Here we have a selection of examples of how not to knock through a tap hole. All examples are how not to do it and is not covered by any manyfacturers guarantee.
On this occasion, the image clearly shows a hairline crack has appeard as too much force was needed to knock through the tap hole, this is a typical indication that the tap hole area to be knocked through has not been prepaired correctly by the installer and the area to be knocked through has not been weakend, requiring excessive force to be needed to smash through the tap hole.
The underside of the sink demonstrates that a 35mm dimond drill bit was not used to further deepen the scorched tap hole area as the sink is approximately still 10mm thick before the knock out. We recommend you drill to around 3mm thickness before considering knocking through. This is a very important process of the tap hole knocking out process as the sink is robust and it minimises cracking the sink.
Here is a tap hole knocking out process gone wrong where the installer has tried to knock through the tap hole in the incorrect position, resulting in damaging the sink.
The piece of granite shows the point of impact where the tap hole was attempted which shows clearly it has been knocked in the incorrect position.