Team MedBots

[Ellis]

Use a single motor and ESC and otherwise ignore Dave, he doesn't know anything, go Hobbyking!

[Max]

Yeah what does Dave know about building a decent drum bot

Below are some Cad drawings of the kind of thing I was thinking of.




Some stats:
-Dimensions: 360x265x80mm with 100mm wheels
- 6mm Hardox armour with 5mm sacrificial HDPE over the top
- Frame/Chassis is all aluminium, would try and get a decent grade rather than the cheap soft stuff
- 6S 60C 4000mah lipo, NTM 50-50 brushless and 100A Red Brick esc, TZ85A escs and either GR02 or RangleBox drive

Then the drum would either be 3" Aluminium billet or HDPE;
The aluminium one puts be right on the weight limit without things like wires, connectors and with just 6mm HDPE/polycarb for the lid/base, most likely I would have to pocket some of the aluminium and find other ways of losing weight.
The HDPE drum would leave me with enough weight too have 4mm Hardox for the lid and baseplate and I would still have almost a kilo left spare. The thinking is that theatrically assuming the drum is spinning nothing should ever get to the actual drum as they should be hit by the teeth first, if they do hit it then it will just take a gouge out and not send me flying, I can then easily melt more plastic into the gouge or replace the drum (each one would only cost around £12 and are easy to machine so I would carry a few spares).

So what are people's opinions? Any suggestions for improvements to the design?

[Eventorizon]

Looks alright over all, you need to radius or chamfer those back corners though. Ditch the HDPE over the hardox, if you have 6mm Hardox you won't need it.
Don't use HDPE for the drum whatever you do, it will turn to a black twisted mess the moment it hits anything. Any of the good Alu grades will to: 7075, 6082, 6061, 2014. Check out 720's battle against NST and look at the hole HST made in its 7075 Alu drum! Now imagine if that was HDPE; it would probably destroy your weapon.
GR02's are no longer in production so you would either need to go Ranglebox or Banebots, either of them will do fine though the Banebots are much lighter but more costly.
All this said, Jamie, Dave and Guilherme are the people to go to for proper drumbot advice, they know their stuff.

[RogueTwoRobots]

HDPE drum? Interesting concept and I can understand the reasoning behind it, but....

The teeth may take the impacts rather than them reaching the drum, but the drum still has to be strong enough to put up with the forces that'll be going through it. Regardless of how the teeth are mounted, those forces will transfer some way or another and, over time, will probably cause deformation of the drum.

Also, can't tell from the pics (on my phone) but would you be running the belt from the brushless directly onto the drum body? If so, the HDPE isn't going to enjoy the heat generated from friction.

Looks a compact, solid design overall though.

[Max]

Looks alright over all, you need to radius or chamfer those back corners though.

Which corners do you mean? You mean the 90 degree foods in the armour at the back?

Don't use HDPE for the drum whatever you do, it will turn to a black twisted mess the moment it hits anything. Any of the good Alu grades will to: 7075, 6082, 6061, 2014. Check out 720's battle against NST and look at the hole HST made in its 7075 Alu drum! Now imagine if that was HDPE; it would probably destroy your weapon.

Hmm, I thought the HDPE drum would attract criticism, in some ways you think if an aluminium drum is going to get destroyed you may as well make it out of something easy/ cheap to replace! But obviously if it falls apart during the actual fight its a probes as then odds are you are not going to win the fight, just needs to be strong enough to survive so maybe aluminium would be the way to go, another option I guess would be nylon for increased rigidity but that may be too brittle and still has a lot of the issues.

At the moment the drum is solid material, there is no shaft going all the way through but a separate, 40mm shaft at each end so the plastic ought to be fairly rigid. This does mean I could use the aluminium and just bore a hole through the centre to save weight but ideally I want the walls to be left as thick as possible to avoid the drum getting crushed/punched through.

Perhaps a composite drum could work, metal centre to give rigidity and plastic outside or visa versa.

GR02's are no longer in production so you would either need to go Ranglebox or Banebots, either of them will do fine though the Banebots are much lighter but more costly.

Yeah I realize the GR02s are out of production so would be a matter of trying to get some second hand ones, depends really on the cost of the RangleBox units which hopefully we will find out soon.

@Jamie: yeah the belt isn't shown but will run directly onto the drum, hadn't thought about the heating issue, didn't realize it would get that hot! And if I were to go HDPE I would have to give some thought to how the teeth where mounted, I was thinking of counterbored bolts in the teeth going all the way through to the other side then into a threaded hole in the tooth on the other side. Alternate bolt direction to keep drum balanced.

[mrsam]

I'm by no means a drum/spinner expert but having seen some of the drums at the 2014 and '15 champs i feel i can offer something.

6mm polycarb for a lid is plenty, no axe is getting all the way through it. Having seen what the likes of LH3, No toy and Hatchet actually do to it you'd be completely safe using it. As for a base, you'd be better off with hdpe or even some 2mm ti/Alu, you could use polycarb if the base is non structural (if you've seen Hatchet's 10mm base last year you'll understand), you won't need hardox top and bottom panels, it's really a waste of money/weight.

As for your outer armour 6mm hardox is way overkill, 722 uses angled 3.2 and i believe Drumroll this year used regular steel brackets with hdpe over the top. Spinners are so powerful nowadays that a decent hit on hardox will bend it and you'll never get it straight again being 6mm, if you went down to 3.2 that would save you a huge amount of weight and wouldn't really leave you any worse for wear.

[robotic_ants]

Conundrum

I've had some past experience using plastic drums back in the day, my first attempt used PVC a brittle plastic and was a total noob failure, it was powered by a "gold motor" and instantly cracked on first impact, lucky the end caps kept it all in one piece.

Second attempt was HDPE this proved to be much more durable, granted it only had a hand full of battles and never went up against the spinners of today but I believe it would still be a viable option if I was still building today. Only issue I came across is when little hitter got a perfect hit on the aerial cover and bent the shell into the path of the teeth, the sudden impact from the collision warped the bar and managed to pull a single bolt through the drum. That time it is was powered by a 1.5hp Ev Warrior motor, I believe a better designed bar and correct internal support for the teeth would work better. I did move onto a saw like disk around the drum but never got tested enough, it was fun lots of sparks but not designed for flipping or throwing another bot.

But these examples are from a old featherweight, modern high power and speed brushless motors would add another factor for the design of a plastic drum, I say why not try it.

[Mouldy]

ha, I remember those robots^ , thanks for making my feel old.

As supreme leader of the drum bots and current champion of champions within the drum bot community my advise is as follows;

The number 1 golden rule- Ignore everything Jamie says, every word he speaks is a lie or hard to understand because he's a dirty scott bag.
The second rule of drum club, is don't talk about drum club.
Third rule, plastic bad - metal good, even if its not up to Alex's standards, 6082 worked fine for me, it took a huge hit from NST and didn't fail, won me the fight... its how you mount your teeth that really matters... and in plastic they are always going to break out.
Final rule, don't base your robot around parts sold by Ellis- i wouldn't want his dangle-box products all over my machine.

Have fun.

[Max]

No link needed, as you're spot on. We have hex 17 wheels.
Larger wheels, like 1/5 or 1/4 scale are hex 24.

If those wheels can take an 2 kg RC car @80kph doing a Dukes of Hazard over 20 meters.....

Hi Mario, out of interest how do you go about connecting this type of wheel to a GR02? If I understand correctly the wheel is designed to take an M10 nut but I guess you make up some sort of custom adapter?

[Max]

Also, couple of modifications made to the CAD:



Main changes are at the front the very front bits are removable so if they get bent I can swap them out and could try out a couple of different shapes for the front to find what works best for getting under people to bring them up into the drum. I've also got rid of the button head bolts and replaced them with countersunk bolts going into a strip of hardox which should mean they wont pull through the HDPE but also should be caught by vertical spinners but it does leave me more vulnerable to horizontal spinners.

But what I have realized was that I had misremembered the density of steel (argh) so all my mass calculations were wrong
So either I have to drop the Hardox down to 4mm with just those front bolt on pieces 6mm or get rid of the HDPE, front bolt on pieces and the hardox strips for the bolts (but these wouldn't be necessary anyway as I could countersink the hardox as the HDPE wouldn't be in the way) then I should just about be able to fit in the 6mm hardox.

Which do you guys think is preferable? The big issue with the 6mm hardox is that if the front wedge bits get bent out of shape there will be very little I can do about it other than replace the entire outer armour.

[Ellis]

Firstly, nice CAD work, you're clearly comfortable in Solidworks!

The main question mark from what I see is the strength of the drum mounting. Looks like the shaft is dead which is good, but it seems a bit skinny otherwise. You have what, based on colour, is aluminium plate mounting the drives. Traditionally these would be brought forward and the drum would mount to them, in this case there is a slim (alu?) plate going across the front, which ties into the relatively thin (hardox or otherwise) sides, which then go forwards to support the drum.

Point is I think a hit from any good horizontal will badly deform the front, either by ripping a side out away from the drum or shunting them both sideways.

It's hard to strengthen that up without making use of the wheel bulkheads (which would also need thickening depending on grade), but then you get that nobody, mouldy or something they call him, accusing you of making a 720 clone. Number one key to success is don't listen to anything he says, ever.

It looks like you could save a good bit of weight if you reduced the chassis height? T3's internal height is just 50mm in order to squeeze the most out of the weight limit. Perhaps you can drop the height here so you can put weight back into other areas.

[Max]

Yeah I have been concerned about those front plates getting bent and thought about using the wheel bulkheads but as you say then it looks like 520 clone, here are some images without the drum to give you a better idea of how it works:



There is a large titanium axle at either end 40mm diameter with a 16mm hole though the middle, the drum spins on bearings on these axles then a 16mm rod made of either titanium or aluminium goes all the way through the centre, each end has an M8 tapped hole so a bolt goes from the outside of the hardox armour into these holes, the idea is that this should tie them togather so they are less likely to be pulled apart and also acts as a spacer to stop them being pushed together.

I have been thinking about making it less tall but it would mean I would have to machine down the flat bar used to make the chassis unless I can get it all the way down to 60mm as I am currently at 3" so it is quite a large step down but may be possible, could mean reducing the drum diameter as well which will mean smaller bearings and so a smaller shaft going through the middle but could save a lot of weight.

[Eventorizon]

Which corners do you mean? You mean the 90 degree folds in the armour at the back?

Yep, they are very vulnerable to horizontal spinners.

In Solidworks you can assign materials to different bodies so you don't have to do your calculations manually.

Asigned Materials C3 01.JPG

If you right click on a body, materials, edit material, then choose one from the list.

Asigned Materials C3 02.JPG

Then you can go to the Evaluate tab and then go to Mass Properties. It will break down all the body masses for you, give you the total and show you the centre of mass.

The reason I said steer away from GR02's is that their will come a point where there are no more left and suddenly your bot wont work. If something breaks you have no way of replacing it. You are best to design around a part you know is still in production and leave what remains of the GR02's to the machines that already rely on them.

[Max]

Ah ok, I've always just gone into mass properties for each part and manually entered the densities but your way is much easier, I knew there must be a way!

As for the folds at the back I will have to see what radius I can bend them, not sure exactly how this hydraulic bending machine works so my plan is to start with the hardox and bend it then adjust the CAD to match whatever radius I get before making the rest of the parts to suit. I will ask if they are able to do a sort of chamfer bend with 2 45 degree bends spaced 10-20 mm apart.

Oh and on the GR02s my main plan is to use the new Ranglebox motors but if they cost an arm and a leg I will have to find an alternative, my hope would be that other people woudo switch to the new motors making some 2nd hand GR02s available. But I was just curious how mario mounted his wheels as I imagine a similar thing could be done with the Ranglebox ones.

[maddox10]

Max, to fit the RC wheels to a GR02 or any other live axle I make a custom adaptor, Hex 17 or 24.

To ensure the hex staying put on the shaft, I glue it with Loctite 638 and tighten the setscrew.