Lots of good packages suggested. I use quite a lot as all my CAM and CNC software seems to have different prefernces on what it wants to import.

The parametric software is probably best for robot designing due to inherent advantages (rhino being NURBS which is slightly differnet- better for free-form modeling, but really down to your preference) 4 common CAD packages that spring to mind are Pro/Desktop, Pro/Engineer (pro desktops bigger brother), Autodesk and Solidworks. Pro/Desktop and Autodesk are very very good lower end CAD packages, and should handle most things you would want to use in a robot. Neither of them hold a candle to Pro/Engineer and Solidworks, but as to which of those to is better, it depends who you ask. CAD types can get quite emotional about it The problem with these packages is that, if youre starting out (as is infered from your post) then they might appear very complex and quite arcane. Much better to start of with autodesk inventor or Pro/Desktop, which, once youve learnt the basics, are very easy to get skilled at quickly. I normally use Pro/Desktop for most things as it can handle most things- included on my profile is a pic of some jet engine parts I built on Pro/Desktop to see how far it can go. Pro/Engineer is lightyears better but complex, as I said, and leaning much more towards the CAM side of things. I rarely use it for robot stuff, in no small part due to the fact that my computer creeks and groans trying to model stuff on it- its a bit of a beast of a program (its installation is spread over 3 CDs :S )

So, In conclusion, Pro/Desktop or autodesk- very good pacakges and they should handle most things you throw at them. As PTC seems to be giving up with supporting Pro/Desktop, and with there being more support for Autodesk, Id go with autodesk.

Glen, Solidworks seem to change their site around on a monthly basis, and downloading anything from their site is a bit of a pain as you practically have to sign your life away in forms before getting anywhere. They do offer a student version for much less, but it terminates after 2 years (whats up with that?) so your best bet is to find some nice person who doesnt mind accidently lending you the CDs and codes. Thats what I did ;-) As for calculating weight, any of the parametric software should do it, but I cant think where Id start looking on Rhino. Thingyma-bob from Ceros seems to know his rhino- hed be the man to ask.