Step Drill and ConeCut
From DT Online
The Step Drill and ConeCut© are both used to drill or enlarge holes in thin sheet materials, including: sheet steel, non-ferrous metals, plastics, Formica and plywood. They are made from HSS and capable of making holes of any* diameter within their size range - usually 3mm to 60mm diameter (*Step Drills are restricted to their step sizes but a ConeCut can produce infinitely variable sizes of hole).
They can be useful to Electricians for example, when boring holes into trunking or cabinets to locate fixings and components.