cutting 175 mm disc out of MDF

Find someone with a lathe ...

screw an oversized square of MDF in the centre, with the router clamped down to e.g. a workmate raised up by an offcut of 20mm MDF, then spin the square to cut the disc.

How about using a holesaw? For your purpose the central hole for the drill bit should not be a problem

Malcolm

Find a disk of something just under size (plate, biscuit tine lid etc). Stick it to your MDF with hot melt of double sided tape. Then put a guide bush in the router and use that to router around the periphery of your template.

Alternatively use the router in the table. tape down a bit of 9mm MDF or similar as a baseboard, and then roughly cut a circle a bit oversize (jigsaw) in your material. Stick a nail though the centre, and then tap it into the board on the table, so that its 175mm from the edge of a straight fluted cutter. Now just rotate the material around on the pivot nail, and raise the router up to make the cut.

Why not just fix the hole in the PB? If as you say there is a noggin there it makes it very simple to graft in a piece of PB and a bit of filler to finish.

Richard

Hammer a panel pin part way in to a scrap board, cut the end off the pin (leave 5-10mm) and file the pin to a point. Attach the scrap board to the band saw or router tables so that the metal spike is the correct distance from the blade or router cut point. Make a small indentation in the centre of your MDF then introduce it to the band saw or router, get the spike into the indent, turn MDF through 360 deg ... job done. Simples!

I like that one, but don't have a bandsaw. TBH I think I would just cut slightly oversize by jigsaw, then dress by hand down to a pencil-drawn circle with a sanding disk in an angle grinder. If you are putting a moulding on it afterwards with a router it doesn't have to be that accurate.

I have a fixed disk/belt sander, with this you can do an even better job using the disk.

Glad I bought that oddball circle cutter tool from Lidl ages ago. Used for speaker cutouts in baffles and will do 150mm. Think it may be Dremel based by the very difficult to get imperial cutter.

But for a one off like this in MDF I'd just cut it out roughly with a saw then file/sand to size.

I bough the TREND circle cutting jig in the end - did a great job and easy to use.