround if you can afford to toast your drill!
Next, chuck the axle in the drill and secure the drill to the edge of your work surface. Grind blanks with course sandpaper to 3-1/8" diameter using a light but consistent force on the sanding block. Remember, uneven pressure can produce flat spots on the blanks
which will ruin your rotors (words of experience; see Step 6!). BTW, don't trash these rejected rotors. If you decide to spray paint your turbine, they can be used as scrap to test various paint colors.
Construction tip: After a minute or two of sanding, remove blanks from shaft, check diameters, then shuffle before replacing them on the axle. Resume sanding. Yes, this step is a time consuming, pain in the feathers; but if you follow this procedure all blanks should be evenly sanded, ground to the nearly the same diameter and dynamically balanced.
Step 5: Don't Make This Mistake!
Pressing too hard on the sanding block and not shuffling discs can ruin your blanks! These rotor discs were tossed in the reject bin because of irregularities.
Step 6: Rotor Assembly
Remove discs from shaft. Reassemble rotor, placing three, 1/4" I.D. nylon flat washers between each disc. Replace steel hex nuts and washers with 1/4" nylon flat washers and hex nuts. Clamp discs firmly on shaft.
Step 7: Verify Rotor Clearance
Insert rotor assembly into shell. Note the six popsicle sticks that serve as shims to verify ~1/8" clearance between rotor edges and the shell -- not too shabby! :>D
Step 8: Drill Exhaust Ports & Contour Edges
Remove all discs from shaft. Take one disc and mark off 120 deg increments along circumference. Extend all three points to disc's origin with a ruler. Insert a 1/4" bolt through a matching 1-1/2" O.D. diameter washer, then insert assembly through shaft hole of disc. Trace around circumference of washer. Remove bolt and washer. The intersections of the inscribed circle and the three radii will identify drill hole centers for the exhaust ports.
Carefully drill 3/8" holes where marked to make exhaust ports for each disc. If the holes have ragged edges (mine looked like they were chewed by a mad gerbil) wrap fine grit sandpaper around a pencil and lightly sand each hole until smooth. Also, sand each disc surface until level. The black index marks on the disc faces and edges ensure that holes line up when rotor is reasembled.
Chuck a disc in the drill and sand edges with course followed by fine grit paper until you have a smooth, sharply rounded edge. Repeat procedure with remaining 15 discs. Use slight but consistent pressure when sanding otherwise, you may need to return to Step 3 to cut more blanks. :>(
Construction tip: Sand discs until index mark on edge is reduced to a ~1/16" width.
Step 9: Re-assemble Rotor
Re-assemble rotor. Line up exhaust ports so they are parallel with shaft, then hand tighten nylon hex nuts.
Step 10: Cut, Drill & Grind Inner End Plates
The rotor support assembly consists of the shell, the inner and outer end plates in addition to the bearing retainer subassembly. These essential structures maintain rotor clearance and stability.
Cut out six inner end plates, drill shaft holes and then grind the plates so they can be press fitted into shell with a slight resistance. Glue three plates together, ensuring that the shaft holes line up. Repeat this procedure using remaining three discs. Finish rough edges with very fine sandpaper.
Step 11: Cut, Drill & Grind Outer End Plates
Cut out two outer end plates, drill shaft holes and then grind plates to 3-3/8", the same O.D. as shell.
Step 12: Cut, Drill & Grind Bearing Retainer Plates
Cut out six bearing plates, drill shaft holes and then grind to a 3" diameter. Glue three plates together, ensuring that the axle holes line up. Repeat procedure using the remaining three discs.
Step 13: Rotors, Inner & Bearing Plates
I lined up the 20 rotor discs, 2 inner end plates and 2 bearing retainer plates on the shaft to show how machined parts should look at this point. The two outer plates were omitted from photo by mistake. :>O
Step 14: Mark & Drill Screw Hole Locations
Determine locations for three equidisdant screw holes on one bearing plate with a protractor. Place a pencil mark every 120 deg and 1/4" in from the circumference. Use this disc as a template to locate holes on the remaining bearing plate as well as on the inner and outer end plates. Making sure shaft holes of each disc serve as the common center. Drill holes where marked to accommodate #6-32 machine screws, flat washers and wing nuts (later replaced by hex nuts in Step 21) .
At this point, glue one set of inner plates to one set of outer plates. Repeat this procedure with the remaining set of plates. Use the machine screws to secure these assemblies until dry. Do not glue retaining rings to plate assemblies!
Step 15: Drill Vent & Bearing Holes
Drill a centered, 1-7/8" vent hole through the inner and outer end plate assembly which permits spent air to exit turbine. Next drill a centered hole through both retaining plates equal to the O.D. of your bearings. Exact hole size will depend on bearing manufacturer.
Step 16: Cut, Drill & Grind Stop Rings
I included this step as an afterthought to keep bearings securely positioned within retainer plates without using glue (too permanent) or tape (not secure enough). It's extra work, but you will have easy access to the bearings should they need replacing or upgrading in the future.
Cut, drill and grind four, 2" diameter bearing stop rings. The diameter of the rings' center hole should be ~1/8" smaller than each bearing's O.D.
Step 17: Assemble Rotor Support Structures
Attach a retaining plate to an end plate using a 3/4" nylon spacer as shown; retainer must face outer plate! Repeat procedure using remaining plates. Take a stop ring, center it and then glue ring to the inside of each retainer plate. The outer ring will be bolted to the opposite side of the plate for easy acces to bearing.
Step 18: Seat & Secure Bearings
The type of bearings used will have a huge effect on your turbine's performance. I used high speed, low friction, hybrid bearings (ceramic balls in stainless steel races) designed for R/C micro turbine engines; these are available here: http://www.bocabearings.com/productDetail.aspx?ItemID=1889. Cheaper bearings would work with a corresponding decrease in rotor RPM.
Seat a bearing into the recess of each retainer plate to verify proper hole size. Most likely the hole will be too large, so remove bearing, apply a thin layer of glue around the sidewall, then line with a strip of paper tape. Trim excess tape flush with scissors and finish plate surface with fine sandpaper.
Take an outer stop ring and mark three equidistant locations which are 1/4 " in from the edge. Use ring as a template for drilling holes through the remaining ring as well as through the retainer plate and inner stop ring. Hole size should accommodate #4-32 machine screws, flat washers and hex nuts.
Step 19: Lt & Rt Support Assemblies
Here are the completed rotor support assemblies with stop rings in place.
Step 20: Cut Out Air Slot
The turbine has a 3/4" x 2-1/2" slot cut lengthwise into the shell for the airstream to enter.
Step 21: Front, Side & Three-Quarter Views
These various photo angles display rotor assembly, supported by end plates and bearing retainers, suspended in shell. Unfortunately, there was too much wiggle room between the rotor shaft and I.D. of the bearings (bearings were metric, shaft bolt wasn't !!). So I used a 1/4" diameter cardboard tube from a discarded roll of dental floss and cut a 1/4" length to make a shaft sleeve, then peeled off a single layer of paper for a snug fit in the bore. I cut and slipped another 1/4" x 1/4" sleeve onto each shaft end so they rested against the bearings. This work-around kept the shaft secure and prevented back-and-forth play between the bearings. Lastly, wing nuts securing retainer plate to spacers were swapped for hex nuts to facilitate removal of outer stop rings.
Construction tip: If inner end plates slip out from shell, coat them with a thin layer of glue, then wind paper tape firmly around inner plate to increase diameter. Trim extra tape flush with scissors. Sand lightly with fine grit paper when dry. Repeat procedure until rotor supports are snug.
Spin the axle by hand. If there's drag, verify at least 1/8" clearance between outside rotor discs and machine screws installed in Step 15. Make additional room if needed by disassembling entire turbine and removing a disc and washer pair from axle.
Step 22: Mounting the Turbine
Essentially whatever works is OK. I cut a 4-1/2" x 6-3/4" piece of cardboard for the base and used thread spools as stand-offs. The tie strip is a wood Craft Stick that joins the mounting screws together for added support. Everything was held together with #6-32 machine screws. Cut or file a notch in each inner end plate to accomodate screw heads; make sure heads don't rub against outside rotor discs when everything is re-assembled. Lastly, glue 4 rubber feet to on each corner of the base.
Step 23: Test Run: This Is How We Roll
Before re-assembling the components, use a metal saw to trim end plate and bearing retainer screws flush with hex nuts. File screw ends until smooth. (BTW, bearings should be removed from their housings before cutting the screws to avoid getting metal dust in the races.)
The assembled turbine should look like this. The red and black disc on the shaft was made from a rejected stop ring. Hold a hair dryer near the slot (use cool setting if available) and direct the airstream at an angle into the turbine as shown to get things rolling.
Step 24: Inlet & Stand-Off Upgrades
Initially, I made a manifold from bendable straws that looked amazingly cool, but didn't work. Next, I taped a rectangular, cardboard flap directly behind the inlet to guide the airstream from the dryer into the turbine. Finally, I tried a slotted injection tube to introduce the airstream. In summary, if you're going for higher-end performance, use the flap; for appearances, consider the injection tube.
The flap option is easy to make. Just tape a business card to the bottom edge of the inlet. Adjust flap angle as needed to produce max RPM.
Here's how to make the injection tube. Construct the injector from a 1" diameter cardboard tube cut to a 5" length and corked at one end. The slot should be about 2-1/2" in length. Cut the width as narrowly as possible (less than 1/4"). Attach tube to hair dryer and align with slot on shell. Slowly turn tube CW or CCW until you locate the "sweet spot" that causes the rotor to spin. Also, try widening slot until you achieve max RPM. Finally, mark proper tube orientation and secure with rubber bands; or you can bolt the tube to the shell. I used a small, plastic funnel as a coupler for the dryer nozzle.
For a more professional appearance, replace thread spools with ceramic stand-offs. Because cardboard edges ding easily, rub a thin layer of glue around the circumferences of the outer end plates and stop rings. Let dry, then sand with fine grit paper.
Your Tesla turbine is now complete!
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