first idea im sticking with it

this is probably my first of many questions but here we go. 
my robot im designing for a 15lb weight class is going to use aluminum as a bottom plate to be a CORE structural element my first question is what should i use for type wise and also a range of thickness i should use like 1/4 inch
 Please someone help me     

Generally what you're after in a frame is stiffness.  A simple plate isn't terrible good for that.  It's also going to be heavy.  Aluminum is very stiff.  I personally like 6061 Al since its relatively strong and its cheap.  2024 and 7075 are often referred to as "Aerospace Grade" and they are stronger than 6061.  I have heard though that the 7075 can be a bit brittle, which isn't a good thing depending on your application.  Something that can bend and spring back to shape is going to absorb energy a lot better than something that can't bend as much (and will be more likely to break).

My designs for the smaller (3 lb and below) tend to be top and bottom plates attached to a lightweight plastic (usually polyethylene).  The plates serve as armor and add stiffness while the plastic can bend absorb the energy.

Check out the riobotz tutorial and look around to see what has been done successfully in other robots.

zwtipp05 wrote:

Generally what you're after in a frame is stiffness.  A simple plate isn't terrible good for that.  It's also going to be heavy.  Aluminum is very stiff.  I personally like 6061 Al since its relatively strong and its cheap.  2024 and 7075 are often referred to as "Aerospace Grade" and they are stronger than 6061.  I have heard though that the 7075 can be a bit brittle, which isn't a good thing depending on your application.  Something that can bend and spring back to shape is going to absorb energy a lot better than something that can't bend as much (and will be more likely to break).

My designs for the smaller (3 lb and below) tend to be top and bottom plates attached to a lightweight plastic (usually polyethylene).  The plates serve as armor and add stiffness while the plastic can bend absorb the energy.

Check out the riobotz tutorial and look around to see what has been done successfully in other robots.

i really like that kind of idea i will make sure i look into that. maybe something like UHMW as the center frame pieces with the aluminum to provide the stiffness. there will be 1/8 inch titanium all the way around so that may help with the stiffness. but otherwise for the two pieces for the weapon support the rest could be UHMW. That advice really helps. sorry it took so long to reply i havent been on her in a while.

I wouldn't use UHMW for anything that holds a thread ( they will strip). Also, don't use it for the "weapon supports" it will easily deform.

yea the idea has more changed to a 1/8inch aluminum piece for the bottom, if the design can have the the weight the weapon support frame will be 1/2 inch

I use polycarbonate as the main frame component on my bot, granted it's 1 lb, not 15.

If you have any vertical chassis structure, I would make that be the main structure, and attach thin plates on the bottom and top.