For sheet metal? I need a hole of 80, so a bit that does 40, 60 & 80?

or can this be made custom?

How accurate does it need to be? You could use an 80mm hole saw. Or a 79mm and tidy with a round file.
Hole saws tend to mark the edge of the hole as they enter the material.

its to go through the floor under the seat on the T6 for the heating duct to come through. The thing is there is already a hole there from origin about 40 i’d say so was hoping to just enlarge this (its just for cables, but not used). not sure if a hole saw would be ideal as centering would be tricky..

sorry, so to answer the question not very accurate!

Tack weld a small strip of steel across the hole then you can use a holesaw with centre drill.

Definitely sounds like a job for a hole saw. Just need to temporarily add something for the centre drill to drill into. A bit of plywood will do it.

If you go with hole saws you may be able to do this (depending on which arbor you have) screw the smaller one on into the large one. It acts a giant pilot. It’s easy to get an egg shaped hole and scratch the metal up with the wrong size pilots! Puts a lot of tourque on the drill with large hole saws! You can ruin the clutch on the drill.

If you take some time to build a robot, enter and win Robot Wars, then the trophy has a suggestively large drill bit attached to it.

A method I have used with success on larger holes but which would probably be fine for this. Scribe the hole. Using say a 4mm drill bit, drill a ring of holes 1mm in from the edge of the hole and 1mm apart from each other. Then enlarge each hole carefully with a 6mm bit. You then need to file the peaks off all the way round, but if you are using a grommet/flange for the hole it doesn’t need to be that perfect I guess.

Yip, chain drill or dremel.

ok i’ll try that! thanks for the tips.

Get a sheet metal punch – far simpler, faster and neater.
Piece on either side, do up the bolt.
Simple.

This

Put the holesaw through some scrap wood, say 18mm ply, then use the wood as a guide for cutting the finished hole, no need for centering.

Do as mrdoberman said, that’s the way I always do it.
With the larger type arbor you are able to screw 2 holesaws onto it at the same time!