It's a problem every rocketeer has encountered: how to achieve a nice, smooth, perpendicular cut when cutting spiral-wound paper body tubes. Whether it's building a new scratch-built model or cutting off a damaged section of body tube on an existing rocket you always seem to get an uneven or angular cut, which means that a small gap is exposed when the nosecone is put on or a stage coupler and second length of body tube are added.
Here's a simple way to produce a cutting guide for guaranteed straight cuts. First, mark the tube where you want to make the cut. Then, take a piece of paper with machine-cut straight edges (such as a sheet of printer paper) and wrap it around the tube as tightly as possible with one machine-cut edge facing the mark (for smaller tubes, you may want to cut this sheet into smaller pieces or ease of handling. Just be sure to preserve one machine-cut edge). When wrapping the paper around the tube, make sure that the machine-cut edge overlaps itself perfectly. Once the paper is fully wrapped around the tube, use one or two short pieces of tape to keep the sheet from unrolling. Slide the roll of paper up or down the tube as necessary until the factory-cut edge is lined up with your mark. Then place one or two pieces of tape at the other end of the paper roll to hold it in place against the body tube. Now you've got a perfectly straight guide for your cut.
Next, take a hobby knife and carefully make an incision in the tube with the tip of the blade right up against the paper guide. Be sure you have a really sharp blade for this step; a fresh, new blade would be best. Carefully start cutting around the tube using the tip of the blade, applying gentle to moderate pressure and making short, saw-like motions, while making sure the blade remains in contact with the edge of the paper guide at all times but not cutting into it. Once you've worked your way all the way around the tube, be careful when cutting the last segment so that you don't start peeling away one of the layers of paper as you pull the two cut sections of tube apart.
Once your cut is complete, take some very fine sandpaper and gently sand the cut edge to remove any burrs. This will make it easier to slide the nose cone or tube coupler into the tube.
We mentioned damaged sections of body tube on existing rockets earlier. If the damage is just a fraction of an inch of tube just behind the nose cone and removal of this short piece won't significantly affect the appearance or performance of the rocket, just cut off the damaged part using this technique. This will eliminate the risk of a layer of paper from the damaged section folding over into the undamaged section when the nose cone is inserted, creating a snug fit that might resist the force of the ejection charge and lead to another crash.
If the damage to the tube is greater, such as a crimp or tear farther down the tube, mark a spot just below the damaged area and measure the distance from the upper end of the tube to this spot (note: if the damaged area to be cut off is close to the nose cone or a mid-body separation point, be sure to allow for enough room within the remaining or replacement tube for both tube coupler for the replacement tube and the shoulder of the nose cone or other part of the rocket). After untying the shock cord from the nose cone and removing the nose cone and parachute, cut off the damaged section of tube at this mark using the above technique, being careful not to cut the shock cord in the process (this only applies when the shock cord mount is below the cut. If the mount is in the damaged section, a new shock cord mount will be needed). Next, cut an identical length of new tube, again using the above technique. Glue the tube coupler into one end of the replacement tube, feed the shock cord through the tube (if applicable), then glue the replacement section into the remaining rocket body tube. Wait for the glue to dry, retie/replace the shock cord, nose cone, and recovery system, repaint, and voila! Your rocket is fixed!