Cutting carbon fiber accurately is one of those skills that separates clean builds from frustrating ones. I learned this the hard way after ruining a perfectly good 16mm tube on my second frame build.
Start by getting your measurements absolutely right. I use digital calipers for precision work, especially when you need arms within a millimeter of each other for balanced flight characteristics. For a typical five-inch racing quad, your arms might be anywhere from 180mm to 220mm depending on your motor-to-motor distance. Measure from the center of your mounting holes, not just eyeballing it. Mark your cut line with masking tape wrapped completely around the tube. This serves two purposes: it gives you a clear cutting guide and critically reduces splintering and delamination during the cut.
For the actual cutting, I prefer a rotary tool like a Dremel with a reinforced cutting disc rated for composite materials. Keep your speed moderate, around 15,000 to 20,000 RPM. Too fast generates excessive heat that can damage the resin matrix. Let the tool do the work rather than forcing it through. A fine-toothed hacksaw also works well for thicker tubes above 10mm, and some builders swear by tube cutters designed specifically for carbon fiber.
The most important safety consideration is the dust. Carbon fiber particles are extremely fine, sharp, and genuinely dangerous to your lungs. I always work outside or in a well-ventilated area wearing a proper N95 or P100 respirator, not just a dust mask. Keep a shop vacuum nearby and wet the area immediately after cutting to capture stray fibers.
After cutting, you need to clean up the edges. I use progressively finer sandpaper, starting with 220-grit to remove major burrs, then finishing with 400-grit for a smooth edge that won't catch on zip ties or wiring. Sand in one direction rather than back and forth to avoid pulling fibers out of the weave. Some builders apply a thin layer of CA glue or epoxy to the cut ends to seal them and prevent delamination over time, especially on tubes that will experience vibration stress.
For perfectly square cuts, a simple miter box or even a jig made from scrap wood helps tremendously. Clamp your tube securely because any movement during cutting creates uneven edges that affect how your arms sit in the frame plates.
