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root-
These days you rout a hole into the floor using a specific template and router blade, then you can drop one of these ready made hatches back in when you've finished. It's a great way to get through solid underfloors but you hope to have a straight run between joists once you are under there.
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Similar to this, but not just a piece of tin covering the hole >>
If your not certain what's beneath the boards, tentatively drill the pilot hole and check for clearance below.
Use the pilot drill to centre the hole saw until that is a few mm into the board, then remove the pilot drill from the arbour and finish cutting the hole through.
Airhead
user69121
https://www.screwfix.com/p/erbauer-armeg-sbc-cover-111mm/10648