Do you have some pictures of the hand so we can see just what it is you’re working on? It’s a lot easier to offer useful suggestions if we can see just what the problem is.
Well, i’m not a materials expert, but here’s what I believe.
There are differant types of stresses. Compression, torsion, and lateral load (i dont know what it’s official name is or w/e).
Titanium outperforms both CF and Alu in all three fields. It’s also the lightest (if used properly but it should be VERY close to CF). That comes at a price however… lots and lots of $$$. Titanium is far from cheap… but it’s extremely strong! We used it one year on our electrical board (we actually used a titanium tennis racket… and it worked great). To make an entire robot from this would exceed the cost limitations.
Carbon Fiber, or CF, is also extremely strong in compression and lateral load. I dont think it’s very good at withstanding torsion though. Again, though, CF isnt cheap, and it isnt healthy. Cutting CF puts small fibers into the air that destroy your lungs =D.
Lastly, aluminum. Aluminum is the most heavilly used material in FIRST, by far. It’s lightweight, and relatively strong. It can withstand tremendous compression forces. It can also hold up to torsion quite well (well, at least Alu box and tube can). It will give quite easilly, though, if it is subjected to lateral loads with a long lever arm. Best of all, Aluminum is cheap, and easy to use. Additionally, it can be welded under reasonable conditions. Welding titanium requires an EXTREMELY controlled environment or the metal will burn. Welding CF is well… impossible. Welding Aluminum is easy (compared to the others. From what I hear, welding Aluminum is still significantly harder than welding steel because aluminum will still melt with relative ease).
For my team, we’ve always used mostly (prob 95%+) aluminum on our robots. We cannot afford to use Titanium, and we don’t see the benefits of CF. Another alternative, steel, which is also cheap and strong, is extremely heavy, so we cannot afford (the weight) to use it.
As for using expanded (extruded, i assume) aluminum, it will not flex back to the original position after it flexes to pick up the ball. CF wont flex either. Titanium will flex, and it has pretty strong memory, but it’s very expensive. Maybe you can try using some plastic material (like polycarbonate or delrin)
I hope that was useful (and correct). Take care,
You can get tremendous stiffness-to-weight benefit from carbon-fiber composites, but there are drawbacks. As already pointed out, composite structures are susceptible to impact damage. Assembly with composites can be a challenge. It’s rather difficult to use fasteners with composites. If you want to use screws, you will need to install inserts or have large nutplates bonded to the composite material. In order to use rivets, you will need to bond sheet metal to the both sides of the composite around the area of the rivet (directly riveting composite will just crush the material and result in a very weak joint). The preferred method of joining composites is bonded joints and to do that right, you need to control the bondline thickness.
That being said, there are some structures that can be effectively constructed from composites. High-stiffness tubular truss structures are a good candidate for composites. Holy Cow’s (1538) forklift arms in San Diego were a work of art, Unfortunately, I didn’t get a photo but maybe the San Diego Imagery Award winners can share this.
From cost, strength-to-weight, ease of fabrication, assembly and repair perspectives, it’s hard to beat aluminum for FIRST robotic applications.
Here’s an example of what I was talking about with the custom fabrication, our 2005 bot. My dad and I made a mold specifically for the second stage of the arm (c-shape). We laid two different configurations: a 3lb heavy duty, and a 1.5lb light duty. We had the weight, so we ended up using the heavier one. the arm though is indestructable. It’s great to see the reactions people have to me hitting the light arm against a brick wall without it even scratching.
So is titanium(Grade 5/9) lighter than aluminum? I thank you all for the advice, we want to use titanium or carbon fiber for the fingers on the hand ( the part where the ball rests). What do you guys think? Also you have to consider the fact that at San Diego our robot hand got pounded and is bearly in one piece and that is why we want to upgrade. That is why I asked if titanium or carbon fiber flexes.
i think that unless you have the ability to make several sets, carbon would be a bad choice. though carbon would be significantly stronger, it would ultimately have the potential to break. for that application i think fiberglass is your friend. it offers greater flexibility and is easier to acquire.
I am not a materials engineer. I’m not a mechanical engineer. What I am is a hobbyist who has built several boats out of composite materials and a few robots. Here are some things you might want to think about in addition to “flexibility.”
Titanium is a pain to work with. It’s hard to cut, nearly impossible to bend, and has to be welded in an inert gas environment. It is springy, though, which meets one of your criteria. The chance of your robotics team being able to work with titanium is low, however, unless you have a mentor with real experience with the metal. I wouldn’t consider it.
The reason people like carbon fiber composites is that they are incredibly stiff, and light for the strength and stiffness. The thing that carbon fiber composites are best at – stiffness – is exactly what you aren’t looking for. It’s probably not your best choice.
I see a lot of stuff on this board where “fiberglass” is used in addition to “composites.” What is usually known as “fiberglass” is glass fiber matt, cloth, or biax held together in a polyester resin matrix. Generically, fiberglass is just one type of composite material. The way you normally experience it, fiberglass is not very stiff, not particularly strong, and heavy for the strength and stiffness. The advantage to fiberglass in boat building, for example, is that it can be laid up by fairly unskilled labor, and doesn’t cost much.
More advanced composites use epoxy (or sometimes, vinylester resins, but let’s keep it simple) to bind a material together. The materials used include glass fiber in a variety of orientations and compositions, kevlar, and carbon fiber. One of the best things about glass fibers is that they are available in a variety of pre-made types – woven glass (in the US, measured in ounces per square yard of fabric – 6 ounce is lightweight, 10 or 12 ounces is heavy), bundles of glass fiber held together in crossing orientations of different weights (called “biaxial”), and other things like mat or chopped fibers which are generally not used with epoxy. Matt and chopped fibers are used to bulk up thicker laminations used with cheap polyester resins.
Since epoxy is expensive, very strong, and impervious to moisture, it is usually laid up with a fiber over a core material. We won’t go over cores in detail here, but for amateur use it’s very popular to use lightweight, high-quality plywood as a core for structural use. Something like BS1088 meranti or okoume plywood is stiff, strong, and lightweight. Some very expensive, very impressive sail- and motor-boats are built up with multiple layers of biaxial fiberglass laid up in epoxy over a core of 1088 plywood.
For really, truly expensive boats (see “America’s Cup”) they will lay up carbon fibers over a foam core with a thermo-setting epoxy to hold the whole thing together. If you want a mind-blowing experience, start pricing things like carbon-fiber spinnaker poles for 80-foot racing boats. For us mortals, carbon fiber is not a great choice for large assemblies – it’s picky, brutally hard to cut, messy, and demanding.
With one exception, if you want to lay up a composites structure for a robot, I would stick to either woven or biaxial glass fibers laid up in epoxy. If you need to make a long, strong pole, you might follow the plans like this: http://boatbuildercentral.com/products.php?cat=36 to build a pole, which can then be bonded to attachment pads for assembly to your robot. This is what I wanted to do with the tetra game a few years ago, but the students didn’t want to mess with composites. By the way, it’s the availability of carbon fiber “socks” that makes this possible for simple CF composite poles. (That link is to a commercial Website to which I have no affiliation, other than as a happy customer.)
Probably more than you wanted to know…
Titanium will eat through most general tooling like there is no tomorrow if you aren’t very careful when working with it.
And there is a small chance, that if you aren’t milling it correctly, it can actually catch on fire.
Although if you want “springiness”, you can also try PVC pipe, a flat sheet of Lexan (polycarbonate), or even use aluminum with a pivot point and surgical tubing to give the springiness.
Mr. McGuire: I want to say one word to you. Just one word.
Benjamin: Yes, sir.
Mr. McGuire: Are you listening?
Benjamin: Yes, I am.
Mr. McGuire: Plastics.
Benjamin: Just how do you mean that, sir?
Titanium is surrounded by this wonderful mythology that seems to supercede reality. It actually is not all that tough to work with. It just requires some appropriate knowledge and slightly different practices when you are working with it to get the results that you want. HSS tooling will work just fine, and carbide tools will do even better. Keep surface speeds low, feed rates high, and use lots of coolant. You want to keep the cutting edge very sharp, cool, and constantly moving to prevent work hardening and/or galling. Titanium machines extremely well (you can basically treat it just like any other high-performance alloy like austenitic stainless steel), and finishes beautifully.
A Uni-directional weave of carbon bent correctly will give you high stiffness in one bend direction, but a “spring” action in the other. Pre-preg is usually easier to work with, but has its own set of caveats in comparison to laying up your own.
Slightly off topic:
What’s the going rate for Titanium tube?
We had 4-5 15-foot lengths of 1/2" diameter tube up in our storage area, as well as a 1" dia. tube. It was given to us back in the day by somebody.
Some of our kids looked up the info for the codes printed on the outside of the tubing. The cost they found was somewhere around $144 per foot!
Is that anywhere near a sensible number?
It might be better if we tried to sell the stuff as a fundraiser lol.
It might be wise to do a bit of analysis on the existing part, before going to the trouble of redesigning the whole thing. What is it made of? We know it’s aluminum, but what alloy? Maybe all you need to do is to go to a stronger alloy, especially if what you have now is just generic hardware store material.
Also I didn’t see any mention of using steel, which might be a mistake, as thinwall chrome-moly alloy tubing is very strong and light, and relatively easy to work with.
Something to remember when discussing structural materials properties:
The terms strength and stiffness are not interchangeable.
Strength is the resistance to breakage, whereas stiffness is resistance to deformation. A nylon rope is incredibly strong, but not at all stiff. A ceramic rod is quite stiff, but has little strength.
Stiffness of structure depends on material and geometry. For instance, a 1/8" x 2" aluminum bar is not nearly as stiff as 1/8" x 1" aluminum angle, but their tensile strength is almost the same.
More things to remember:
Three important properties of a material are it’s modulus of elasticity, it’s yeild strength, and it’s tensile strength.
The modulus of elasticity is how much stress it takes to deflect a certain amount. Polycarbonate has a very low modulus of elasticity, steel has a high modulus.
Yield strength is how much stress it takes to permanently deform the material. A soft aluminum alloy such as 3003 has a low yield strength, while a hard alloy like 7075 has a high yield strength.
Tensile strength is how much stress it takes to break the material. You’ll find that different alloys of the same material (aluminum, for example) will have different ratios of yield strength to tensile strength–that is, the stronger the alloy, the sooner it breaks after first bending.
Do you want your part to be more flexible, or do you just want it to flex further before becoming permanently bent? A material with a lower modulus of elasticity will generally let it flex a lot more before bending, but a harder material with a higher modulus will take a lot more stress before it finally bends.
Do you have a photo of the damaged mechanism? That would be helpful in visualizing the weak points.
Alum is what we use to make our frame, and mostly anything that is made out of metal. Because it’s light weight and strong. But the cons are that if you weld it your welder has to know what they are doing. Carbon Fiber is good, but it has to be molded or casted into that shape. We use a carbon fiber knock off for spacers on our wheels.
From experience, I’m going to have to agree with Dave Lavery about titanium. It’s not as hard to work with as some may think… We made two additional forks that help us grab on to the ball during one of the fix-it windows. These two forks are made of titanium tubing and are bent to shape and welded. Our welder said that it welded really nice and gave him no trouble at all. As for bending it, it was no more difficult to do than bending steel tubing.
As David Brinza said our forks are made of carbon fiber (pics are being uploaded to CD media as I’m typing this). In 2006 we used CF rods that we bought online and were breaking them every match or two.
This year we had the forks custom molded, with the help of a sponsor. We started off with a foam model of the forks. The foam models were hand shaped using a band saw, grinder and a sander. From there two fiberglass molds were created and the CF was laid inside the molds. After being laid out a thin epoxy coat was applied to the outside.
There was some tweaking done to strengthen the forks. In the picture that I uploaded to CD media they are completely hollow on the inside. We ran a small rib along the outer edge to increase its rigidity. This set weight 9oz and was rammed into steel I-Beams, through dry wall, twisted them in the over pass and even lifted the robot on several occasions during testing. The set survived all of our qualification matches in San Diego. One of the forks did break during the quarter finals, but since we molded a second set we were able to replace it with in a couple of minutes.
In the second set we molded we foam filled them and then milled pockets to take out some of the weight. This set weighs over a pound, and has received just as much abuse as the first set.
The third set that we molded are similar to the first set, but we increased the rib size and added another couple layers of CF. This set survived all of the LA regional.
In conclusion I think that Dragon Ninja is correct about carbon fiber… Don’t use it unless you have the ability to make multiple sets. There is definitely a weight benefit to using it (we weighed 107 and 112lbs at the San Diego Regional and LA Regional Respectively, not that all of that was a result of carbon fiber), but do be prepared to replace those parts at some point during the season.
I’m going to reiterate that. My Aero Design team always has two fuselages per plane minimum, both of carbon fiber. The reason for this can be summed up by: Crashes happen. (We tore a carbon fiber fuselage today in a crash landing when the internals broke loose. It wasn’t bad, but it did hurt us.)
If you break carbon fiber, you can’t fix it easily. Broken metal, on the other hand, needs only a weld or some extra bolts.
To agree with everyone here about the carbon fiber. Our team uses carbon fiber claws, and while the look nice and are quite resilient to damage, we have broken two, one against a wall, the other from another team getting caught and going the other way. So if you ever use carbon fiber BRING EXTRAS. We had spares made and have plenty lying around the pit at all times, just incase we get another breakage. If we had not brought extras, we would not have been able to compete.