Design update:

http://imgur.com/BRwHRlY

Micro Magnum finally has its top and bot cover (4mm aluminium)

Retracted the wedges a little bit to ensure a better bite.

Changed weapon shaft to 20mm.

It's now 11.7kg.

And as always, if you have any suggestions, plz feel free to speak up.

Well, I guess I will be focusing on adding details to the drum over the coming few days...

And I have a few questions to ask.

1.What kind of bearings should I use? (I am going for a hollow drum here, so contact surface with the shaft will be limited, so im thinking about maybe using brass sleeves will be better? But idk, this is my first combot)

2.I have pretty much decided on using timing belts, but here is the problem: it seems really hard to find a timing pulley that has a bore larger than 20mm (shaft will be 20mm but with the bearings in there its gonna get bigger than 20) So what should I do to make this work? Do people usually buy a timing pulley and machine it to get a bigger bore?

Bronze bearings suck for high RPM, high impact loads. Their good points are low cost and compact size, but they just won't last for a high power spinning bot weapon. Needle roller bearings are a step up and will work at high RPM but their relatively soft and thin outer bodies can't take impact stresses for long.

Deep groove ball bearings are the sweet spot for most builders; they work well with high speeds and are fairly good with high impacts, although they will eventually wear out. If you get shielded bearings, they are low maintenance as well.

Angled roller bearings are probably the ultimate for dealing with high impacts. They need special care with the bot design and are usually not sealed so they are higher maintenance.

You are correct about the timing pulleys; most builders end up drilling or boring them to the required size.
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Australian 2015 Featherweight champion
UK 2016 Gladiator champion

All the feather spinners I did build use bronze or whitemetal bushings.

I don't think Twisted Sister, Valkiri 2 or 3, and the halfbrothers Surveyor and DX11 can be called weak.

Also, the drive of Bullfrog, uses bushings for the 2 stages.

But yes, they wear out after a few events.
So what? Most of the time you will have to rebuild the machine before that happens, and the lower price, weight and way easier tolerances are a boon there.

Perhaps you get a better class of bronze bearings in Europe? I definitely agree its possible to use bronze for weapon bearings, just with a few caveats. Back in the day, local builders had many problems with sudden failures and high friction; I recall one bot's disk seizing up mid-event and there was another bot that converted from bronze bearings to ball bearings and getting noticeably higher RPM afterwards,

I am a big fan for bronze bearings for drive trains; they have been bulletproof in Mr Mangle and Shiny, but I figure that for high speed combined with high shock loads, its hard to beat a shielded ball bearing.
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Australian 2015 Featherweight champion
UK 2016 Gladiator champion

Thanks for the info guys.

I am not entirely sure at this point...

But I guess I will assume using a ball bearing for now cuz its bigger and leaves space just in case I want to switch to other types of bearings.

Well...

The next problem I run into is, where can I find aluminium tubes with a big diameter (about 10cm or more o.d.) and a medium thickness (about 1cm)?

I have searched for those but most results are all somehow in inches which could cause problems due to the fact that the rest of the bot is in cm. (except for the gearbox, which i really cannot find replacements for)

For the drum, using an 10mm or 3/8 wall aluminium tube most likely won't be able to store enough energy. But if you want to use it you can get 1/2inch wall here:

http://www.onlinemetals.com/merchant.cfm?pid=19929&step=4&showunits=inches&id=71&top_cat=60

It would probably be best to start with a 100mm round bar of something like 6061 or 7075( 6061 would be cheapest, but 7075 will be a lot stronger) and making it yourself.

1 inch is 25.4mm so it is quite easy to convert between the systems. Although it can be a bit confusing at first, getting a good grasp of the imperial system will set you in good stead for both building combat robots and in your future career in engineering.

Also if those end caps supporting the tooth are 10mm aluminium, they will get destroyed very quickly. Also the tooth will get broken on the first decent impact.

Are you planning to get the S7 machined by Whyachi?

Hey thanks for the advice!

Yeah this drum sucks and is more of a placeholder than an actual design (in fact the drum design hasn't been changed for around two months because I was busy getting everything else set up properly XD) But hopefully I left enough room for a decent drum.

I will try to work with the spare weight I have on hand right now and see where I can get.

Also about the teeth mounting methods, will cutting grooves on the aluminium barrel and fitting the teeth into the grooves work? (kinda like the featherweight Touro, but I have a hollow drum instead of a solid one)

Well... haven't really digged that far into the manufacturing process that much, but good to know that Whyachi also does S7 stuff. Btw are there any local S7 machining service that you know about?

You could tap some m12 high tensile socket head cap screws into your drum they work pretty good on some bots-and easy💥
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-Pickasso- Vivid Sportsman champion 2015

Here are some pictures of an old drum from the UK bot 720:





I don't know what alloy it is but it took quite a bit of abuse. The grooves for the teeth should probably have been deeper and the main problem is that the M6 screws were not strong enough and all sheared off - that would be a royal PITA to fix at an event! The replacement drum had several upgrades and has lasted for years; the grooves were replaced with deeper T slots and the teeth were machined in a matching T shape so they slid into the groove. The teeth are held in place by end caps - no screws required .

Machining a T slot isn't really much harder than a regular slot, it just needs a special cutter and a few extra passes on the mill.

Machining S7 isn't hard either; it comes annealed so its not much tougher than other alloy steel. The problem is that local heat treaters have NFI how to harden it! I once tried a large treatment place in Sydney and they totally stuffed it up; the part came back so brittle that it broke after just a light tap in the arena. The Whyachi guys are quite reasonable for costs but the heat treatment is going to cost you at least $100 USD.
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Australian 2015 Featherweight champion
UK 2016 Gladiator champion

@ Nick. The bronze bushings I normaly use are standard d20 D25 L30 sinterbronze bushings.
Most of the time using 3 of those to cover the complete width of the disk/pulley running over a surface hardened steel shaft.

That locking , we have seen that in DX11. But one run of the reamer trough the bushin gs and a shim later, problem solved.


For disks and eggbeaters , Monoblock Hardox all the way. Simple and strong.
Drums is another matter. The best ones seen to now are 7075T6 ali with T groove , or the hybrids as used by Rango and Equinox. 2 or more lasercut monoblock disks set appart with ali spacers.

Yeah using a T-slot will be a great way to mount the teeth.

And since it's not hard to make a T-slot, is that also the case for a ducktail? (ducktail will have a bigger angle at the joint so I was thinking that might relieve the stress better, if it is still easy to make)

Ducktail??? Did auto correct just kick in when you meant dovetail? That should work better in theory but I can't recall anyone trying it. What might happen is the dovetail acts like a wedge and either jams the tooth in the slot or levers the slot wider - or I am maybe just looking for trouble...
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Australian 2015 Featherweight champion
UK 2016 Gladiator champion