Wobbly 9" cutting disc

[Nozzle]

Just a thought to make sure the inner flange piece is properly engaged with the shaped keyway on the driving shaft. You would still be able to clamp the locking ring down if not, but it would then only drive through friction rather then being properly engaged. There's is an off-round drive shaft with corresponding off-round hole.

Nozzle

 

[DIYalot]

Follow-up: The seller has arranged for a refund. I think they inspected it and found the fault I described (wobbly inner flange). Looks like I should use the £44.99 towards another brand of grinder. Still got the Neilson disc though, I did not return that because it was the grinder that had the fault. I suppose I'll be safe with the Neilson disc.