Moulding New Gears For A Micro Helicopter


So you’ve got a broken gear for you model helicopter, and don’t have a 3d printer handy. If you need your little helo flying right away, [James] wrote in to tell us about his solution. As you may have guessed from the title, he made a tiny mould and produced a copy of the gear he needed with it. Given the complications of printing or some tiny subtractive method, this little gear turned out really nicely!

The video after the break shows all the steps for doing this procedure. If you’d rather just skip to the results, check out around 10:00 to see the finished gear, and eventually the little guy in flight. As noted, he did have to drill a hole in the middle of the gear after the mould process, but this was the only machining operation.

The helicopter gears worked out nicely, but be sure to check out some of the other really interesting projects on the [xrobots], some of which we’ve featured here!


36 thoughts on “Moulding New Gears For A Micro Helicopter

  1. first thought (havent watched the video):

    if your gear is broken, where do you get a working one to make a mould from? Last time a gear broke in a RC heli i had, it was a broken tooth, and that would be copied into the mould, unless i was very dexterious and could add the missing shape manually

    1. Most of those tiny helicopters are coaxial, and that means they have two motors, with identical gears on them, and two similar (but not exactly identical) gears on concentric main shafts. So if one gear breaks, you have a second good one to make a copy.

    2. Make the mold before you start flying your heli :) Or buy a spare and then don’t have to buy again.

      When I started this flying hobby 8 years ago, parts were really expensive back then. $10 – $15 for each tiny brittle plastic gears or plastic parts. Nowadays, parts are a lot cheaper from China.

      I really like this hack for the reason that I can make my own stuff instead of waiting for parts in the mail and disposing of all the packing material that come with the order.

    1. Could have removed the shaft from the heli or used a drill bit the right size if the shaft isn’t removable.

      Using the shaft from the heli would make the gear even more precise than the original because the hole in the gear would be exactly the size of the shaft.

      Still cool to make your own parts. That urethane resin is likely stronger than the Nylon or whatever cheap plastic the original gears are made of.

      One potential problem with making one gear stronger is the next piece to break will likely be a different gear. Make them all stronger and it might bend shafts instead of breaking off teeth.

  2. What I’ve learned from years of HaD: You can’t build parts without a 3d printer, and you can’t make them do anything without an Arduino.

    Does the polyurethane hold up? Is it possible to cast in nylon without industrial vacuum equipment?

    1. You could melt Nylon into a ‘high temperature’ silicone mould which can also be used for metal casting. The Polyurethane seems to have held up ok though, I’ve had a few crashes since I posted this and the teeth are still on so far.

    2. Nice and to the point, but well, thats just showcasing and mostly eyecandy. A tinkerer should choose the tools according to his abilities not to the depth of his pockets and should grow with the problems to solve instead of the first thing seen.

      However there are various branches of tinkerers with their respective skill set, therefore there are various branches of tools and skills needed and shown. No problem with that one, at least to me…

  3. This is epic win! There are so many frigg’n proprietary gears, props, etc. in remote control aircraft hobbies…it used to be something breaks and you don’t have it in your kit you have to order a new one which takes forevvvver.

    Awesome job here!

  4. ugh. I am pretty sure I have posted this exact fix in comments oh so many times before but yeah, silly putty- take gear and press in. lift out and turn so teeth match up and press in again so you have a “full” set of teeth aka the broken tooth is sitting in a full notch. Dump in whatever epoxy or magical substrate and let cure. Peel the gear out and (i use a nail file) smooth out the bumps. Works great and glad James got things working again :)

    1. I suspect you are using snark for snark’s sake. Either that or English isn’t your first (or second) language.

      Clearly in the very first sentence it states “don’t have a 3d printer handy” that would insinuate you don’t have a CNC machine or full resin casting shop available either.

      Should you find not using the ‘right tool for the job’ uninformed, I suggest changing your handle as hacking is not the hobby for you.

    2. Yes, for a cost you can use a pressure pot and a vacuum chamber to remove bubbles in the mould and cast, but for a one off this is the most cost effective way. For a cost I could have just bought a new helicopter, or the gears from the manufacturer for that matter, but the key message here is the principle of making inexpensive replacement parts without specialised equipment.

      1. The pressure chamber is completely redundant if you paint your item with silicone as the first layer. Dab out any bubbles and you’re fairly safe. There’s also silicone types that don’t require degassing.

  5. This reminds me of the time I fixed my friend’s old VW odometer by making a new gear that’d broken (in half) inside. I took a half of the gear and moulded the teeth into hotglue, let it set, rotated a little , added more hotglue until I had made a complete rotation and had a full negative of a whole gear. I then filled this with epoxy, let it set and drilled out the centre hole. Slid it on the original shaft and It worked!

  6. Nice work. I first did this about 10 years ago to replace a gear in an old projector – luckily it had 2 identical cogs. Then I used waxed plaster-of-paris for the mould. Since then I have copied gears for slot cars very successfully. I have used polyurethane and polyester (fibreglass) resin and epoxy – all of which break much easier than the original nylon or acetal part. Incidentally I usually use the shank of a drill bit in the mould so I don’t have to drill out later.

  7. I have had some success moulding replacement parts from casting alloy (aka BiSnPb) or BiInSn for the envirowhiners.
    Might work for something this small, just use the 100C alloy then fill with carbon fibre filled Epoxy and once cured remove from mould.

    Another worthwhile hack is to make the inner section of the gear from metal wire (cheap) to reinforce the delicate teeth then mould as normal.

  8. One thing to watch out for, Epoxy does get hot when curing. To overcome this use the 3 hour variant and whatever you do don’t use Superglue + baking soda with 100C alloy unless you want a meltdown on your hands.
    Here speaketh the voice of painful experience.

    Another worthwhile trick is to make some Polyliquid.
    This is a mix of Polymorph ™ and acetone, which gives you a slurry that can be dripped into the mould then allowed to cure.

    1. Polymorph ™ and acetone? -That sounds interesting. Does the Polymorph™ still set as hard once cooled? Is its melting temperature the same as it was, or lower? How much detail would this give- could it be used to form a small cog as above for instance?

      Thanks for any useful info bothersaidpooh!

  9. I have the same problem – same cog with three teeth missing. I am trying to source a new one but will mould if I need to. BTW I have sourced new rechargeable internal LiPo batteries from China. 250 Ma Hrs instead of the weaker originals. They work great. $17 US for ten with free tracked postage to UK. Let me know if you need details or just want a couple of mine. Cheers – Jerry;

  10. Hi James, following your project I will try to replicate a broken gear from an Apple floppy drive. I was wondering why you used Smooth-Cast 65D and not the 300 series. On their site 65D is defined as semi-rigid.
    Thank you

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