The key to turning rubber is the hardness, referred to as durometer reading 0-100. Unless the rubber is very hard, say 90 or more, it can be easily turned with HSS that is ground with a large side relief and top relief. Make the tool as sharp as possible. You don't say what finish is required of the rubber surface, but if you play the speeds and feed, you can achieve a pretty smooth finish. The rubber should leave the tool as smooth rubber tape. Be careful to not loose control of this waste, as it will try to wrap around the tool and the roll at every opportunity You can smooth the tooled finish with a tool post grinder turning against the rotation of the roll, or you can use a slack belt grinder mounted to the saddle for finishing. The only time I have ever seen carbide used is on long, large diameter paper mill rolls that were covered in very hard rubber. If you don't have a Pi tape to measure the OD, get one before you start. Rubber is loaded with elements that can be very abrasive, so detail clean your lathe and surrounding area carefully to avoid problems. Rubber dust can cause damage to vented electric motors by re-vulcanizing on the armature. Good Luck, and Regards, Clark