I read an article on waterjet cutting of laminates recently, which demonstrated several advantages of using computer-controlled equipment for trimming and adding features to composites structures. Robotic-guided CNC routers have also been used with much success for large 3D shaped parts with much success.
While technology is great, it is expensive. With the rest of composites processing being relatively inexpensive, the trimming operation usually follows suit. Most bimetal saws that cut steel can handle fiberglass, but for large production runs where they will wear down, diamond coated tools offer longer life.
Making a cutting guide for a production composite part is relatively easy. I always called it a “splash”, and it can be done similarly to building a part. Just wax up the gelcoated surface of the finished part, lay down a coat of gelcoat, then build a 3/16 laminate. Then mark out the desired cutout shape and allow for the tool setback for the router collar or saw guide. An aluminum channel edge should be added for wear resistance. Drill bushings can be located on this splash to mark where holesaw work needs to be done as well.
Using a guide collar on a router is essential. I was in a shop where they had a 1/4 inch steel plate guide that was the exact size of the hole, and required running the router bit against it to make the cut. There was TONS of wear on the guide and they were going though router bits like crazy. I set them up with a router collar and compensated for the offset on the jig, and it was a whole new world for the trim operators. The router bits lasted several times longer and the cuts were much faster to make.
Damaging the laminate is one of the dangers when cutting or machining composites. Delamination is the biggest risk, ans is diagnosed by a separation of the layers of fiberglass. The resin bond becomes too weak to hold them together. Delamination can be caused by several other things too, but very often it can happen around machined areas from dull tools or operators that are pushing tools too quickly along a cut.