The DeepHole Drilling

Deep hole drilling is the machining of holes with a relatively large depth to diameter ratio, whereas normal drilling techniques produce holes where the depth is rarely more than five times the diameter (5 xD). In deep hole drilling the ratio may reach 150:1, and any hole deeper than ten times the diameter (10 xD) should certainly be considered a deep hole, requiring a specialized drilling technique.
Deep hole drilling can employ various machine set-ups: rotating workpiece, rotating tool, or both tool and workpiece rotating. The most common, however, is for the workpiece to rotate, while the tool supplies the linear feed movement.

Whichever set-up is employed, the basic principles of drilling still apply, and the correct choice of cutting speeds and feeds are still crucial. Satisfactory chip breaking, and removing the chips from the cutting edges without damaging tool or workpiece, is essential.
Gun drilling is capable of producing smaller holes than the Single Tube System (STS), but the STS system is far more productive (4-6 times) and should always be the first choice when possible. The Ejector system is an alternative to STS when drilling smaller batch quantities as it does not require a special machine.

In deep hole drilling, a combination of tool design and cutting fluid pressure is used to flush the chips out of the hole. Two different drill systems are common.
All two systems can produce holes with excellent surface finish, close dimensional tolerance and concentricity.

High pressure pumps supply cutting fluid down the outside of the drill tube, between the drill and the drilled hole.
The drill shank itself is hollow, and the fluid pressure flushes the chips into the drill body through chiprooms in the drill head, and back out through the drill tube.

The high cutting fluid pressure makes the STS-system more reliable than the
Ejector system especially when drilling materials where good chipbreaking is
difficult to obtain, i.e. low carbon steels and stainless.
The STS-system is always the first choice for long series production.