It is a very simple circle cutting tool I have been using over last 20 years . I use it for cutting circular opening for speaker on front panel and making vent holes on chassis for air circulation.
You can make it using mild steel rod with little bit of lathe work, drilling holes and tapping.
The cutting bit can be made using a 3mm drill bit and grinding it to desired shape and size.
Useful for making circular openings in mild steel sheet, Aluminium sheet, PC Boards, Flex boards, etc.