Best metal to make cheap lightweight frame and how to secure motors

[ignatius_gim]

Hi, I'm looking to make a robot on wheels that will be able to pick up tennis balls. I have a chassis made of steel with gears and everything that came from a kit I won in a contest. Now I need to build the frame upwards and am looking for a lightweight fairly cheap strong material. What would you recommended and where could I find them?
Also what is the best method to mount a motor.  Should I use a gearbox. Its a large 5A motor.

[jwatte]

My favorite material is 6061 T6 aluminum. It's strong, lightweight, and easy to work with metalworking tools/mills.

Another option is Plexiglas-G, which is easily cut on a laser cutter, is lightweight and reasonably rigid, but not as strong as metals.

Motors are typically used with gearing, because tying a wheel directly to the output shaft is likely to spin the wheel way too fast, which would give you uncontrollable speed if it worked, but it won't work, because the torque will be too little to effectively accelerate the load.

5A may be a lot or a little depending on the voltage, and the weight of the 'bot it's moving.

[Sylvestre]

I second JWatte's suggestion. 6061 square Al tubing has worked wonders for me.  Plus you can say its partially constructed out of "aircraft grade" aluminum, which has a nice ring to it.   

[jwatte]

I think "Aircraft Grade" is typically 7050 or 7075, but at this level, who's counting? :-)

[Sylvestre]

I think "Aircraft Grade" is typically 7050 or 7075, but at this level, who's counting? :-)

Lol My mistake. Google is quite misleading. When you search 'aircraft grade' aluminum, 6061 is the first link. 

[ignatius_gim]

Thanks for all of your answers.  How expensive is aluminum and do you suggest going to a scrap metal yard or a mill, what about consumer stores like Home Depot?  Also, could you just connect the motor righ tot the gear box and will it stay?  Any other mounting options and where could you buy gear boxes?  Thanks a lot.

[waltr]

Aluminum is not that expensive and can be found at lots of places which is reasons it is a good material to use.
Scrap yards can be a good place to get aluminum cheaply, Home Depot, hardware stores and other consumer places can have aluminum but it is hit ot miss for them having what you need.

Also look into old computer stuff, floppy & cd drives tend to use thin aluminum chassis and can provide good pieces to use on a Bot.

McMaster-Carr has a good selection of aluminum so check their online catalog if you need a specific size.

[jwatte]

Raw aluminum is typically easiest to buy at a "supply" place, where builders and machinists go to get their material. Expect to pay about $4/pound retail for simple extruded forms of Aluminum -- L profiles, rectangular bar, sheets. Piping/tubing may be more expensive. Here's the place I use, which might be convenient if you live between San Francisco and San Jose:
https://plus.google.com/101102587570788380732/about

The Home Depot has a small selection of 6063 aluminum profiles for use in building -- 1/16th inch thick and 1/8th thick L and sqare profiles in 1/2, 3/4, and 1 inch sizes, 36 or 48 inches length. The L profile is actually quite easy to work with; chop it up into lengths, and join it up with screws or epoxy glue.

Amazon/Small Parts also has aluminum available, and if you're Prime, it will ship in two days. Slightly more expensive than my local supply yard, but oftentimes convenient:
http://www.amazon.com/gp/product/B00CNLSDE4/ref=as_li_ss_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B00CNLSDE4&linkCode=as2&tag=enchage-20

McMaster also has aluminum, but I've found them more expensive than both the local supply, and Amazon.