RE: Question for anyone that knows drillingI am not an expert, but I do know a little bit (excuse the pun) about some of the techniques used.
Core drilling is different than convential drilling as the name describes.
For example, if you were to drill a hole in a piece of wood, you would use just a standard drill bit. Now if you were to drill a hole for something like a door, you would actually use a core drill. The core drill has a hollowed out center with a ring of teeth. This allows a solid core to be extracted.
Once a hole has been drilled, it is difficult to follow the same track as the smaller hole(at depth)because it would grab and twist the drill everytime it hits the original hole.
Hope this helps.