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).
Features and Applications
They can be useful to Electricians for example, when boring holes into trunking or cabinets to locate fixings and components.