Unit 4.3 of our course The Fascination of Crystals and Symmetry
Additonal resources at: https://crystalsymmetry.wordpress.com/chapter-4/
Screw axes are - as glide planes – symmetry elements, which require two combined symmetry operations, carried out consecutively. First a rotation around an axis and then a translation parallel to this axis.
in crystals, the rotational part, likewise as discussed for the pure rotations, is again limited, a) because it is composed of concrete entities, and b) because there is this special thing, that rotational symmetry in crystals is restricted to the order one to four and six.
--------------
If you prefer books instead of videos, have a look at:
https://www.amazon.com/Introduction-Crystallography-Frank-Hoffmann/dp/3030351092/r
