From DT Online
Features and Applications
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 (but considerable effort would be required to drill a hole 75mm diameter!). 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.