this instructable will show you how to make 3d print robot arm with 4 forks to design it you need Autodesk 123D Desgin it is free app here link http://www.123dapp.com/design.
you can use Autodesk inventor or Auto CAD too but this instructable is for 123 design
if you want to know how to use it here link :
Warring read this: do not start your design before read the full instructable
i hope you enjoy your time
Step 1: Get Started
*first thing is to draw this shape then draw rectangle at the end of at as shown on the figure u can use any length you want that does not matter you can use any numbers and change the length but make sure you use the same ratio you can also draw as mine as shown there.
#1_ Extrude the first shape with length 6 mm, (Extrude is from Construct menu ), now Extrude the rectangle 1.5 mm
now as shown on the figure that is the shape of the solid