Extrusions are made of 6061 T6 aluminum.  The frame shapes have a 3/8 inch square top and bottom edge and corners with a thin plate between them.  Sort of an I-Beam you can bolt to.  The 3/8 inch square is beefy enough to drill and tap up to a size 1/4 inch or 6 mm thread into the sides, top, bottom or end.  Tapping the end is particularly useful for constructing frames without use of welding or corner brackets.  Of course 6061 is a weldable alloy, if you want or need to weld to it.   The downsides of welding include significant loss of strength near the weld and distortion of the parts from welding shrinkage.  If you are countersinking socket head cap screws into the squares,  the head of a 1/4 x 20 screw is too large and weakens the frame structure.  So 10 x 32 or 5mm should be the largest socket head cap screws used if countersinking the heads.  

The  slide shape fits over the 3/8 inch square.  If it is to be used as a slide, a small amount of lubricant should be used to keep the aluminum to aluminum contact surfaces from binding. 

The frame and angle shapes will allow direct and robust mounting of many of the gearboxes from AndyMark, Banebots and Vex.  For example, the 3 inch frame fits the Nano gearbox well.  Mounted against the hollow side, it clears the bearings and screws and bolts up flush. 

The frames may be lightened by drilling the web.  I recommend not cutting into the squares and leaving at least 5/8 inch material between the holes for shear strength.  A metal cutting hole saw works well but should be used in a drill press or mill and the shape should be clamped down to the table.  I recommend the Blair hole saws.   Use a slow rpm.  Or you can clamp the part in a bench vise and use a hand drill, again at the slowest Rpm with a light feed.  Always use plenty of cutting lubricant.

We also sell drilling and tapping guides to facilitate assembly.  Using the guides, you can construct a robot frame and mechanisms with a good woodworking miter chop saw with a carbide tipped blade and a hand drill.   They are sized for 10-32 screws.  If you use Socket Head Cap Screws, they are alloy steel and heat treated similar to grade 8 bolts.  So they are tough and strong.