If you're still rolling tube under a knife blade to cut it, try the easy way and use a K & S Tubing Cutter. Perfect for cutting copper, aluminium and brass thin wall tubing and with a capacity of 1/16" (1.5mm) up to 5/8" (15.5mm) outside diameter.
The K&S Tube Cutter handles round brass, aluminium and copper up to 5/8 OD. Nylon body helps reduce friction. Cuts tubing without crimping. If you've ever tried to cut brass tube by rotating it when you use this.