illustrator cad line drawings not printing properly

I've created line drawings with both straight and curved lines using the pen tool and also square and oval shapes with the rectangle / shapes tool. on my screen these lines are all smooth and clear. But when I print, the lines are all jagged, with steps, creating a 'step up step down' effect. 

I get the same result when printing when i save to pdf... they look fine on the screen but dont print smoothly! Where am i going wrong please!?