The tool works famously. The holes about diamater 20 cm/8 inch don't pose any problem. This device has a lot of power to make hole about 25 cm/ 10 inch or even 30 cm/ 12 inch.
This project involved some work but the effect is better than we assumed. One thing which I should make is the extension of the drill so as to have the opportunity to drill deepest holes than 80 cm/ 32 inch.