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weight probs
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Spockie-Tech
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Joined: 31 May 2004
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Location: Melbourne, Australia


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just a quick tip for those looking for weight savings..

UHMW plastic can be had very cheaply from Safeway.. it come in packets of 3 different "Knife cutting Boards" for your Kitchen bench for just a few $. You can even get it in different colors IIRC. Look near the cutlery section.

Post Thu Jul 08, 2004 10:43 am 
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Nick
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You might want to check that - I used two sorts of polyethelene plastic in basilisk; one was industrial UMHW in the wheel mounts and the other was the chopping block stuff. There was a HUGE difference in the sawing, cutting (handheld knife) and drilling characteristics that makes me think the chopping block stuff is not good for structural work. It would still be great for things like spacers, electrical insulation, and other lighter jobs - can't beat the price though.

I think that was the reason that Hellbringer didn't stand up to the spinners so well. It was a really clever design with sandwiched chopping boards to get the weight down. When I looked at the damage, the cuts looked like it was relatively more brittle than the "real" UMHW.

The Dotmar web site has a pretty good explanation of the different grades if anyone is interested.

Another cool thing with assembling UMHW parts; you can use sheet metal screws and the result is completely shake-proof Smile. I use long 10G screws with great results.
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Post Thu Jul 08, 2004 11:30 am 
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Spockie-Tech
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thats quite likely.. I'm sure the cheap stuff is not as likely to be as structurely sound as "quality" UHMW. In general you get what you pay for Wink I hadnt compared them myself though, so thanks for the info.

Post Thu Jul 08, 2004 11:53 am 
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kkeerroo
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quote:
Originally posted by Nick:
I think that was the reason that Hellbringer didn't stand up to the spinners so well. It was a really clever design with sandwiched chopping boards to get the weight down. When I looked at the damage, the cuts looked like it was relatively more brittle than the "real" UMHW.


I don't understand that comment. Did or didn't stand up well? Only one hit managed to get though the UHMW and that also went though the battery and aluminium. I think the way Hellbringer was bouncing around the arena during the annihilator, including on hit of Balisitic where Hellbringer knocked itself around 180 degrees while airbourne, made those $10 chopping boards well worth it.
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Post Thu Jul 08, 2004 12:49 pm 
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andrew



Joined: 16 Jun 2004
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Location: Castle Hill, Sydney. N.S.W


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Well considerinb u drilled teh heck out of it and all I also agree with u. for 10 dollars worth of chopping boards they did pretty well against the evil spinner of nick. I may get some in case i ave to face basilisk of Glens new project again as they r quite alright for the price and saves your frame or armour getting mangled all for alittle extra weight.
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Post Thu Jul 08, 2004 2:37 pm 
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Glen
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ah i think that may be incorrect. i remember reading on team nars site (of belladonna fame) that he used chopping boards and that they where not UHMW. i will chase up the link.

the UHMW though is very good at certain things. mounts for one as nick said they basically are there own locknuts. i use the 12g screws with excellent results.

the chopping boards for their weight and price weren't to bad. of course nothing like aluminium etc but you cant complain for $10 and a few hundred grams. id be inclined to go kevlar and foam though for nearly the same price and weight with a big increase in strength.

Post Thu Jul 08, 2004 5:05 pm 
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Nick
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@ the Keerroos: Don't get me wrong, I thought Hellbringer was one of the cleverest designs at the annhilator. I noticed that there was a deep cut on the back left side where a spinner blade had gone through the outer side wall.

To me, the plastic had a bit of a fractured look, where the industrial stuff would have had more of a flowing look as the UMHW tends to stretch before is fails. For $10, the chopping boards are great performers - all I was saying that for more money, one could get a bit more structural performance.

BTW: just how many lightening holes did you drill? you must have a lot of patience. Smile
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Post Thu Jul 08, 2004 5:39 pm 
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andrew



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Yeah, that amazed nme as well
COuld Jake or someone put up all details on where they buoght it from, for how much and what u have to do to it to get teh super light yet strong material that they had all over and inside their robot.
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Post Thu Jul 08, 2004 5:40 pm 
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Glen
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a place called FMS and you buy the kevlar for $10 i was told.

i assume making it is largely the same as fibreglass with the matting and then just coating it with the resin.

i will definitely be trying it some time.

Post Thu Jul 08, 2004 6:17 pm 
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chris



Joined: 18 Jun 2004
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Location: Brisbane


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quote:
Originally posted by Nick:

I think that was the reason that Hellbringer didn't stand up to the spinners so well. It was a really clever design with sandwiched chopping boards to get the weight down. When I looked at the damage, the cuts looked like it was relatively more brittle than the "real" UMHW.


Nick, when you say they used cutting board stuff to save weight, well, i know they drilled holes in it but actually it would be heavier than a sheet the same size of polycarbonite
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Post Thu Jul 08, 2004 8:09 pm 
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Glen
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not unless it was over 6mm thick. in which case youve lost your weight advantage.

anythiner and its just too flimsy.

Post Thu Jul 08, 2004 8:25 pm 
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kkeerroo
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I never intended the polyetylene to be used as armour but unfortunatly my sprokets took all available weight (1.25 kg each!). I wanted a strong robot that could be run into opponents repeatedly and bounce back and the UHMW boards where all I could afford (including cost of tools I would need). All up the chassis weighed in at 2.5Kg including all the ali barackets and nuts and bolts. Armour (3mm ali on the front) = 400g.
All up the robot weighed 16.5kg before the holes and 13.9kg after. I estimate that I drilled over 500 5/8 and 1/2 inch holes into the chassis, armour, wheels and sprokets of the robot.

What have I learned: -UHMW polyethylene is great for structual elements but is easy to cut making it bad for armour.
-Be careful when ordering sprokets and gears from catalouges as weights stated in the catalouge may be incorect.
-Use plenty of lubricant when drilling high tensile steel.
-Nicads would be nice if you have enough money left over after buying sprokets and plane tickets
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Post Thu Jul 08, 2004 10:10 pm 
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Nick
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@ Chris: No the UMHW PE plastic (.95g / cc) is lighter than polycarb (1.22g /cc). You might argue that the polycarb could be thinner for the same tensile strength but the Keerroos made the right choice for structural plastic as UMHW is far less brittle than polycarb.

If I was rebuilding Hellbringer, I would use industrial UMHW with thinner sheets on the inside, 2024 alloy Aluminium on the front & back, and lighten up the sprockets some more. Much easier to talk about than to do of course...
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Last edited by Nick on Fri Jul 09, 2004 7:19 am; edited 1 time in total

Post Thu Jul 08, 2004 11:13 pm 
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Ajax
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quote:
Originally posted by Glen:
a place called FMS and you buy the kevlar for $10 i was told.

i assume making it is largely the same as fibreglass with the matting and then just coating it with the resin.

i will definitely be trying it some time.


There are a couple of different types of kevlar. There is one the is very similar to fibreglass. The there is the real stuff. with that stuff you need to form it to ths shape you want do some things to it (Not sure sorry) then shrink rap it and put it in the oven and bake it for so long.

I was told the basics when at Dunlop pacific.
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Post Fri Jul 09, 2004 2:15 am 
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3Faze



Joined: 26 Jun 2004
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Location: Lincolnshire, UK


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A good tip for building bots is aim for the finished product to be underweight.

Eg - when I built omicron, I wanted a 12kg feather with a 6kg spinner. So I planned a 5kg chassis and motor mount, which came out at 5.5kg, and a 6kg cage which came out just short of 6.5 kg.

End result - bot is 6 grams under Very Happy

Post Fri Jul 09, 2004 6:17 am 
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