Re: Tips on loosing weight (Not spam)
 

I liked the two-hockey-stick arms, too (practically solid fiberglass) but our arm was just a disaster in general. It was too long, too thin, and too holey. I plan to stick with thin-walled aluminum and wise design for this year to avoid any possibility of composite catastrophe.

Re: Tips on loosing weight (Not spam)
 

I love thin wall aluminum.

I don't want to tell teams to not use composites, but most of the time I see them used "in place" of aluminum they are used in much more volume so their lower density provides no weight advantage, and a thin-wall aluminum member would be more efficient.

I see them used right time to time, and it makes me happy every time to see something different used effectively; but that unfortunately seems to be the minority.

Re: Tips on loosing weight (Not spam)
 

[quote=AdamHeard;1090563]I love thin wall aluminum.

I'm surprised I haven't seen a team make a round tubular frame similiar to a bike frame or go cart frame.

If you guys have never seen a tube laser cut tube check it out on youtube

Roy

Re: Tips on loosing weight (Not spam)
 

[quote=roystur44;1090601]It's more difficult to work with when compared with other methods.

67 is about as close as I've seen.

Re: Tips on loosing weight (Not spam)
 

67 and 469 routinely make the top half of their robots out of thinwall aluminum tubing. It is VERY light, thin, and strong stuff. Instead of using 1/16 box on an arm, .0200 aluminum tubing could provide a lighter alternative. The warning that comes with this is that you need to make sure you properly design so it is not loaded in the wrong direction. Our entire 2007 double jointed arm was made out of this and it weighed maybe 5lbs max.

Re: Tips on loosing weight (Not spam)
 

Our arm this year was around 0.065 or 0.050 1.5"OD tube, worked great.

Re: Tips on loosing weight (Not spam)
 

1024 uses thin wall tubing a lot as well... they have access to a good tube bender, so that's what gets used and, IMHO, used well.

Re: Tips on loosing weight (Not spam)
 

You're off by a decimal point, about.

We've used .035 thinwall aluminum for very high stress applications (arm in 2007), it's just really hard to weld...

I believe 469 uses something around the same thickness, in various diameters, for basically everything, with rivets holding everything together. Very very lightweight.

We've used material as thin as .050 and .063 in drivetrains, back in the days before mandatory bumpers (we had no bumpers, they lived).


As to tubular frames, how about this:http://www.chiefdelphi.com/media/photos/12949 and http://www.chiefdelphi.com/media/photos/12952 (same robot) - Chief Delphi 2000

Re: Tips on loosing weight (Not spam)
 

It just takes a patient welder :D

This is a weld I did in college, but one of my students two years ago was welding soda cans together like this for fun in her shop class. It is certainly non-trivial, but definitely doable.

Re: Tips on loosing weight (Not spam)
 

Al, it may have happened once or twice when you weren't around :D

On a serious note:

Weight management is a subject that is very near and dear to my heart after spending 15 years building spaceflight hardware. When the contracts include penalties that run on the order of $60/gram for being above the spec limit, you learn to pay a lot of attention to weight.

The first thing I tell rookie teams in any of my presentations is to buy a decent scale. It's an investment, just like any other tool. One I like is this one from Grainger. It's just over $200, has a 400 lb capacity, and has a remote display so you don't have to try and get your head under the robot. The extra capacity means you can set a plywood platform on it so the robot will fit, or that you can weigh your crate before you ship to Championship after your well designed under-weight robot wins your regional:) If you really can't afford to buy a scale, try the one the wrestling team uses. Or find a local company (sponsor) that will let you use their shipping scale a few times. Buy a small scale and weigh all the parts of your robot. There really is no excuse for showing up at the competition being 10's of pounds overweight.

The rest of the secrets have been mentioned several times in this thread already:

1) Make a weight budget.

2) Enforce the weight budget.

3) See #2. If a subsystem is not going to make their part of the budget, make them negotiate with the other subsystems to see if anyone has any excess. I usually withhold 10 lbs as "Engineering Reserve". The team members would rather negotiate between themselves than try to get any of that from me. If we have any left, they get to add decorations, etc. Or I get to add weight to put the CG where it needs to be.

4) Use the CAD tools not only to pay attention to the weight of parts you design, but to decide how much work to do to to purchased parts. We use the CAD models from AndyMark, etc to evaluate different lightening schemes vs. the amount of work. Keep in mind the added benefit of reducing the rotational inertia of moving drivetrain components such as gears and wheels, just don't get carried away to the point that structural integrity is compromised.

4) Design from the beginning with weight as a design parameter. Think about how you can make each part lighter while making it strong enough at the same time. It is much easier to put holes in a part before it is assembled than after the robot is together. Avoid the "we'll just put it all together and see how much it weighs" trap at all costs. It never seems to come back apart to do the necessary work later. Making shavings before you put the electronics in is much preferred to getting metal bits inside them. The holes also look a lot nicer if they are carefully laid out, nicely finished (deburred), and if the part is painted, the holes are too. And if you design them into the part and lay them out properly, you can avoid having an over-enthusiastic freshman drill holes too close to the edge of a critical structural part.

5) Take advantage of the bumpers. You get to rigidly attach a 5" wide strip of 3/4" plywood to the side of your robot. That can provide a lot of structural support if you take it into account during your design process, and it's free weight.

6) See #2.

7) Use creative materials. Composites. Plywood (yes, really. We built most of our 2011 robot from 12mm Baltic Birch plywood). Plastics. Avoid steel unless absolutely necessary, and only for small parts where needed.

8) Pay close attention to the number of fasteners. An ounce here, an ounce there, suddenly you are talking about pounds.

9) Same goes for wiring. Plan your layout to minimize the number of inches of high current (large gauge) wiring runs. This has the added benefit of keeping the resistance down. Just make sure you don't cut everything so short you don't have any slack to work with or that things are so tight they pull out of the connectors.

10) See #2

As I have mentioned on CD before, the three things I say most often during build season are (in no particular order):
1) Is your homework done?
2) That's in the rules.
3) How much does that weigh?

If you pay attention to weight during weeks 1-5, then it is a non issue during week 6. Nothing pains me more as a Robot Inspector than to have to help a team cripple or remove their well thought out game mechanism, rendering them unable to do more on the field than drive, just because there is no way to lighten up their robot enough at the competition any other way.

My final comment on the subject: Thinking you will just drill a few holes in things when you get to the regional if you are overweight is a recipe for disaster, or at least a very non-inspirational event. If you ever think about taking this approach, please sit down and calculate how many 1/4" diameter holes you need to drill in a piece of 1/4" aluminum plate to reduce the weight by 1 lb. I promise you will be surprised.

Re: Tips on loosing weight (Not spam)
 

Airplanes have entire teams dedicated to track weights.

Like Burt Rutan said when building Voyager, "If you can throw it up in the air and it comes back down it's too heavy!"

As others have said, but holes in everything from the get go. It requires a little bit of for thought, but honestly it makes robots a very distinctive look. The robots of team 40 were a great inspiration to us as a 15+ lb overweight rookie team, they made everything light from the get go with predetermined wholes that look great, compared to our hodgepodge of holes all over our superstructure.

If you do end up overweight, it is very important to recognize that volume (and as such weight) goes with the square of the radius. A 1/2" diameter hole removes 4 times the weight of a 1/4" hole. Bring big bits.

We had a love/hate relationship with pneumatics. I think it is worth it if you use them for several functions, but if you only need one or two things look long and hard at using motors and limit switches instead. In the two years 1276 didn't use pneumatics we came in well under the weight limit, but we also came in well under the year we used pneumatics for most of our actuators.

Re: Tips on loosing weight (Not spam)
 

It's good to know the training stuck!

Re: Tips on loosing weight (Not spam)
 

If you search through our belt vs. chain evaluation to find pictures, you will find that we made a round tube chassis for this robot test. However, the closest it got to competition was being used as a "box on wheels" robot to drive around with an attached trailer at the Muncie Scrimmage back in 2009. We used the kitbot chassis for our 2009 robot for simplicity.

Re: Tips on loosing weight (Not spam)
 

We weigh everything every time we change something. this way we can keep track of weight and get our workout in.