We R Makers 3-Way Corner Punch - Round
This compact cutter punches 4mm, 7mm, and 10mm corner radiuses, which are great for any size project. Its small and sturdy design fits perfectly in your hand, and makes for easy storage, and its high-quality metal material punches clean corners every time.
Note: as with all punches, this tool works best when used with a thin cardstock (eg Bazzill) rather than paper.