(FW)Nibbler- Horizontal Spinner

[Cameron_S]

So with parts starting to arrive and things starting to happen, I thought I would start a thread for the build of hopefully what will be a horizontal bar spinner. The reason for documenting the build on here is so I can ask for advise from you guys and for you to hopefully catch any beginner mistakes I’m about to make before they happen. Especially considering I’m kind of running before I can walk jumping to a spinner instead going for normal wedge/rambot first. To make sure I don't end up with a half finished robot I plan to first build it as a wedge then add a weapon, so if things do go belly up I should at least have a working wedge robot. Any way here the parts i’m planning on using.

Mechanical
Armour- 20mm HDPE for sides with 10mm top and bottom
Wheels- 125mm Nylon wheels with bike tread and hub inserted with Ellis’ melting in a nut method.
Motor pulley- HTD5 15mm 12 tooth
Weapon pulley- HTD5 15mm 40 toothed (Giving a ratio of 3.3:1)
Shaft- 20mm silver steel
Spinning bar- 500mm*50*15 s7 or hardox. Should weigh about 3kg.
rectangular metal(attaching bar to body)- 1.5mm 20mmx10mm rectangle aluminum as shown in the cad.
Bearings- 20mm standard deep groove ball bearings

Electronic
Drive motors- 18volt drill motors (already brought and had the clutch bypassed with grub screws).
Weapon Motor-NTM Prop Drive 50-60 Series 380KV / 2665W planning to run it on 7s (Should spin the bar at a theoretical 3000RPM after reduction)
Drive ESCs- 2 x 30 amp ones from Rangle Box
Weapon ESC- ESC- 125 amp red brick
Weapon Batteries- http://www.hobbyking.com/hobbyking/s..._5S1P_40C.html plus http://www.hobbyking.co.uk/hobbyking..._2S1P_40C.html in series to give about 26volts and 3000mAh
Drive battery- http://www.hobbyking.com/hobbyking/s..._5S1P_20C.html
18.5volts + 3000mAh
Connectors-XT60s

Maths
These are done with the assumption the bar will be at 2500RPM (15% loss from motor)
Energy-2.2 kj
Bite(5 mph impact)- 5.5cm (quite good right?)
Tip speed- 145mph (scary!)
Questions (sorry there’s a lot)
Am I completely mad to try and build a spinner with no previous robotics experience?
Is hardox a good choice for the bar and does the grade matter?
Could i get away from only having spares of the cheap things e.g bearings/drive motors? What parts are prone to breaking?
Do brushless motors have mounting holes at the bottom? Is it a good idea to mount only to the bottom, or is supporting both sides a better idea?
Is the stall current of a drill motor over 30amp? Is the ESC i’ve mentioned be able to handle the stall current or is the Tz85a a better choice?
Where do people normally buy hardox from? Struggling to find any online sellers that sell something other than digger wear plates.


Cad (minus wheels)

[maddox10]

You're mad to ,start with Robot Wars whatsoever. Build something that takes a lot of time and efford, and then hope it won't get smashed in 5 seconds.
Join the assylum.

Hardox is a fine material for the spinner.
The grade, as long it's higher than the opponents armor, it's good. so aim for 500+. Valkiries disk is hardox 550, 12mm thick.

Expect everything to break sooner or later.

Brushless motors have mounting holes on 1 side.
The NTM outrunner has threaded holes on both sides. Mounting to the frame is done with the 4 M4 holes on the side with the wires coming out. The other side, on the spinning body, has 3 M3 holes to mount propellers to.
Oh, and ruggedize the NTM. The weak point are the bearings and the magnets. bearings can be replaced with decent brand bearings, and the magnets really love a lot of glue.

Hannibalito4 and Valkiri3 used a 25A per channel esc, and they moved fairly well; but with higher graded esc's it went better. the 30amps could work. But take the advice, go for the TZ85's.

Hardox, small bits of high grade Hardox I buy at a place that makes excavator buckets and simular.
Expensive, compared to buying a big sheet and cut from that, yes, but a full sheet goes a few £1000, and a 350mm diameter disk goes about £100 in raw material.(and around £150 in lasercutting cost)
The proven and renowed Hardox 450 3.2mm thick we buy in bulk with the DRG, and then we get a pasture sized steel plate of 2.5*6 meter (last time it was even a 2.5*8m), And then the cost is "less". If I recall correctly, divided in sheets of 1.5*1m it was about £160 a sheet.

[typhoon_driver]

Carry out your bite calc at 1m/s. The vast majority of collisions are at low forward speed.

[overkill]

That NTM motor may not have enough low speed torque to get the bar spinning up in a competitive time and will likely get dangerously hot. The NTM motors are good quality but I would try to use something with a wider diameter and closer to 4KW peak power (or at least leave room for an upgrade).

The main concern to me are the aluminium arms supporting the weapon - With the energy on tap they will very likely bend on the first full speed impact. Try to increase the wall thickness and diameter as much as you can.

[Cameron_S]

Thanks for the advice so far.
So 30 amp ESCs out and Tz85a in. Thanks for the information on mounting brushless motors, understand that a lot better now. For hardox I might make it a problem for future me and sort it out when I need it.

At 1mps collision speed there would be about 2.4cm of bite. Much more realistic and thanks for the spot.

As for the weapon motor, shame the NTM wont do as the next step up gets a bit pricey. The outrunners seem to have a huge jump in price once in the 4k power level, so I might end up having to go down the the inrunner path. I originally didn't want to do this as the increase Kv makes single stage reduction harder, but it looks like I have little choice. Would something like http://www.hobbyking.co.uk/hobbyking...Motor_4kw.html be more suited for the job? Or I could be irresponsible and use the NTM motor and have it hate me and look to upgrade it as you mentioned. The aluminium is a bit of a shame as this was one of the few parts I've already brought. Never mind I could also just keep it for when the bot is in wedge configuration.

[maddox10]

Valkiri uses an NTM5060 with a gearratio of 1.3. Spin up is slower than wished for but it has a very high top speed on 6s.

a bigger gearratio will solve that. On getting hot. Not really. We measured it and it stays inside any reasonable limit.

[Eventorizon]

That is a damn good start! You have your list, you have a design and you have a plan!

"Am I completely mad to try and build a spinner with no previous robotics experience?"
- Yes, but that didn't stop me from building Conker 1 ;-)

"Is hardox a good choice for the bar and does the grade matter?"
- The higher the better, 450 minimum - 600 at Best. After tossing a basic bar design through Solidworks, you can get your 500mm bar in at 12mm thick, 60mm wide, with a 90mm dia hub in for around 2.6-2.7Kg.

"Could I get away from only having spares of the cheap things e.g bearings/drive motors? What parts are prone to breaking? Do brushless motors have mounting holes at the bottom? Is it a good idea to mount only to the bottom, or is supporting both sides a better idea?"
- In an ideal world you would swap out the stock bearings in the brushless, glue in the magnets, put in a Titanium shaft and then have a bearing to support the back of the motor as well. That said many machines run stock motors and just have lots of spares. The NTM's are so cheap that its more effort/cost to reinforce them than replace them.

"Is the stall current of a drill motor over 30amp? Is the ESC i’ve mentioned be able to handle the stall current or is the Tz85a a better choice?"
- TZ85A every time. If you look after them they will last you several years and several robot iterations. Just check the insides every so often for swarf and loose components

"Where do people normally buy hardox from? Struggling to find any online sellers that sell something other than digger wear plates."
- That is a tricky one. I got the Hardox for my first weapon from JL Steel in Scotland. They were very kind and provided me with a plate for my weapon (20mm Hardox 500) and my armour (4mm Hardox 450) and asked I donate £60 to MacMillan Nurses rather than paying them. However - http://3.bp.blogspot.com/-65GE6TQ2E_.../s1600/041.JPG - That is what I did to a 12mm thick bar spinner last time I met it in combat so you will want a spare.

[GlenR]

Hiya, Nice start. You are crazy indeed, but get stuck in and make it, then find what fails and improve it till the whole lot eventually works! That's half the fun.

For a bar spinner like that, 500mm is in my opinion too big, the robot will be huge and in the robochallenge arena at least, will barely be able to get up to speed + will throw itself out immediately if it ever does. Decimators was about 350mm effective diameter for reference

The aluminium section on the front is way too weak sorry, that will crumple the first time it hits something 25x25x1.6 or 2mm thick steel would do though, very cheap too.

The NTM is fine to get you going with the aforementioned mods for reliability as long as the weapon sits on efficient bearings. They have decent enough torque especially the super low Kv ones like that. Check out the SK3 motors too, they are substantially better made and tend to have more power IMHO.

Probably have way too much battery as well. Lots of weight can be saved by using 1x for the entire lot, or at least a way smaller battery on the drive. 4s 2000-2200mah is plenty, the drive will be fast especially on 125mm wheels (they might be too big btw, probably will break the gearboxes) and you shouldn't burn out motors. Or go for a bigger 6s pack and run the weapon and drive off it.

Absolutely go for the TZ85s over the 30As on drills, trust me on that one.

Good luck Keep us posted

[harry hills]

For the record, that bent bar wasn't hardox, it was a different kind of steel which turned out to basically be only just better than mild. It was also 8mm not 12mm. So, don't build your bar from 8mm mild steel and you'll be fine. Also wasn't that the hit that slightly bent your 20mm hardox beater which caused you to shit even more bearings up through the rest of the comp, Alex?

Echo Glens comments, esp the ones about the tz85. I was an idiot and designed myself into a corner meant I had to use 30a. Don't do that lol.

[Eventorizon]

For the record, that bent bar wasn't hardox, it was a different kind of steel which turned out to basically be only just better than mild. It was also 8mm not 12mm. So, don't build your bar from 8mm mild steel and you'll be fine. Also wasn't that the hit that slightly bent your 20mm hardox beater which caused you to shit even more bearings up through the rest of the comp, Alex?

I hit you so hard I bent my own beater, which was under designed because I had estimated at top speed of 8000rpm with losses, not 11'000 :P And I was using tapered thrust bearings which had to be in compression (something I didn't know at the time) so that's why they kept exploding. If I ran that again I would use regular deep groove and I doubt it would break/explode. As for the thickness, I just looked at the Pic and guessed And I failed :P