What sort of excavator are you wanting to build? Wheeled, tracked? Any certain brand? How detailed are you expecting the drawings to be?
How about a 3D model for free?
Go to
http://sketchup.google.com/3dwarehouse/ and type in excavator in the search box. You may have to look at a few different ones to find the one you are looking for, but should be a start. Sketchup is pretty easy to use CAD software and is also a free download if you've never used it.