CAD is the system used by all professionals for the creation of the products you see and use everyday. It allows you to make 3D computer renderings, seeing how things fit together on the computer before you build. This step can revel potential problems on your bot which will reduce your time and cost overall.
It is a common thought that CAD systems are difficult to use and build if you are not an engineer or have been trained to use them through some class. Recent CAD software has been shifted from even five years ago so that they are easier to build models with a user interface that anyone can pick up and learn within a few hours.
Within industry the three most popular pieces of software are Autodesk Inventor, Solidworks, and Pro-e. Each one of these has advantages and disadvantages to their own right but all are comparable for this type of design. I will not be going into how to use CAD in this instructable but there are many resources online for using this type of software.
Buying CAD software can be very expensive but fortunately there are many opportunities for free licenses of software if you are a student, or if your company has licenses of the software.
Students can get autodesk inventor for free from http://students.autodesk.com
All you need is an email with a .edu ending
You can also get a copy of student version of solidworks very cheep/ free from time to time online.
They also have a great tutorial for robotics design located here. http://www.solidworks.com/pages/products/edu/Robotics.html?PID=107
For robot design with little to no CAD experience I recommend Inventor or Solidworks both provide a simple interface, and more importantly there are lots of models available for free download. Stock parts like bearings, screws, motors, etc can be found. Using these models will save time when modeling.
The most important thing about CAD design is that you have your dimensions right. Now that may seem like a straight forward piece of advise but I see loads of people trying to make realistic renderings and spend too much time making their parts look nice instead of focusing on the real goal of CAD to make models which are accurate.
I am going to leave this step because if you take the time to learn CAD the process steps for design in the software become more apparent. If you choose to skip this step due to the inablity to run the software or the lack of interest I recommend a "cardboard template" method. Take cardboard and cut out scale models of each one of your parts for layout, before you cut your real material. A good example of this method in the webshow by revison3 called Systm located here http://revision3.com/systm/robots/
Ultimately the purpose of this design step is to minimize the mistakes with your expensive.materials.
*modern CAD software can assign weight properties so you will know how much your bot should weigh before you build
*Ensure that you have sized things correctly so they fit together, for example a 1/2" shaft will not fit through a 1/2" hole. For exact machining you are dealing with thousands of an inch (.001") .