These sets are manufactured in steel, so they will last long enough (you get a spare disc, anyway). I checked the sets TE Z018 and TE Z030, referring to 20 and 30 mm in the discs' diameter, respectively. Strangely, the numbers printed on the frets are Z014 and Z018, but the printed diameters are correct. Voyager has smartly etched holes and recessed areas inside the discs to make room for the material removed. This helps to produce a cleaner cut. The 20 mm disc is 0.15 mm thick while the 30 mm disc is 0.25 mm, therefore you will get very thin cuts. In my opinion this is the best feature of these tools when compared to similar items.

Observe the turning direction: they are designed to rotate counterclockwise in the photo above. Remember to adjust the discs in your rotatory tool as centered as possible, since the securing screw may be considerably smaller than the hole in the discs' centers. Also, adjust the speed of your tool when cutting plastic to avoid melting. And of course, always wear eyes protection. 

This is a useful tool. Recommended.