how we can design a 3D CAD model of text writing mechanism which holds a writing device like pen/pencil and writes the required text according to the electronic input given.
we need not show the electronic circuit and the digital input devices,but we have to show all the mechanical linkages and parts such as screw, bolts, pulley, chain and sprocket, gears etc. if used.
First of all you need to get a CAD software. I offer the Solidworks. (It has studet version: http://www.solidworks.com/sw/education/student-software-3d-mcad.htm ) It's simple for beginners and it has really good help function with tutorials. And you can find other and very useful tutorials on youtube, just enter the right words (part, assembly, mechanism, animation, etc....+ solidworks) and ready.
Later, if you are very good in the basics, you can start the Electrical function and the Circuitworks in the Solidworks.