I assume you mean footed carbons? I cut mine to length, clean up the edges with a file. Then rotate and file a taper on one end. Another way is taper with file, cut and clean up edges. This way you have more old shaft to hang on to. OR clean them up, mount on shaft, install insert. When glue is dry take a 175 grain field tip and chuck up in drill motor. Screw on shaft and use file to make taper. This last way is tricky, you do not want to file the shaft.
Also Harbor Frieght sells a chamfering tool for pipe. It works pretty good, about 10 bucks in plumbing section.