Safety

Using FR4 is not recommended for milling. Getting into contact with fiber could cause health issues. The following resourcen I found:
https://enviroliteracy.org/how-long-does-fiberglass-stay-in-the-air/
https://indoorscience.com/blog/should-i-be-concerned-about-fiberglass/
https://doh.wa.gov/community-and-environment/air-quality/indoor-air/fiberglass