Quickly I would like to discuss how amazing desktop 3D printers are. So far I have been able to make two versions: the first is blue and printed by my schools 3D printer (6.07 grams), the second is clear plastic and made by a Makerbot Replicator 2 (4.729 grams). For a reference point Cornell's weighed 3.89 grams. Between my first and second designs the weight of the wings has gone from 4 grams to 2.6 grams because the wing thickness went from 0.02 inches to 0.008 inches (200 microns). After receiving comments about more flexible wings, I have made a new version with a total wing weight of 1.968 grams making my ornithopter a total of 4.10 grams(the total weight of 4.729 grams minus the difference in the wings (2.6 - 1.968) grams) making it only 0.21 grams off of Cornell's weight! I am not done with this project yet and would really love to get a design that flies, so please give me your comments and ideas!
Step 1: Parts/Tools
The majority of the parts for this design are 3d printed. The rest are as follows.
Motor - The exact one Cornell uses I coincidentally already owned and is ideal. It is a 1g pager motor with a planetary gearbox that can be found HERE.
Paperclips - Try and find very thin (diameter wise) ones.
Batteries - Can use anything around 3V to test and 7.4V for test flights. The arduino's 3.3V pin or a power supply can also work. Power supply is definitely the best but not needed.
The rest of the parts are 3D printed!
3d Printer - Pretty vital.
Drill - With some smaaaalllllll drill bits.
Step 2: 3d Printing
There are five parts to the ornithopter that need to be printed: the body, two driving arms, and two wings. I would like to note that there are two versions of these parts below. The 'Ver3' ones are much more refined and precise with the wing thicknesses of 200 microns and 100 microns and holes drawn into the sketches, so no drilling is needed. If you don't think your printer can print to that quality level then just download the parts that don't say 'Ver3' and they are the exact same ones used on the version 1 (blue model) I made. Apparently some programs have had trouble loading the files to the correct dimensions so I have added pictures to this step that show dimensions (in inches) for all the first version parts. This should allow anyone to scale them properly.
Cornell, as far as I know, doesn't offer the files for the parts they 3d print (this was a little irksome as it seemed like the point of Cornell's project was that they could be easily manufactured with 3d printing). Theirs has a resolution of 16 microns and the Makerbot Replicator 2 has a resolution of 100 microns, but that's plenty close for this project. I wasn't sure of the precision so the first version parts are a little beefier than they probably needed to be. Anyways they both work and are great places to start out before modifying the design!
Step 3: Motors
The motor for this project is quite cool. Weighing in at only 1.2 grams it packs a lot of power. Recently there have been several more planetary gear pager motors that are available at robotshop and other sites so feel free to experiment with those. They all run on 3v (although Cornell ran them at 7.4V for the flight tests) and will need the motor nib to be drilled out.
The one thing to note and pay attention to is how fragile the wires are. They pull out EXTREMELY easily. My advice is to use a zip tie or tape to keep them against the body and take the stress of the wires off of the base connection with the motor because that is where they lose connection and stop working (see my picture if you are confused what this looks like). Also soldering very thin wire to the leads of the motor helps from pulling on them too much. It is also necessary when flight testing so the ornithopter only has to lift its weight off the ground and not the wires.
I took one apart to see if I could get it working again and found out how cool the insides are. If the top and bottom pieces of the motor separate it's no big deal because the bottom is the coil of wire and drive shaft while the top part is the gear box. They can be placed back together with no trouble. Unfortunately I found fixing the motor by soldering new wires into it impossible. My one semi-solution is to drill two holes where the wires came out of the motor and if you slip two wires into it and jimmy them around a bit you can get the motor to work, albeit with an intermittent connection. Just things to keep in mind!
Step 4: Drill
NOTE: If you used the 'Ver3' parts with built in holes then you can skip this step.
I didn't print the holes for the paperclips into my design for the ornithopter parts because of the issue of precision. To ensure a tight fit I pulled out my tiniest drill bit and went to town (I feel there's a few jokes here). There are many holes that need to be made, one through both wings, ones on each end of the forks, one directly above the motor mount on the body, and two through the chamber for the wings. Check out my photos of the assembled and drilled parts because it clears up a lot of questions. The last thing you need to drill is the top piece of the motor (the part that spins). This is quite tricky and leaves little room for error so I first suggest making a little groove that the drill bit can sit in nicely by stabbing it with a needle or something similar. Make sure when doing any drilling to use a vice to hold the fragile parts steady.
What I've found to be the best way to do this is to spin the bit at the highest speed possible and use very little force to push it through the plastic. This prevents stress fractures in the thin layers of plastic. For the fork that drives the wings you will probably have to drill a few holes at different positions and test to see which gives your wings a full range of motion.
Step 5: Assembly
Now you are going to have to put it all together. Pull out your paperclips and straighten them. For the offset crank you should put a hook at the bottom for the motor then bend it out slightly making two offsets for the driving arms, they should be approximately 30 degrees apart. Leave a short straight segment at the top of it after the offsets to go through the top of the body. Check my pictures, it's much easier to see than explain. Bend away. Expect lots of trial and error. The size of the offsets depends on where you drilled your hole in the wings. It should be half the distance between the hole when the wing is at its mid position and the hole when it is at its fully extended position.
For pinning the wings to the body and the driving arms to the wings use little L shaped segments of paperclips (tiny, straight sewing pins also work well). Check my pictures for a clear example of this.
I would like to note in my first design for the body I didn't have a way to get the wings into the section of the body where they would be mounted so I cut a slit through one wall as seen in the pictures. This allowed me to slide the thin center beam of the wings through it. You may want to trim the body anyways to reduce weight.
For mounting the motor just get some sand paper and slowly increase the size of the hole at the bottom of the body until there is a tight push fit. I didn't have to use anything other than friction to keep the motor in place.
Step 6: Wings
Alright this is the part that I would like lots of suggestions on! Obviously the wings have to be super light and Cornell did a special 3D printing technique to create theirs so I'm not sure how to replicate it other than by printing very thin layers. The Makerbot Rep 2 is capable of 100 micron thickness and Cornell used wings 40 microns thick so it's impossible to get the same resolution but we can get close!
I have also included files for my open wing title 'EmptyWing' so you can try using cling wrap or whatever you would like instead of the 3D printed filling.
Step 7: So Fly
You are ready to flap. Make sure when you are holding the ornithopter that your fingers don't come in contact with the wings, driving arms, or offset crank. I just tried to hold it but a clamp would be ideal. For power I used an arduino uno and hooked the two motor wires up to 3.3v and ground (doesn't matter which goes to which, it just changes the flapping direction) but this is optional, three AA batteries or anything around 3v would also work.
For flight tests you will need to suspend your ornithopter. I did this similar to how cornell did, by using fishing line through two holes in the frame that is nearest to the ornithopters center of mass. Finding the center of mass is all just guess and check. Try and make the fishing line as tight as possible, in my video you can see it vibrating, allowing the ornithopter to move around too much. Tie a small knot in the fishing line for the ornithopter to sit on so it won't be in contact with the table. To try and actually get it to fly I trimmed the frame down and put very thin wires (individual strands from stranded wire) connected to the motor so it wouldn't have to haul up the weight of the wires up. I also used a 7.4V battery like Cornell did. Cornell recorded a rate of flapping around 30 Hz and while I couldn't give you an exact number for mine it was definitely MUCH higher than with 3V.
Step 8: Final Thoughts
I sincerely hope you enjoyed this, if it got you thinking then it has done its job. There are a few difficulties I had that I wanted to discuss. I think the precision of the 3d components can definitely be improved with iterations, but unfortunately my limited access to 3D printers forced me to make beefier parts to ensure they worked.
I have a few ideas for variances, the most interesting one would be using a 2g brushless motor I saw on some hobby site (capable of much higher torque offering around 28g of lift) in a slightly larger over all build. This would eliminate a great deal of the challenge of such flimsy, tiny parts and would perhaps even be able to carry a payload. The other thing I was hoping to see was the use of a small camera and/or use of motor control to perform wireless controlled flight. The last thing I thought would be just plain cool was if rather than carrying a payload of batteries a laser could be used to give energy wirelessly to drive the motor. Really how awesome would that be?
I will post when I am able to make my next iteration, but if you have any ideas for existing modifications please let me know! This means you all need to take it up and show me what amazing places you take it. Let me know. Thanks!
Step 9: Updates
Since posting this there have been many great ideas from the comments! Here I will post my trials of these ideas:
So far I have drawn up two revised wing designs, both with tapered thicknesses to allow for more flex when flapping and thus more upward thrust. I will plan to have test videos of them soon. I have put the stl file for the new wings down below, they work with all the 'Ver3' files. The wings weigh under 1 gram each bringing the total weight of the ornithopter down to slightly over 3 grams!
I have also done a few experiments trying to print straight onto plastic wrap so I wouldn't have to print out the filled in part of the wings. This would reduce weight, but so far the plastic has not bonded well with the plastic being printed onto it. I will try using some light adhesives and hope to have an update soon.