First Build - Featherweight Bar Spinner

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!

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:

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2. I've been looking at using a round belt for the weapon drive - does anyone have any experience with these?

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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.

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4. Does anyone think the ball transfer units at the front are a bad idea?

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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.

@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.

"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.

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

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..

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.

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Originally Posted by Team Gremlin

I disagree that a spinner for your first build should be out of the question.

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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

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.

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.

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 -

https://www.youtube.com/watch?v=FXey3Phs3eQ

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

Attachment 8517

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 - https://gimsonrobotics.co.uk/categor...tary-gearmotor - 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 - https://www.technobotsonline.com/rob...alculator.html - 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 - https://www.technobotsonline.com/tim...alculator.html - 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!

Thank you everyone for the feedback!

In all honesty, I originally wanted to build an axe bot but the idea of working with high pressure CO2 frightened me more than a spinner!

The plan for testing the spinner was to clamp the bot, put it in a shed and film it with an old phone, does this seem okay?

Thanks very much for the feedback on the drive motors - pity I already bought two but hey ho. This will no doubt require a change to my bulkhead design as I plan on bolting them to the bulkheads and using square ‘clamps’ per my design.

The main reason I switched to a round belt is because I couldn’t find an aluminium pulley supplier for timing belts that would customise the bore to order - as I mentioned I don’t have access to a pillar drill to bore them myself. Is anyone aware of such a supplier? The motor shaft is 6mm and the weapon shaft is 20mm. I was looking at using bore reducers but again I couldn’t find any with reasonable specs.

i totally take into account all of the comments regarding safety and I do understand the risks involved. Ideally I would like to create a more simple bot for my first go, however with the lack of available facilities to fabricate and machine parts myself I can’t keep the cost down. Therefore I would rather make something vaguely competitive if I have to go all-in.

Thanks again!

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

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.

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.

I bought the Turnigy 9x TX as well. Just making sure you know that as is, it won't failsafe. I had to fit an FrSky module with one of their Rx's. I also flashed the Tx, not sure if that was necessary but the software is a lot better.

Great, thanks everyone for the help!

I will get a request off this week to sort the pulleys out.

In the meantime I have ordered one of the housed bearings which I am using on the drive shafts and the weapon shaft so that I can actually get a look at it. I’ve only actually modelled it from the spec sheets thus far and they are a little bit ambiguous with some of the measurements (to me anyway). The hope is that I will come up with an idea to simplify the bulkheads as they will currently require quite a lot of cuts - hence the large CNC estimate.

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

1. Is it okay for the pulleys to overhang the motor shafts as long as the grub screws make contact?
2. Are keyways absolutely essential or will grub screws generally hold up okay?

Thanks!

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.

Hello - hope everyone enjoyed the holidays!

Thanks to the kind advice above, I have bought new Gimson motors as suggested. In order to accommodate these larger, heavier motors my design is now larger across all three dimensions.

This then presented a weight problem and so I have designed some cuts in the chassis to try and save weight - I'm estimating the chassis to weigh in around the 4.5kg mark (based on a 2.71g/cm^3 density).

The good news is I now have access to a makeshift and somewhat limited workshop but it does mean I can do some of the machining myself.

I have a few more questions and would, as always, be really grateful for any advice:

1.I currently have one 5S 4000mah 30-40C LiPo - is this going to be enough to power the drive and the weapon or am I going to need two?
2. From my attachment "Overview" v2 - I have never worked with aluminium and so I am wondering if parts of the chassis are likely to bend under force given I have now cut weight-saving holes?
3. From my attachment "Motor Bracket" - I am wondering how I can drill these horizontal holes into the bracket? I am also looking to thread them. It is 1/2 inch thick (note in my CAD the holes go all the way through just for ease but I'm only expecting to drill enough to bolt it to the bulkheads).

Thanks once again and a Happy New Year to everyone when it comes!

Cheers

Hello all,

I’ve made a few changes to the design - mainly reducing the thickness of the chassis from 1/2” to 3/8” or even 1/4” where I think it will allow.

The aluminium has arrived! I spent an hour making the attached cut :lol: The marked section is one of the bulkheads.

This is the only remaining 1/2” section though so that’s as hard as the blade will have to work. They aren’t super fancy blades so I imagine I might go through a few but will see how it goes.

Ive sent off a piece of 3/8” sheet to be cut by waterjet - this is for the V shaped arms at the front which are now much simplified. The shop is miles away and only open Mon-Fri so will be a while before I can pick them up! Lots to do before I need them anyway...

Plan is to get the bulkheads cut and then start drilling holes for attaching the drive motors. I have a cheapy pillar drill which I am hoping will prove to be good value!

As im at the startup phase - the biggest expense seems to be buying lots of little bits like drill bits, saw blades, files, lubricant etc. - all adds up!

I’ll get some more cutting done this week(end).

cheers,

P.S. does this thread need to be moved to Build Diaries? Can a mod do that or do I need to start a new thread?

Looks like this thread is coming along well. I can certainly sympathize with being a newcomer & having the tools drain the budget! Don't worry about the thread, it's fine here.

Good going! Cutting 1/2" ally on a table saw looks scary.

Has the design changed much with the move to home-building? I find my designs for robots feature a lot of rectangles, since I have to cut them myself, whereas stuff I design for work gets sent to a machine shop so it's all fillets and funny angles.

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Originally Posted by McMullet

Good going! Cutting 1/2" ally on a table saw looks scary.

Has the design changed much with the move to home-building? I find my designs for robots feature a lot of rectangles, since I have to cut them myself, whereas stuff I design for work gets sent to a machine shop so it's all fillets and funny angles.

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haha I’m actually surprised at how well it’s done considering it’s a £20 blade!

It was a fairly simple design to begin with, but I have made it even more simple since I got some equipment to cut it myself.

The main worry now is there are some large-ish holes to cut (up to 56mm diameter) that I’m not sure how I’m going to cut.

thanks for the feedback!

Perhaps you could use a hole saw?

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Originally Posted by Eidg

Perhaps you could use a hole saw?

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I was thinking this but it’s 1/2” ally and not sure how good (expensive) a saw would be required!

I've cut through some thinner aluminium with a cheap hole saw. Took a while but worked fine and the saw managed OK. I think I did 3 pieces of 4mm thick sheet which is 1/2" total. That was only a 20 mm hole saw though. The other problem would be accuracy, you probably won't find a 56 mm hole saw and if you do it will probably cut 59 mm holes...

I guess ideally you would want to use a milling machine and a boring bar to cut a hole that size, if you can get access to one.

So the blade gave up on me before I could cut out one 120x210 bulkhead! it came in a pack of two (40/80 teeth) so will put the new one on and see how it goes!

Ive also realised cutting straight lines even on a table saw is a little bit difficult :lol: but maybe that’s just me!

I therefore bought a file yesterday so when I’m done cutting I can take the last few mm off. Is this viable with aluminium? I know it’s a soft metal but can I realistically take say 5mm off an edge without breaking an arm?

im hoping this is super difficult mainly due to the thickness of the metal! As I said I’ve got 3/8” and 1/4” for the rest of the parts so it should get a bit easier lol

cheers

You would do better to invest in an small angle grinder if you are cutting

metal out and a variety of blades for slitting/grinding.

The thing about alluminium is that because its so soft , five minutes with a

file and the file gets clogged and you spend more time cleaning it

than filling-better to use a flap disk in a grinder.

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Originally Posted by team death

You would do better to invest in an small angle grinder if you are cutting

metal out and a variety of blades for slitting/grinding.

The thing about alluminium is that because its so soft , five minutes with a

file and the file gets clogged and you spend more time cleaning it

than filling-better to use a flap disk in a grinder.

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thanks very much for the tip. I’ll take a look at some cheap grinders tomorrow.

I managed to finish cutting out the two bulkheads.

Thats the thickest piece of ally I’ll need to cut for this build so glad the blade held up!

i also got myself an angle grinder to clean up the edges. I am really pleasantly surprised at how well the flap disks sand it down.

next up is the long horizontal bars which will weld to the bulkheads. Four of them at 3/8” so gotta keep up the momentum!

Flap discs are my favourite angle grinder accessory, don't know how I lived without them previously :p

If you don't already wear a dust mask when using one, I'd advise on one. Used a flap disc to remove a sizeable amount of aluminium from a pulley once, in the absence of a lathe. Didn't wear a mask and felt quite unwell afterwards. It didn't amount to anything thankfully but I always wear a mask now whenever I'm using the grinder; can't be great inhaling fine metal particles!

Thanks for the advice Jamie! I’ve been wearing a cheap dust mask when using the grinder and very thankful for it - the stuff that comes off is not nice at all lol.

aluminium is so messy! Didn’t quite expect the sheer amount of chips/shavings that come off it. I was out hoovering my driveway the other day lol - neighbours must think I’m daft.

the other thing I didn’t expect was the noise - especially the table saw. I mean I knew it would be noisy but it makes my ears ring and that’s when I’m using it outside! Ear plugs on the shopping list.

so I have managed to cut most of the pieces I will be needing and the ‘A’ shaped bars have come back from the waterjet cutter - they look awesome. The guys also gave me a tour of their shop so got to see the waterjet and laser cutters in action which was cool.

the pieces are still very rough so need filing and cleaning up before i move on to drilling the holes.

i do think I have overdone it with my thicknesses - I didn’t realise how strong aluminium actually is!

Hi all,

Most of the parts are ready now - just a few more holes to drill.

im now turning my attention to getting these parts welded together. I have done a lot of research and basically discovered that MIG welding aluminium is an absolute nightmare lol

i do want to give it a go though so I’ve bought a second hand Clarke 150te MIG welder off Gumtree. I wanted to stick with the MIG welder even though TIG is recommended for aluminium because hopefully I’ll be using it on other projects later on.

i have now just been reading that MIG welding thick aluminium is not possible unless you have a 200A+ machine which I cant justify investing in - is this correct?

I’ll basically be welding bits of 1/2”, 3/8” and 1/4” Ali pieces together. Is this at all possible with the equipment I’ve bought? Anyone have experience welding thick aluminium?

Advice is much appreciated.

Cheers!