Anything under 5mm or 1/4" and it wont hold properly. For these sizes and up it is good value for money. So if you put a piece of flatbar on at 3mm thick there is a good chance it will move. You will need something else for thinner material.
I've been making sculptures with plasma cut sheet metal. This chuck is the best thing to hold my small parts for wire wheeling off the dross and clean up for welding.
okbassfisher 
