A few years back I designed a small axial flux generator of about 100W to be used for generating wind power, but I ran into the problem of making a suitable turbine for it. The methods I found online, like carving the blades out of wood or cutting them out of PVC pipe, did not appeal to me. I wanted something more precise and efficient.
The answer came to me after I started 3D printing, more than a year ago. The maximum height my printer can do is 150mm and my blades needed to be 500mm long, so I designed the blades to be printed in 4 sections that are to be fused together.
At this point I must make it clear that these blades need to be reinforced with a coating of carbon fibre or fibreglass, to be able to withstand the centrifugal forces that otherwise would rip it apart.
The cross section of the blades is NACA 4412 aerofoil and the TSR (Tip Speed Ratio) is 7. That means, in 10m/sec wind, the tip speed is 70m/sec and that translates to 1336.9rpm rotational speed. The Betz limit for a 1m turbine in 10m/sec wind is 278W, so it should be easy to extract in excess of 100W at this wind speed (36km/hr).
Step 1: Download and Print the Files.
Download the six STL files from Thingiverse, http://www.thingiverse.com/thing:1636095 and print in ABS with 3 perimeters. The inner piece, that bolts to the hub, needs to have high percent infill, the rest doesn't matter as they are not structural parts. The necessary supports are part of the models, so no extra supports are needed. Good adhesion to the print bed is essential as the bottom ends of the sections must remain perfectly flat.
Step 2: Cutting Away the Supports.
Using a sharp blade, trim off the brims and side supports. No need to be too fussy at this stage, just make sure the ends are nice, flat and smooth..
Step 3: Fusing the Blades
Make small dowel pins out of whatever you can think of, such as toothpicks of matchsticks or pieces of wire.
The material doesn't matter but they have to be a good fit into the holes that serve to align the sections.
Once you are happy that the whole blade is in nice alignment, with minimal gap between sections, just soak a few drops of acetone into the join lines. I used a small syringe to apply the right amount.
At this stage make sure the leading edge of the blade is perfectly straight and give it a couple of hour for the acetone to evaporate.
After that use a flat file to sharpen up the trailing edge of the blade and smooth any remaining support remnants.
Finish off the blade with some sand paper. Repeat the exercise for the other two blades.
Step 4: Reinforcing the Blades
Unfortunately I don't have images for this step but it is fairly straight forward if you have any experience with composite materials fabrication. There should be heaps of online information about laminating fibreglass.
I happened to have some carbon cloth left over from another project but you could use glass cloth with epoxy resin, although in that case one layer might not be enough. I cut my carbon cloth jut large enough to be able to fold it over the blade with a bit of extra. I painted the blade with epoxy, applied the cloth and painted some more epoxy on the outside of the cloth, just enough to saturate it. You don't want the epoxy to be dripping and you don't want to fill in the 4 mounting holes in the blade either. When ready, I put the whole blade into an improvised polyethylene bag and pulled a vacuum on it with a repurposed fridge compressor. I left the bagged blade in the sun, with the compressor (vacuum pump) running for a couple of hours, until the epoxy had set. It is very important to make sure the blade is straight before the epoxy sets, otherwise you will have a bent blade that cannot be straightened. After the blade comes out the vacuum bag it will require quite a bit of sanding and repainting with epoxy, but this time no vacuum bag required. Make sure you use a dust mask when sanding composite materials. The mounting holes of the blade have to be reopened by piercing through the reinforcement skin. Use the 3D printed hub for guiding your drill bit.
Repeat the exercise with all 3 blades. By the time you are doing the third one, you will be an expert and wishing you could redo the first one.
Step 5: Finishing Touches
To mount the turbine to a generator, you will need to have a machined metal disk, on the shaft of the generator, that has the same diameter as the 3D printed hub of the turbine. The 3D printed part only serves to position the blades and to locate the spinner. The mounting bolts must go right through into the metal disc including the ones that hold the spinner.
So you need 12 M3x16 Stainless steel bolts for the blades, out of which 6 must have flat heads, and 3 M3x20 Stainless bolts for the spinner. The mounting holes on each blade that are closest to the outer edge of the hub must be chamfered so that the flat head screws don't interfere with the spinner.
I sanded the spinner and gave it some coatings of UV resistant paint, but this is optional.