Beadsmith has made a very versatile tool with this 5-step Mandrel. There are 2 available – one has steps from 1.5mm to 5mm, the other has steps from 6mm – 10mm. They can be purchased separately, or as a set. The set retails for around $12.
These stainless steel mandrels with a sturdy plastic handle are perfect for making uniform jump rings and other loops.