Construction materials
 

I'm curious about what everyone thinks is a good material for frame construction. If you have a specific please feel free to mention it. I'm more curios about the strength than cost.

Re: Construction materials
 

Aluminum (Or if you're RAGE/571, wood ;))

Don't use steel. Ever. We did in 2002, for some insane reason thinking we would need the extra strength with all the robot on robot contact. Well, a frame made of 1 1/4" steel angle 0.125" thick ended up taking almost half of our allotted weight--with nothing on it.

Needless to say, aluminum would have worked just fine, and we would have had the weight for a lot of components that got scrapped because the thing was so darn heavy.

P.S. Perhaps you want to make this a poll.

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Just like the commerical said: plastics make it possible. If it were cheap enough, I would make everything out of plastic.

BTW, what possible reason would they have to make those plastic propoganda commericals. Are there really enough people that feel strongly enough against plastic that they have to launch a propoganda campaign?

Re: Construction materials
 

Aluminum's done the job for 1293 on both robots.

Although that lexan splash guard we put on underneath the bubble maker (to keep the Victors and RC below nice and dry) does look pretty nice...

(By the way, this is why you shouldn't throw out all of those scrapped ideas immediately--that same piece of lexan was going to be a mounting board for electronics, until we changed the design on the arm. And an earlier version of our winch side plate became our extended battery holder.)

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Personally we didn't like our aluminum because it bent. It wasn't even at the joints it was the beam itself!

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Team 836 likes to use a combination of aluminum plate, angle and square tubing, plus 80/20. Very sturdy, easy to work with and tons of ways to use them. and for everything else lexan.

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Its not the type of material you use, but how you use it... remeber those bridges you build out of 1/8x1/8 balsa wood and elmers glue that would weigh 50 grams and hold 50lbs?

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If you want favorite material to work with as far as budget goes aluminum by far but if money was no object i would be using a titanium or carbon fibre. both of which have their disadvantages (titanium-dulls drill bits and other cutting tools fast)(carbon fibre-expensive and must be custom made for the most part)
So aluminum is easy to work with light and fairly strong.

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What type of aluminum were you using, specifically, and what was it used for?

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Yeah but a 1/8x1/8 titanium is going to work much better then balsa. However i agree engineering has a HUGE aspect on your strength. Also making frames out of carbon fiber is not as expensive as it seems.... ;) But yes you do have to form it yourself (its not as hard as it seems). I probably should have included fiberglass too but i don't think anyone here used it.

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Titanium is not all it's cracked up to be, at least from a FIRST aspect. Its extremely strong, but so is steel. Titanium is is like 30% as heavy as aluminum. I can have the bragging aspect of making a bullet resistant robot, but if someone is coming into a competition and shooting my robot, I have more important things to worry about!

Check this PDF out: http://www.engr.ku.edu/~rhale/ae510/titanium.pdf

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However it does make painting your robot easy! You can anodize aluminum with a 6 volt battery and soda. Who wants to paint the Guggenheim red? Anyone? Wouldn't that be amazing! robots doing drive bys on other robots... except for the whole safety thing.... I'm just trying to picture a gangster robot.

Re: Construction materials
 

Aluminum box tubing (square and recangular) has been the material of choice for 696 since 2004. Previously we had used Bosch 30x30 extrusion but then we realized that it was too heavy especially with all the corner brackets and fasteners.

I'd say if your base frame is more than 15lbs and it's not a crab drive, you need to rethink it.

mechanicalbrain, what exactly did you have bend and what were the circumstances under which it bent? Any pictures?

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Please someone correct me if I'm wrong but I think you are not allowed to use exotic metals in FIRST which would include Titanium.

Also my team has used 8020 and .125 aluminum, but next year we are thinking box aluminum or 90 degree corners depending on the game.

Dave

Re: Construction materials
 

They changed the rules.

You can use titanium, but you have to account for the price of it in your BOM.

$3500 isn't going to go a long ways towards building a robot, and buying Ti.

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You are wrong. That rule hasn't been in place since 2003 when they made illegal Titanium, Carbon Fiber, and Beryllium (and I believe Diamond too). Check section 5.3.4.2 of the 2005 Manual. The only rules to be conserned about really are

<R35> Additional Parts shall not be made from hazardous materials or be unsafe

and then the rules about Cost Accounting and suppliers. Follow the flowchart and you will see that Titanium will pass.

In 2004 we successfully passed inspection with titanium parts on our robot which the inspectors were made fully aware of. :)

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I wish i did have pics but we straightened the bend. A robot hit us hard at the clash and bent the side piece (we used the kit frame). Anyhew our robot was on three wheels for a while.

I think FIRST classifies exotic material as something not easily obtainable like a frame made of carbon nano tubes or radio active materials. (Battle Bots seriously lists this as a banned construction material) I think it applies to anything that you can't go to a store or online and just buy or make. I still don't think anyone would make a frame out of titanium. However molding fiberglass or carbon fiber could make it onto a FIRST robot.

Hey Sanddrag what parts were titanium and how did they work out weight wise.

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Thanks for the correction, i truly though they were not allowed. Well i guess no i have more things to play with ;) . Also how did you machine your titanium parts and where did you buy the material?

Dave

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I love google! Titanium suppliers how to machine titanium how to make titanium turn into groovy colors

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696 used 1/8" titanium plate which was lasercut. The plates had a 1/2" hole with an 1/8" keyway in them. They were used affix an aluminum sheet metal arm to a keyed shaft so the arm moved when the shaft was rotated. Plates were keyed to the shaft (by the small 1/8" thickness) and bolted to the arm. Also, we used them in a similar method to affix pulleys to the shaft.

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But isnt steel almost twice as heavy as titanium?

190 used titanium shafts for a lot of stuff last year, as well as some 1/16 or 3/32 plate for protecting some parts.. Definitally helps cut down the weight - I don't have exact #'s but I would estimate it saved atleast 3-4 lbs.

Re: Construction materials
 

Yes. Titanium is about 1.66 times the weight of 6061 aluminum and mild steel is about 1.74 times the weight of titanium.

So, titanium isn't your "strength of steel with the weight of aluminum" that a lot of people make it out to be. With regards to weight (density really) it is between aluminum and steel.

Nominal Densities in lb/in^3

6061 Aluminum - .098
Titanium - .163
Mild Steel - .284

Re: Construction materials
 

The idea is since it's stronger, you use less of it, thereby saving weight over aluminum.

Re: Construction materials
 

Just so we're on the same page... I think the term that many of you have nearly hit on is "strength to weight ratio."

This varies widely depending on the type of each material (hot vs. cold rolled, heat treating, carbon content, etc) ... but here's a rough guide:

Average Off The Rack Steel (1020 Carbon Steel)
Density: 0.284 lbs / in³
Yield Strength: 50,800 PSI
S:W Ratio: 178,873

Aluminum (6063-T6):
Density: 0.0975 lbs / in³
Yield Strength: 36,000 PSI
S:W Ratio: 480,000

Titanium (Grade 2)
0.162 lb/in³
Yield Strength: 40,000 PSI
S:W Ratio: 246,913

Titanium (Grade 5)
0.162 lb/in³
Yield Strength: 106,000 PSI
S:W Ratio: 654,320

I'll let you guys do the (Strength to Weight) to Dollar ratio, but the end answer is that steel ends up being the big big winner in that category.

Matt

Re: Construction materials
 

Over the summer I did a lot of design and fab work for Berkeley National Labs. Even though it wasn’t robotics you could adapt the concepts and use something like this:

-Cast, porous silicon carbide
-Heat treated aircraft aluminum (surprisingly cheap and easy to make, terribly hard)
-Bronze aluminum alloys

But those are really expensive or hard to do. Unless you get them donated by Berkeley, I don’t recommend you use them. If you’re creative, you can always.. um.. cast your own silicon carbide? I don’t recommend it to anyone with little casting experience. For obvious reasons.

Especially in using aircraft aluminum, if you have the tools to draw the beams in slight arcs (say, radius 200ft) and weld them together with the curve outward, welded as if they were straight, they give your chassis a bouncy feel to it. Kindof makes your chassis into a giant spring. But you have to be VERY careful when you weld it. Tac it here and there to keep it together, and then weld it SYMETRICALLY. If you don’t, your chassis will bend. It takes a lot of practice to weld like this.

If you don’t exactly have a drawer (I expect you don’t) you can always go to an exhaust pipe shop and have them heat treat and bend your tubing for you. It’ll cost a bit, but your chassis will bounce instead of bend. It has worked very well in the past.

Re: Construction materials
 

We use L channel Aluminum makes for easy construction using our Hacksaw & Drill method, our entire frame was pop riveted together. We tried using box channel Aluminum in 2004 which we welded together (most of those welds broke and needed to be bolted back together) Also when we tightened stuff down the box channel tended to crush, we don't have that problem with the L channel. We built our main frame around the KOP drive train and just bolted it down. We did use wood as a base plate in the bottom of the frame because it lighter and easier to mount things to than metal (plus we have had base plates that were supposed to be Aluminum magically "transform" in to 1/8 inch thick steel plate)

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I don't see nearly enough PVC.

Wetzel

Re: Construction materials
 

While not particulaly stiff, PVC is reasonably close to low grade aluminum in strength and has terrific elongation with a minimal weight impact. This enables it to absorb a terriffic amount of energy without breaking.

PVC is great for arms or other appendages. We used it for our arm last year and it proved to be extremely durable. Team Hammond ah also used PVC with great success in the past (2002) to build arms that survived tremendous forces.

But these applications did not require the PVC to help maintain a reasonably close alignment between components. Frames usually serve exactly that purpose, keeping the wheels and gears in reasonably close alignment, among other things. So I'd be a little leary of a PVC frame. Such a fame would probably weigh more than an aluminum frame that does the same job.

Re: Construction materials
 

Aluminum sheet metal is the ThunderChickens' preferred choice of material / configuration.

A couple of things to consider when weighing the steel vs. aluminum design choice.

1. Aluminum is ~ 1/3 the weight of steel

2. 6061 -T6, 6063 -T6, etc. aluminum is >1/2 the strength of your average steel. The exceptions are your alloy steels like 4140, 4150, etc.

3. Manufacturing constraints. If you calculate that a steel tube only needs to be 1/64" thick to be strong enough for your design, chances are you will not be able to use such a thin walled tube due to manufacturing limitations. So, by definition, your steel tube will be over designed due to the fact that you had to select a more readily available tube size. Aluminum, for example would need to be twice as thick (almost), but that still is pushing it for availability (1/32"). The aluminum would be over designed, too; but the weight penalty is much, much less.

4. Part geometry is more important than material selection. How your part handles the external load is much more important than what material you should use. In most cases on FIRST robots, we have more material than in we need in some places and less than we need in others. That single phenomenon is the leading cause of robot failures.

I can tell you this from an industrial point of view: Aluminum is the preferred material for housings, cases, etc. for economical, environmental, and cost reasons. Gears, shafting, and anywhere that needs small space, but high loads will usually be steel or titanium.

Composites are at a whole different level that I will leave to the spacecraft and aircraft people.


-Paul

Re: Construction materials
 

I agree with the the aluminum angle and pop-rivets. The frame can be crafted with only hand tools. T6061-t6 aluminum is readily available and cost effective. Do not use 6063. Use steel pop-rivets with backup washers. We usually use 1 x 1 - 1/8" wall. We cover the sides and top with poly carbonate attached with velcro. Mcmaster Carr sells velcro with real good adhesive. The poly adds structural strength. Welding the aluminum frame up is probably a bad idea unless a team has professional resources. Welding removes the heat treatment from a area next to the weld and can induce stress in to the metal from the expansion and cooling. Few teams will have the resources to stress relieve and reheat treat the whole frame. If a weld breaks during competition its a big deal. Drilling out some pop-rivets and fixing a frame is much faster. We have taken some massive impacts and not had frame problems. Buy some double end drill bits for the pop rivet holes.

Re: Construction materials
 

I think my personal favorite is a carbon/Kevlar composite plain weave with a heavy duty epoxy. Yes you do have to form it yourself but its actually pretty easy and not horribly expensive. The shaping is actually good because you can customize the shape of the frame without having to lose strength. Also It has a good strength to weight ratio (the exact ratio escapes me).

Also one critical thing Ive failed to see mentioned are the types of strength materials have . A materials type of strength defines its function. The Kevlar gives a great tensile strength and torsional strength. Yet the carbon gives the weave good transverse strength and amazing impact resistance. However I recommend practicing the process and familiarizing yourself with the materials. If you don't form it evenly or use the right types of epoxy and weave you will create weak points.

Re: Construction materials
 

696 has used welded 6063 1/8" wall aluminum box tubing frames for the past two years. 6061 was not available. Anyway, we have had virtually no problems with the 6063 and have only cracked (very minorly, no repair needed) a weld once ever. The 6063 polishes very nicely too. :) As with any welding, be prepared for warpage and have a plan to correct it (or prevent it)

Re: Construction materials
 

I wouldn't use carbon fibre as the sole material in a frame.

There was a thread about using it in different robot applications, about a year ago. I noted then that if you scratch something made of carbon, even cosmetically, you can cause it to catastrophically fail, with no warning like with metals that will bend.

Seeing as a frame is probably going to get the hell scratched out of it, that doesn't sound too appealing to me.

It's not an easy material to work with either. It can be very hazardous to your health if you don't have a good understanding of what you're doing and take safety precautions.

Re: Construction materials
 

Ive worked with carbon fiber on a multitude of modeling projects and Ive never had the problem you've described. Actually if you use the correct epoxy it will be hard enough to scratch it. Actually the structural characteristics of carbon fiber are completely altered by the weave, material, and epoxy. As to being hazardous. Epoxy is essentially a glue. Use a respirator and work in a ventilated area and you'll be fine. It really isn't dangerous as long as your not directly breathing in fumes (I think thats the case with almost all chemicals). Oh and make sure to use gloves and long sleeve shirts.

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Yes, Ti is about the perfect medium between aluminum and steel, but as for frame building, not so much. The more I think about it, it would be useful for small parts though.

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http://en.wikipedia.org/wiki/Scandium :)

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I know they use that in bicycles but i don't know anywhere where you can buy it. I tried to google it but nothing promising showed up. Any good links?

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It's way too cost prohibitive for FIRST use. I don't know where you can buy it in it's raw form.

it's original use was on USSR ICBM's on submarines under the polar ice caps. The missiles would have their fins ripped off, so they started making them out of an alloy of scandium and aluminum, giving them the strength to punch through the ice.

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"Scandium metal powder is combustible and presents a fire hazard."-wikipedia

haha, great, electrical fires and chemical fires! :D

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That's why you wouldn't use it in it's powder form :)

It's no different than magnesium--it's highly flammable when in strips/powder, but it's used for various other things in a more solid state.

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http://en.wikipedia.org/wiki/Magnesium cool stuff really. What would FIRST say if we told them our robot was made of magnesium (I know, I know, Its near impossible to burn the bulk stuff) Much lighter than aluminum.

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whilst looking for an aero seat tube for the composite TT bike i'm (hopefully) going to build, i noticed that easton sells scandium alloy tubes used in high-end mountain and road bikes.the selection is very limited, but if you absolutely have to scratch that scandium itch, here's a trustworthy* place. they also manufacture components made from carbon nano-tubes, but i don't see any tubing made of CNTs...yet.


*i assume since a large number of distributors and manufacturers trust them, we can too.

Re: Construction materials
 

I don't think Easton sells these directly to the consumer. As far as I know, they sell them only to the manufacturer of the bikes (of which, only one or two companies use it).

Easton is most certainly a trustworthy company as far as quality of product goes. I don't know much about their baseball bats/hockey sticks, but their bicycling products routinely exceed their strength parameters by 3-4x.

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i assume that you'd be able to get materials and components from one of their international distributors. in the US, it appears to be a company called Veltec Sports Inc. I plan to contact them in the future to see about buying some materials, and if it's still relevant, i'll post what kinda response i get.

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When they say "distributor", I think they're referring to finished parts, not semi-raw materials.

At any rate, bicycle tubing is probably fairly hard to work with. I know Scandium isn't easy at all to weld. It's also extremely cost prohibitive. A plain bicycle frame made of the stuff is around $2000+. After all is said and done, for FIRST exotic metals probably wouldn't offer much of a tangible advantage over plain aluminum when used properly.

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since they've gone and put part numbers on all of the tube sets, i'm hoping that's an indicator that they're available to the general public. either way, i suppose i'll find out when i contact them.

i agree that exotic metals probably won't give me an upper hand; i'm still trying to get the most out of Aluminum 6061. but it sure is fun to dream of 'bots made of scandium and carbon nanotubes...

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or some reason I thought that magnesium was a banned material... not sure though. I have a pair of magnesium snowshoes actually, and they are pretty neat.