The tubing cutter is an essential plumbing tool if you are going to be doing any work with copper pipe. With copper pipe it is important to have smoothly cut and squarely cut ends and well fitted joints.
When using a tubing cutter, hold the copper tubing and clamp the cutter around the pipe where you want it cut. Then spin the cutter around the pipe, tightening the knob slightly on each revolution to increase the cutting wheel pressure on the pipe. After making the cut you may find some burrs in the pipe's cut edge. Remove these with a small file or de-burring tool for this purpose.