The Rotary extension drill bits works on different principle then the above described percussion OVBD systems. It is connected directly to the drill string - no DTH hammer is used. This means the principle of drilling is rotary only, no percussion. When used in combination with casing, the extension drill bit juts over the casing crown and the extended elements pre-drill larger diameter then that of the casing. After the pre-drilling the casing must be pushed into the borehole by the drill head or the extension drill bit can be used in combination with the double rotary head. Because the drilled diameter is larger then that of the casing, the friction between the casing and the rock is significantly reduced, which is advantageous for both pushing down and pulling up of the casing. When the overbuden layer is passed and hard rock is reached, the extension drill bit is simply pulled against the casing crown, the extended elements are retracted into the bits body and the extension drill bit is simply pulled up through the casing.
There are several advantages and disadvantages compared to percussion OVBD systems. The advantages are: lower price, more simple retraction of the extended bits, no need of the DTH hammer, the maximum diameter of the casing is not reduced by the casing shoe.
The disadvantages are: the casing is not driven to the borehole by percussion, the drill speed is lower when drilling through hard layers due to lack of percussion - the drilling principle is rotary only.
There are two types of the extension drill bits, see the picture. Type 1.: Button drill bit for usage in sand and gravel. Type 2.: Blade drill bit for usage in clay. The standard dimensions of the extension drill bits are listed in the chart below. For the given casing diameter the extended/retracted diameter is indicated.

Rotační rozšiřovací korunky_ta
Rotační rozšiřovací korunky