Experience needed regarding FW Beater and Armor

[R9000]

Yeah, you want a barrel nut, not a barrel bolt I think. It's just a small metal cylinder with a threaded hole in the side. You get them quite a lot in IKEA furniture and the like.

[Runsler]

Guess i've found "a lot more expensive" on my quick search.
Anybody know what K-Cut or any watercutting service like that takes?
And if they do calculate for the finished product, or the sheet it is cutted from?

for 5kg*70€/kg it would be 350€... expensive, but payable. add some bolts as teeth to work around titanium being too soft it could actually work... maybe i don't have to start over again that much.
But i will try some designs with the motor to one side, too... even though i don't like to have a "weak" side, but seeing all these troubles with the mid solution i should at least test it.
Regarding the galling, i've seen some plastic bushings. they seem to be teflon coated, are even lighter and come in the right size.
Would those be working, or would i still have the same problem, and possibly even one more since they are not made from metal?

Also got me thinking... would it cause yet again new/different problems to make the axis non-rotating, and put the bushings actually into the weapon part? would have to strengthen that part, since obviously the material gets a lot thinner when i put a hole into it, and the pulley would have to be directly connected to the weapon instead of to the axis. could possibly even just make a ring as a pulley instead of an actual disc. This way i could make the axis from steel (that won't be soooo much weight), the weapon possibly from titanium, but won't have the galling, since the bushing stands relative to the titanium still, and only rotating to the steel (which should be no problem, i think).



Ah, and thanks, google actually found the dwarsmoer. now i recognize them, used those a few times when working with wood. Logical they would work with HDPE, too.

Another idea would be thin plates with a threaded hole inside working quiet the same way, only they are thinner and might be lighter. But the thread would also be more likely to get ripped out.
Got a small plate HDPE in the post today, will do some testing to that one looking what is working the best.

Oh well...
So much to rethink and research and so little time and knowledge beforehand

[maddox10]

Forget plastic bushings. Those are for light loads only.

It also seem that I'm one of the few that uses sinterbronze bushings on the spinners (Valkiri for example, that's 7.8Kj of spinner)

Titanium for a spinner. Not only is the base material a lot more expensive, but the machining is a lot tougher(or expensive) and the vurnability is another matter. Securing the teeth/hammers/cutters isn't easy.

It's more cost effective to make a smaller monoblock beater in decent steel like high grade hardox.

Nick has a lot of experience in compounded spinners, bars and beaters, and in his last machines, it's all monoblock.
I would defer to the master if it comes to spiny things.

[Runsler]

Okay, so no plastic bushings.
But what else (if not bronze) do others use? ball bearings also don't seem good.
Also, how about that idea with the bushings in the rotating part (let's say bronze bushings, no matter what is rotating there) on a fixed steel axis? which problems would that cause?

Since i need to redesign now, anyway, i could as well try to go a complete different way for the weapon design.


The problem is with a smaller diameter, my complete design and idea is useless. The important thing about that is, that it is wedge-shaped and invertible, with a vertical spinner to the back of the wedge. now if i make the diameter of the spinner smaller, it is hanging somewhere in the air (at least from one side), and a lot of ground-clearance isn't something too good in a spinner, i think.

So i have to stay roughly with that. And with titanium seemingly a bad idea, and Ali possibly even worse (at least when it comes to durability, sure not cost- or machining-wise) i have to search for a way to make a steel spinner with that diameter.

Or, if that is impossible (as it seems) even have to downgrade from such a wide weapon to a thinner one, like a disc. But with that, i'd have to redesign the mounts (or have a disc on a 30cm axis o.0), so yeah, a lot of work to do for me.

[Andy]

Could you not create a steel beater of a smaller diameter and adjust/redesign the bulkheads accordingly? If the bulkheads are still in the design stage im sure itll be worth the extra bit of time

[Runsler]

not really. the whole bot as seen from the side is a triangle.
the idea was to make it invertible, so no matter on what side it drives, and having a wedge as "backside", and the spinner on the (almost) vertical side of the triangle.

Now the diameter of the spinner is fixed by how high the bot is at half of that as distance to the bot.
For an angle of 45° for the wedge i got (17+r)*tan(22,5)=h
with 17cm the length of the rest of the bot including a small safety gap to the spinner, r being the radius of the spinner and h being the height of the bot at the point where the axis is.
The left side of that equation would be the overall length from tip of the wedge to the axis of the spinner.
Not included is some extra height since the wheels would of course make everything a bit higher than just the chassis. Only ways to change this would be:
use smaller internal parts (i couldn't find any) or making a sharper wedge (and the wedge-part is getting ridiculous long then, with a lot very flat, unused space on the inside, making weight for the armor much higher).

So to make the spinner scrape the ground no matter on which side i drive, i would need a value for r that gives me an x a little less than 2r would be.

(hope this works...)
https://cad.onshape.com/documents/4b...ad700dae0c2145
you should be able to see a view from one side.
the small square are gearboxes/drive motors as seen from the side (wheels would be there), the bigger rectangle are both ESCs (brushed for drive, brushless for weapon) and the battery seen from the side. then the back wall (1cm HDPE), and those lines are the outlines to where the chassis would go, depending on weapon size and position.

Maybe this could make it a bit clearer if anybody is still interested in the overall concept.
Well... seems to be a concept not working (this way) anyway.

Will likely have to swap battery/escs and drive train, put everything in a box shape and go with a normal beater or something like that. Actually, that will only make me lose the wedge and the unique look. And i'll need a new name.
But will be way easier to build and drive, and likely more effective, too.
Maybe i can come back to that when i am more experienced, technology is advanced, and i could possibly have a few ideas how i could actually pull that off. Oh, and maybe even enough money to just try it and another bot to enter if it fails^^

Still, learned a lot just from this thread, will try to put it into use when planning this new bot (won't call it another version, too much change for that). Maybe it'll be something completely different.

[Andy]

but does it need to be that high? could you not lower the whole thing with the diameter of the weapon? or you could lower the beater as much as you make the diameter smaller so it is still on the ground when up right, since when you're upside down its kind of useless anyway since you're flipping yourself

[overkill]

One thing with having a stationary axle is that the rotating part has to be much thicker around the axle; with a 20mm axle, a 25mm OD bearing and at least 15mm of beater material around that and the beater will need a 55mm thick block of material, most of which ends up as waste. The advantage of a stationary axle is that it ties opposite sides of the bot together and ensures that bearings are aligned. Drum bots almost always use stationary axles for this reason, while beaters seldom do; they rely on the strength and rigidity of the beater to provide a 'virtual axle'.

With bearings, there are not many choices to use with titanium. One possible idea is to machine & polish a short steel tube and press fit that over a solid titanium axle - that would remove the galling problem with a bronze bushing. Apart from Mario, most vertical drum and beater spinners use ball bearings very successfully.

General design: Waaay back there was a builder called Daisy Robotics, who made a series of successful bots similar to yours. they used a short, wide drum but the concept is the same. To improve resistance to side attack he cut off the front corners at 45 degrees like this:



Turning a sharp 90 deg. corner into two 45 Deg corners adds a heap on strength, even if the weapon goes the full width of the bot.

[Runsler]

That might be a possibility, just making the weapon end a bit "thinner", so the weapon would be smaller and lighter.
Maybe i could even go with less reduction then to speed that thing up a bit.
Have to do some drawings and calculate the new weight of a front end with that style and if that would help.
maybe get the weapon more to one side, so i could get the motor on the other side of it... well... long day(s) of scetching this will be.

Also good to know drums often use stationary axles, somehow that didn't get clear from most build diaries.

Somehow i am also thinking a bit more of a wide disc or short drum now, since a short beater wouldn't be much different from a bar with a long hole in it, and i'd think a more round shape could be more efficient, distributing the incoming forces better. but will have to see how the dimensions and the weight left look when i redesigned at least the front end.

Would it be a possible way to basically build two discs, also put the teeth into those, and just connect them with a (toothless) tube?
the middle part of that tube would only be for added stability and to get more weight from the inside to the outside of the weapon.

[overkill]

Having two disks with a tube or solid block is not exactly common but the idea works well. The advantage that I see is that a hefty set of bearings can be placed in the tube section, which would otherwise be wasted space. The tube can also have a pulley groove cut into it so you can place the motor in the gap between the disks. Its back to your old plan on steroids . Another advantage of a stationary axle is that you don't need space for large diameter bearings in the side walls, so the axle can be closer to the front and the weapon projects further out from the body. That helps protect your bot as your weapon is a bit more likely to contact your opponent before it gets to your armour.

From my experience in fighting vertical disks, they should be at least 12mm thick Hardox or similar steel or they can be bent by horizontal spinners. Increasing the weapon speed is much more effective than increasing it's weight. In the KE formula energy increases with the square of the speed, so doubling the RPM gets you four times the energy.

[daveimi]

Nick you have to be the single most knowledgeable person I've ever seen.

[overkill]

<blushes> Thanks ,but not true - there are heaps of other builders with deep reserves of knowledge; Mario, Ellis, Mouldy, Jamie, just to name a few. I like to think my forte is a willingness to do research and be proved wrong - just a few posts ago Mario pointed out that I am being ripped off on titanium costs.

[maddox10]

The €70/kg for titanium remark , that's a decent price for offcuts at a good supplier.
New material cut to fit the requirements , that will be a lot more, as the supplier will have problems selling the rest of the standard sized in a reasonable timeframe (from the accountants pov).

[overkill]

And of course THE most knowledgeable guy in combat robotics; Marco from team Riobotz, who literally wrote the book.

[overkill]

Mario - I am paying the equivalent of over €100 per Kg, plus shipping from the US. The local Ti price is even more and suppliers only sell full sheets. In 4 months our government is going to stick a 20% GST tax on all imports - everyone should hang on to the EU with both hands to keep costs down!