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Thread: First Build - Featherweight Bar Spinner

  1. #11
    Welcome to the sport!

    Given this rather reminds me of my first build and parts choices I figured I'd weigh in here. I'm going to try and not go too in depth, but rather than simply telling you, I feel its better to explain why so you can apply the logic to other things in the future.

    First of all, yes it is generally considered a bad idea to build a spinner as your first robot, however that doesn't mean you can't provided you take the necessary precautions during the build and testing phase. I did what many said not to and built a vertical disc spinner on my first attempt -

    - and like it has been said, it did go home in (not quite) a bin bag, (but close).

    Overview NST.jpg

    Testing was done in phases, were first the motors were tested, then the weapon on one motor, then on two, all of which had the robot G-Clamped to a work bench and I went out the room and use a mirror to watch the tests through the doorway. If you also check the system fail safes before attaching the weapon mechanism, you can stand right next to it in case something goes wrong without being in any danger.

    Second, looking at the parts you've listed, you aren't far off. Main issue is your drive motors, they are to small, but the 800W brushless motors are not necessary either. While that is becoming the norm, Conker 1 you see in the video ran 4x RS550 motors at 4S/14.8v, on a pair of 30A ESC's, each with a 16:1 gearbox. Run the numbers (P = V x I) and at 30A continuous, each side was using ~430W; that 215W per motor. While Brushed and Brushless wattages are not directly comparable, It gives you an idea of the power needed.
    Two geared 12V drill motors will happily move a FW robot, and used to be the norm. They are cheap, easy to obtain, and even come with a clutch if you need that facility. I'd suggest getting a pair of £30 drills, or if you have a little more money having a look at these - - If you look after them they will last years, and if the motors burn out you can easily replace them or upgrade to brushless in the future.

    Next, this applies to your weapon speed, but will also help you pick your drive belt ratio - - This calculator is very useful for getting an idea of what speeds or ratios you are going to need; i use it all the time to double check my calcs for weapon and drive setups.
    For the weapon, I'm going to assume you will have a 250mm Diameter bar, and will run it on 6S/22.2V. Kv time Voltage gives us ~6000rpm. On a 1:1 ratio the tip of the weapon will be doing 175mph which is a pretty solid speed. 1.5:1 Would give 117mph at the tip. For your first machine I'd say this is a better option as it should make it more reliable and you can change it in the future if needed. While the speed limit for weapons is 250mph, most robots don't actually get close. For example Binky II tops out at 175mph, my robot Conker 3 maxes at 200mph and even the crazy drum spinners doing 12'000rpm only reach ~200 due to their small diameter.
    If we apply the same logic to your drive, I'd say to aim between 12mph and 15mph. If we go with those Gimson gearboxes at 13.7:1, 125mm wheels, and on 5S at 18.5V - We can look up the No Load after gearbox speed which is 1536rpm, put in the 125mm wheels, put in our wanted 15mph, and it says we need 1.5:1. Easily done with a belt. While you will loose some speed due to load and resistance, it should still drop you into that 12-15mph range quite nicely, and have quite a lot of torque.

    Last note on this giant block of text. Some may disagree but I'd advise using Timing Belts on both your weapon and drive. At FW class, 5mm pitch belts are the norm, and most people tend to use HTD profile heavy duty belts. I'd say use a 9mm wide belt for your drive, and probably the same on your weapon as it will be protected by the frame of your robot and the reach of your weapon. We use 9mm on Binky's 3.6kg disc, and have only snapped a belt once; and that was due to the grub screws in the weapon pulley coming loose and eating the belt. You can use this calculator - - to get your centre points, no tensioners required. Just look up the available belt lengths and tooth counts and you can pick the right belts and pulleys to get that 1.5:1 ratio on your weapon and drive.

    I could easily keep going but I won't, that's a lot to digest, but hopefully this should give you resources and knowledge you can apply now and in the future. Happy building!

  2. #12

  3. #13
    To build a axe bot at fw use a 350w scooter they are cheap and simple and fun as all hell to drive

  4. #14
    Theo's Avatar

    Axes are really fun, especially in featherweights.
    But your plan for the shed is really smart; never test a robot in the same room you're in. We've had a few incidents here and there of people powering up heavyweight spinners up close without realising the dangers.

  5. #15
    For your pinion pulley, you can use BeltingOnline. They will modify the bore for you for a little extra cost, but be warned; this can make the order take a lot longer (even longer than they say on the website). Same applies for getting them to put a grubscrew in it. For your weapon pulley, you'll want to get one specially made, most likely. This is because it's usually best to have a smooth, flat pulley with no teeth. Not only does this mean your brushless weapon motor won't stall out as easily when starting the weapon, it protects everything downstream of the weapon from shock when you hit something. So yeah, having teeth on your weapon pulley (though a bit counter-intuitive) isn't usually the best thing. What's more, a custom pulley can have all the mounting holes for your weapon in it, and a bore sized specifically to fit whatever bearing or bushing you want to use. You could talk to your machinist about getting that made, but there are some people in the community who could make one for you too.

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  9. #19
    Ocracoke's Avatar
    Team Kaizen

    To answer your questions directly:

    1. Ish. My axe motor does this on my middleweight I think but not by very much. I would prefer the pinion gear to be wholly on the axe motor itself since I don't really want to end up bending the shaft.

    2. In my experience, no, not on a drive train but keyways do help lock in a gear or a wheel hub onto a shaft. The axe motor I have on my middleweight has a small keyway which I drilled a specific point to allow the grub screw to sit in. Depends on how much force is likely to go through it. In the lighter (i.e. Beetlweight) classes, I don't bother with this and simply screw the grub screw onto a flat on the motor shaft.
    Team Kaizen - Build Diary for all the robots

    AW: Amai, Ikari, Lafiel, Osu, Ramu
    BW: Shu!, The Honey Badger
    FW: Azriel
    MW: Jibril, Kaizen

  10. #20

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