Construction materials

[sanddrag]

Originally Posted by David Guzman

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.

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.

[mechanicalbrain]

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.

[David Guzman]

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

[mechanicalbrain]

Originally Posted by David Guzman

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

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

[sanddrag]

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.

[BrianBSL]

Originally Posted by Andrew Blair

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 almost twice 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

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.

[sanddrag]

Originally Posted by BrianBSL

But isnt steel almost twice as heavy as titanium?

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

[Cory]

Originally Posted by sanddrag

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

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

[Matt Adams]

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

[Veselin Kolev]

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.

[Joe J.]

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)

[Wetzel]

I don't see nearly enough PVC.

Wetzel

[ChrisH]

Originally Posted by Wetzel

I don't see nearly enough PVC.

Wetzel

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.

[Paul Copioli]

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

[Gdeaver]

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.