Expansive Bit
From DT Online
Expansive Bits (aka Adjustable Bits) are available with a Tapered Square Shank for use with a Brace or a Hexagonal Shank suitable for use with Power Drills.
They have a small central pilot drill and adjustable cutting blades which enable holes to be drilled at any size between approximately 20mm and 75mm diameter. It is the ability to drill odd size holes which is their main use today. As with most Auger Bits they can be sharpened with a file.
Note: Most wood bits will cause splitting if an attempt is made to drill straight through the material. Best practice is to drill from one side until the tip emerges on the reverse then turn the wood around and complete drilling from the other side.