Materials; Laser cutting

[emisnug]

So, I'm almost at the point in our design where I'm happy to send things off to be cut with a laser. It's a basic enough design, everything is sketched out where I want it (the construction lines on the image), I'm waiting on the motor mounting pattern before I send it off. The question now comes down to material.

By my reckoning, I've got 2 solid, cheapish options, and 2 expensive options.

Mild steel. Heavy, but cheap as anything.
5083 Aluminium. Slightly lighter, but still pretty cheap.
7075 Aluminium. The grade I actually want, but price quadruples over 5083.
500 Grade HMWPE - Prices vary; I've never worked with it before.

By reading through the build diary for Gabriel, I know that HMWPE can be used without too much issue, but I'm worried that it might not withstand impacts all that well. And I have no idea how well it holds a thread, which would be key to making sure the bot is stiff enough.

What would everyone's preference be? Any recommendations on laser cutting places?

A couple notes about the design:

Two frame sections, held together by screwed in crossmembers, with a total height (not including armour) of 180mm. Each frame section is 25mm in depth (if steel, 12.5mm); the construction lines show drive motor position and mounting, wheel position and battery location. I'm still working out the joining functions on Inventor, so unfortunately no isometric models for now. If there's anything glaringly wrong, please do let me know.

Big Dave 3 Frame MK1.PNG

[Eventorizon]

Unfortunately that won't work. Even at 25mm thick, the frame work is far to thin and would bend/snap.

Carbide used Hardox sheet braced with tubular steel to make their frame strong enough and its much shorter than yours and thus much stiffer.

If you are going for something like Tombstone/Last Rites, its made out of tubular steel, not solid bars/plate steel. The stiffness:weight ratio of tubular steel is far greater than solid bar, that's how it can be built like that.

NST. the FW spinner that Carbide is half based on, uses Aluminium to make its 'A' frames but that is 12mm thick and uses a fixed shaft to make sure it can't flex.

HDPE is not suitable for a HW spinner, its to malleable and flexible. It can be used in FW's but you are looking at at least 20mm thick.

It is hard to tell but it also looks like you could make the components much more compact, saving you weight and thus allowing you to make the frame stiffer. Your bar also doesn't look thick enough. Carbides bar is ~25mm thick and ~100mm wide I believe and made of Hardox. Yours looks like its 50mm wide at most.

As much as it feels great building HW robots, I'd advise looking at the FW class first. I've been building robots for 5 years now and only just begun building my first HW. It is possible to jump to the HW class but you need quite a bit of prior knowledge to pull it off.

Keep goingthough, pretty much no one gets it right first time. For example, first motor/gearboxes I bought were actually meant for model vehicle applications like replica steam engines/scale busses: totally useless!

[emisnug]

A healthy dose of reality is always welcome, cheers!

The bar in the picture is literally just a construction line, we were intending 1250 x 125 x 25 for the bar, with bolt on impactors to take it to 1300mm.

Tubular steel - I've used and cut and welded it before, but never to an actual design. Back to the drawing board! I'll scrap the HDPE idea.

[maddox10]

On HDPE. Craigs machines like Gabriel and Saint get away with it as it's used in a 'lose" configuration. Meaning it can flex and bounce to its hearts content making it incredibly resilient.

If you use it in a stiff setup, as needed for a spinner heavy, it loses most of its advantages.

Ali, stay away from the 5000 series.
If you want to weld it, the 60XX series is the optimum , if bolting or riveting, even glueing are your possiblities, look no further, and 7075 is your friend.

Yes, it does cost more to start, but it will pay back very very fast.

Mild steel. If used well, and seen as sacrificial, you can't go any better. But again, fork out a tad more money, and get a lot more BANG for the buck if you go specialised steel.


Hardox, due its nature seems about perfect for our application.
Pity the new hardox 500 tubes start at 76mm diameter... If it was 40mm with a wall of 4mm , ideal for Beast/Tombstone exosceleton designs.

[bigsi]

HDPE is not suitable for a HW spinner, its to malleable and flexible. It can be used in FW's but you are looking at at least 20mm thick.

Excuse me?
Our drum was made from 10mm thick HDPE. Although it took some cosmetic damage, it was only actually pierced once and then only a 5mm hole.

As someone else pointed out, if it is mounted in such a way that it is allowed to flex, it will distribute the force over a larger area meaning the energy is absorbed by much more material. With rigid armour, the energy is concentrated, giving a higher probability of puncture / fracture.

Polycarbonate and Polypropylene are less suitable as they will fracture at lower energy conentrations than polyethylene. To determine how much energy (pressure) a polymer will withstand, it's inversely proportional to it's freezing point (the point where it becomes solid (rather than plastic) and brittle. A low freezing point material will absorb more energy before it fractures.

It's worth looking at Armox instead of Hardox for similar reasons. Hardox, despite the name is intended as a wear resistant steel (things like teeth on digger buckets). It is tough, but will fracture at relatively modest impact energy concentrations. It's not considered ideal for Armour.
Armox is intended for balistic protection. It's available with the same hardness as Hardox but will absorb higher impact energy concentrations without fracturing / penetration.

For structural elements, my preference is Domex / Docal. It has 3x the tensile modulus of mild steel, is easily welded & folded - It is less likely to suffer stress fractures than Hardox as it's elasticity & ductility allow the force to be distributed over a wider area (lower stress concentrations). It's cheaper too! Compared to mild steel, you can achieve a 66% weight saving. It is generally about 3x the price of mild steel - but for the same price you achieve the same strength with 1/3 the mass.
Quick guide here:
http://www.ceweld.com/sites/default/...e%20steels.pdf

[Eventorizon]

Excuse me?
Our drum was made from 10mm thick HDPE. Although it took some cosmetic damage, it was only actually pierced once and then only a 5mm hole.

I don't want to be stating the obvious but the weapons have come on a long way since Barber-ous was fighting. A good modern FW can slide through 20mm+ of HDPE with no problem. Granted, being able to absorb hits and deflect in an impact will help prevent it from being punctured, but if a spinner gets a good hit will will be doing straight through it. I was also talking about it not being suitable for as chassis, not the weapon itself.

Armox has only recently become available to us, which is why most machines are Hardox. An issue that has been spotted with Armox is that it doesn't bend at all which can cause knock on problems. When Photon Storm fought Minotaur, the killer blow dealt to PS caused all the welds to break on its new front Armox panels meaning all the force was transmitted back into the robot resulting in huge amount of internal damage. If it had still been Hardox, the front would have bent absorbing the energy there rather than transmitting the force through the robot. While it still would have been a killer blow, probably, less energy would have made it to the internal components and would have resulted in less damage.

Basically, because Hardox bends it cab absorb hits rather than just moving the point of failure, or being so stiff the welds break. I'm not saying don't use Armox, more its probably only any god in key locations. Also, I am not sure of its cost but I would be willing to bet its much more expensive than Hardox.

I agree with you about the Polycarbonate and Polypropolene, though Terrorhurtz is proof that Poly can be used if done correctly.

I also agree with you on Domex/Docol. Great stuff! Used it when I was working for a company that designed gritters and other road maintenance vehicles. Some of the shapes we were able to make with it were amazing wile still being structurally sound and not weak at the bends.

[bigsi]

Were this universally the case, you would expect vehicles with balistic protection to fall apart under fire. It is more difficult to weld and form than Steel, but actually very similar to Hardox of the same hardness / grade.

It could be that the people you mention procured a less suitable grade. However, without knowledge of the grade, it's impossible to make any useful comparison between Hardox & Armox.

Like all materials, you have to pick a grade that is suitable for the application and follow the recommendations for welding / forming.

Armox can be folded and will bend - but it is both tough and hard so will transmit shocks unless this is mitigated in the design.

Cost wise for the raw material it's similar to Hardox. However it's harder to find so you may pay more due to supply/demand.

Perhaps it is a little too specialist in this market - but balistic protection is what it was developed for.

"A good modern FW can slide through 20mm+ of HDPE with no problem" - It's funny, that's what everyone said back then too. I remain to be convinced - and you may get your chance to convince me!

I agree HDPE is mosly unsuitable for the structure above, but not universally so, depending on the design.

Si

[Eventorizon]

Photon Storm was a team comprised of Ed Hoppitt of Storm 2, Andrew Marchant of Tornado and Alan Young of Cronic/Robots live. If you were trying to assemble a dream team of robot builders that would be a pretty good combination. Given Alan and Ed's connections I would reckon they got the Armox direct from SSAB.

I'd also bet that Minotaur drum packs far more energy than a 50cal round; which is probably what Armox is designed to stop along with other small arms fire, and is probably going to be in contact with an opponent for longer, imparting more energy into what it hits. I'd link you a video of the fight but it seems ABC haven't got it up on Youtube which is weird.

[bigsi]

I'm really not trying to pick a bun fight with you.

I'm sure you and the 'dream team' are more knowledgeable about this than me.

[Eventorizon]

Armox is on the same level of unknown as HW brushless drive: some people have tried it and the results have been mixed.

I'm not trying to pick a fight either, I just thought you'd like to know that current experiences with certain materials are not what you'd expect, or the same as they used to behave in the past. With Robot Wars back, there is going to be a new arms race, and its going to be fun finding out the results one way or another.