Steel Frame

We’ve been kicking around the idea of making the robot frame this year out of steel, or a combination of steel and plywood. Thinwall (1/16") square steel tubing is a bit heavier per unit length than thickwall (1/8") square aluminum tubing, but it offers several advantages that might make the few extra pounds well worth it.

Like plywood, steel tubing is easy to get locally, and also like plywood, it’s relatively easy to work with. Welding it together with the MIG process does not require as much skill as welding aluminum, and the equipment for welding steel is more commonly available (I have a 140 amp gas MIG welder in my home shop, as do many of my friends).

To mount brackets, axles, and such just requires welding on a piece of steel. Cantilevered axles can be made by simply welding a bolt to the lower edge of the tubing. Brackets for mounting transmissions can be steel strap with holes or slots cut in them.

If you see some weird steel chassis prototypes showing up in the next week or two, don’t be surprised!

(I did search for other posts on the subject, the previous one that seems to be most applicable is locked)

I love your idea, thinking outside the box, allows you to do many things

Steel does sound attractive this year. The robots will take quite a beating with the whole defensive part of the game. Remember 2007? I do not know about you guys, but our robot was falling apart in the end with an 80-20 aluminum chassis. Some other metal than aluminum is probably the best way to go this year.

I don’t know about that… We used 80.20 that year too, and it held up alright during the competition. It wasn’t until the next year when it started falling apart :slight_smile:

Since then, we haven’t used any 80/20, but everything has still been aluminum (except for our steel weights to get up to 120 lbs), and none of them have had a problem with falling apart. Welding aluminum can make it plenty strong!

Steel is probably not a good choice for teams that have the equipment and ability to make a nice welded aluminum frame. There are plenty of teams that do not.

Completely agree with you there, squirrel. Teams should go with whatever they have the ability to make. My point was that the problem Cyberphil had wasn’t with aluminum - it was with 80/20. That stuff was designed to be very flexible in configuration, but you lose a lot of durability for that.

For that matter, I’ve seen teams build durable robots with kitbot frames and plywood. Play to your teams strengths and abilities :slight_smile:

1501 has used steel tubes before and they worked very well. Search for picks of their frame on the RacknRoll bot.

I have some pictures of their steel frame that I took in Atlanta that year…they used round, strong alloy tube (chrome moly) and gas welded it. I think we won’t get quite that fancy…

Steel is good for many things and a frame could be one of them. I always choose what to make mechanisms out of based on what the design needs are not necessarilt what is lightest, because sometimes stell is lighter than aluminum for the given purpose. One of my favorite tricks is to us 4130 aircraft grade round steel tubing for a structure if it’s under high loading because it would take a lot more mass of AL tubing to get the same strength and durability. Just remember to consider wall thickness…

Squirrel: You may want to use this as well as it is easily weldable. We usually TIG but you should be fine MIGing it.
Also McMaster carriers it:

I would guess that using 4130 would allow us to use a thinner wall, for a lighter assembly with the same strenght. The downsides are that it’s kind of difficult to MIG weld structural parts with less than 1/16" wall thickness, and that it’s not available in town…which is a concern for us, especially during the prototyping phase.

I agree that it’s wise to pick the best material for each part of the robot. There are a lot of things to consider…weight, strength, ease of fabrication/machining, ease of attaching other parts, cost, availability, repairability, etc.

I think steel is perfect for the robot who decides to be elevated and then who decides to lift two other robots a couple inches off of the floor. With a safety factor of two, that’s 900lbs* the tower connectors will need to support. It’s well worth the additional weight for a bot who’s lifting other bots due to the compactness of strength alone. Score 8 points for the home team!

*120lbs (robot) + 14lbs (battery) + 20lbs (bumpers) = 154lbs… time three robots = 462lbs pre-safety factor

Started on the prototype steel frame, we’re moving in slow motion this year…

One axle is welded on.

We used chromoly steel (most bicycles are made out of it) and it welds very nice with a MIG. The welds are nicer than steel and it is stronger.

Did you normalize after welding?

I don’t understand the question

1218 has been using 8020 since our second year and it has been wonderful to us. While we could do a welded aluminum frame, we often find ourselves making HUGE changes in the end of the build season. Plus, it provides so much flexibility and is easily reusable and i is easy to prototype with. We have learned the art of lexan, 8020 and luan (where needed). Its all about what fits your needs, and we have some crazy ideas that wouldn’t work without 8020

Normalizing in simple terms is post heating the weld to relieve stress from the weld process. Actually when you weld chrome moly tube you should pre and post heat the weld joint but i know alot of chassis builders that do not and have no problem. Another way you can normalize the frame is to totally finish the frame , drill tiny holes by each weld joint and have the whole frame heated in a oven , this will take out all stresses built up by the welding process.

thanks for explaining that :slight_smile:

Did some more welding. The transmission and chain tensioner mounts are welded to the top of the frame, the axles to the bottom. Center four axles are welded to 1/8" thick plate, the plate is welded to the frame, to drop them down a bit for easier steering.

I must admit I’m a little confused…Why add unnecessary weight? Just because you can’t weld aluminum doesn’t mean you shouldn’t use it. Rough estimates would say steel is about 3 times the weight of an equivalent piece of typical aluminum. So with 1/16 inch wall steel, and 1/8 wall aluminum you have half as much volume with the steel. So for a size that would weigh 16 pounds in aluminum, the same volume will weigh 24 pounds in steel. Do you really want to waste that much weight into a frame that’s strength would suffice in something that is much lighter. Plus the fact that you can also use 1/16 aluminum in places of low contact. My team used half 1/8 aluminum and half 1/16 and we never had any problems at all. True we welded it, but there are certainly ways to bolt aluminum that are very structurally sound and are still much lighter than welded steel. It’s your call though. There are some advantages to steel as you mentioned, but I don’t think the added weight is worth it.

The assembly shown has support for the wheels and transmissions, and weighs under 12 lbs.

If that’s too heavy for your robot, then you should definitely use something besides steel!