When holes are drilled they usually contain helical feed marks on the inside and have sharp entrance edges with burrs on the exit side. The holes are normally drilled perpendicular to the surface of the workpiece in order to minimise any deflection of the bit, which may lead to the holes being misplaced.
The process can create a layer of stressed and disturbed material around the hole leading to corrosion. To counteract this a finish can be applied.
Spot drilling creates a guide hole for the drilling of the final hole, thus its depth is only a partial thickness of the workpiece.
Deep Hole Drilling
A hole with a depth that is greater than ten times its diameter is classified as a deep hole. Such holes require specialist equipment and tooling methods to ensure the hole remains straight.
Otherwise known as orbital drilling, this process uses machine cutters which rotate around their own axis whilst simultaneously rotating about a centre axis off-set from the axis of the cutting tool, in order to create holes.