What type of fiberglass are you going to cut? Does it have a name or number like G10. G10 is a vacumed, layered fiberglass that is very tough and needs a diamond edge cutter. So the type and density of fiberglass varies a great deal and all have their problems. Are you cutting all the way through? Fiberglass causes heat that will destroy your tooling so a low rpm and high feed rate are necessary and you have a good strong hold on the part or it can move