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Thread: First Build - Featherweight Bar Spinner

  1. #1
    Hi everyone - I am totally new to anything engineering-related but after many years of watching and admiring the robots on Robot Wars/Battlebots I've decided to get involved!

    I have spent probably a year perusing this forum and looking through the various build diaries etc. and thanks to the endless wisdom and generosity of this community I think I've picked up quite a few pointers.

    My main obstacle (besides my lack of knowledge) is that I have pretty much zero access to a workshop and no space in which to build my own. To this end, I am hoping to spend lots of time on the CAD and then get a CNC machinist to machine most of the parts. I'll then be bolting it all together like a jigsaw and doing the soldering etc.

    Moving on to my concept - it's a featherweight spinner with a rather unoriginal design but my main goal with this first build is robustness and reliability over innovation.

    Frame: 1/2 inch 6082 aluminium
    Drive: 2x Nidec DMN37KA motors (3600rpm) - geared down about 2-3:1 depending on the size of the wheels.
    ESCs: Drive - 2x tz85as which I've already reprogrammed (that was quite an adventure!) Weapon - not sure yet
    Weapon: 2kg Hardox Bar Motor - 270KV Brushless Outrunner from HK Geared about 2:1 Shaft - 20mm titanium
    Batteries: Drive - 5S 30C 4000mah LiPo Weapon - another 5-6S LiPo
    Radio: Turnigy 9x
    Wheels: - ~120mm Nylon with rubber treads bolted directly to the pulleys, 4 Ball Transfer Units at the front
    Belts: 9mm Timing belts for the drive with aluminium pulleys, round belt for the weapon with aluminium pulleys
    Armour: Depends on the final weight

    I do have a few questions and would be grateful for any help!

    1. When calculating the final gearing, I was wondering if there are any good ways to estimate the rpm a motor will put out under load. For example the 270KV outrunner - I have no idea how well that would spin a 2kg bar.
    2. I've been looking at using a round belt for the weapon drive - does anyone have any experience with these?
    3. Following from Q2 I was thinking about possibly using a chain but I'm not sure how to create a clutch (I found one in the RioBotz book which uses a belleville washer but I don't really understand how it works).
    4. Does anyone think the ball transfer units at the front are a bad idea?
    5. Finally, does anyone have any ideas how I could simplify this design? I got a quote from a CNC machinist nearby and I'm probably looking at £500 at least to get the bulkkheads and 'V' shape frame cut (one V is made of two parts - £60 + VAT per part).

    Thanks for reading!
    Attached Images Attached Images

  2. #2
    Ocracoke's Avatar
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    Hey there, welcome to the forum!

    Specs look OK to me, perhaps a bit of a query on the drive motors as to wherever they'll have enough oomph to move this but otherwise looks fine. I will have to note that it is generally not a good idea for a first time roboteer to build a FW spinner - if they go wrong, they go wrong quickly and, as you can imagine, the consequences of that going wrong can be very hazardous. Also, not all robotic events support spinners due to their specific arena safety features - you'd have to get confirmation that you could use this.

    To answer some of the questions you have:

    2. I've been looking at using a round belt for the weapon drive - does anyone have any experience with these?
    Not directly but if under enough tension, that should be OK. The only problem I can see is that after a few hits, the belt is going to wear out very quickly methinks but does avoid the need for a dedicated clutch.

    4. Does anyone think the ball transfer units at the front are a bad idea?
    Not particularly. I've used them before in The Honey Badger to help with turning. The problem with them is that they do have a tendency to fail under hard use and then it locks up. If you needed to save a bit of weight, you could replace these with some Oilon plastic feet to save a bit of weight.
    Team Kaizen

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  3. #3
    @DR.Crane Your drive Motors are too week.
    I am using 800W Brushless Motors for my drive, the robot is also a spinner. Next time I will go for 1200W.
    Your Motor has around 10W...
    I would recommend you to use multi v ribbed belts, with the pj profil.

  4. #4
    Maxamuslead's Avatar
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    "I am totally new to anything engineering-related" so let's build a dangerous spinner with 2kg bar spinning at 2500-3000 rpm.

    please just make a nice simple wedge bot. making a robot which can move well and make it through 3 mins of madness is an achievement in itself.

    For drive just grab two cheap cordless drills and using the gearbox and motor. the drive for most FW. plenty of power and cheap.


  5. #5
    Theo's Avatar
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    Basically what he said. A spinner is a very bad choice for a beginner to combat robots, moreso someone with no engineering experience. I've been doing this for nearly 2 years now, and still haven't felt confident in my abilities to make a spinner.

    Plus, cordless drill motors are a fantastic option - cheap, rugged and powerful enough

  6. #6
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    I disagree that a spinner for your first build should be out of the question.. I'm a carpenter and my team mate is a mechanic, we had never built anything like this before and jumped straight into a heavy weight spinner that worked a treat in battle.. aslong as it's done safely when testing etc there shouldn't be a problem.. sometimes it just comes down to the person and if they're competent or not. As for the price £500 seems alot for just part of a frame?.. my heavyweight cost about 1500 or so all in. All this said I'm no expert so take the advise from whoever you feel confident with..

  7. #7
    firefly1's Avatar
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    As long as the robot is planned well enough, since when building a spinner there is much less room for error, I feel a spinner as a first build is completely doable. You will defiantly learn a LOT if you go through with this and future builds may be very different but there is nothing wrong with challenging yourself on a first build. I would happily bodge bits on one of my featherweight builds but never if i was building a spinner. Also as everyone else has said those drive motors wont do, drill motors are the cheap option that everyone uses or if you want even more of a challenge try getting your head around a brushless drive train. Also remember to factor in how your locking bar would fit before you start building it.

  8. #8
    Maxamuslead's Avatar
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    Quote Originally Posted by Team Gremlin View Post
    I disagree that a spinner for your first build should be out of the question.
    The strange thing about this sport is that the lower weight classes are arguably much more dangerous than the large ones due to how much power they still have at there size. Yes, people have built spinners with no past experience in combat robotics you are a great example of that (gratz on coming third) however I would never recommend it to anyone for their first bot. making something move well and make it through a match is great. Something about walking before running

  9. #9
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    If you are confident and safe fine. But remember there is a good chance that you will be going home in a bin bag!! Test it for every eventually you can think of.

  10. #10
    Ocracoke's Avatar
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    I can see where you are coming from. Because of the lower weight classes and the smaller overall size, I sometimes get the impression that folks don't take account how dangerous these things can be. Doesn't matter about the weight class, if it goes wrong, it goes wrong quickly and the amount of energy in one of these things... well you get the picture.

    I've had catastrophic failures happen to me like this, even with a Antweight. It was still a case of me putting it in a sealed container rather than test in the open because like heck I was OK with having a ~5g bar flying at my eye if it got loose.

    Spinners are dangerous and they are dangerous for a reason. There is really nowhere safe to run them apart from an appropriate arena and they have to be built properly, hence the recommendation to build something simpler in order to gain engineering experience first before taking the next leap.
    Last edited by Ocracoke; 2nd November 2018 at 18:19.
    Team Kaizen

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