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What are the methods of deep hole drilling?

There are three traditional deep drilling methods: gun drilling, single tube system (STS) and double tube system (DTS, also called Ejector system). Is it necessary to start with a pilot or starter hole when drilling deep with each of these methods? In the case of gun drilling, guide bushings are always used when operating on dedicated gun drilling machines (usually multi-spindle).
When a gun drill is used in a machining center, a pilot hole needs to be punched in the part, on dual tube systems (injectors) a guide bushing is sometimes used in front of the part, but a pilot hole in the part is very common. On single tube systems (STS), guide bushings are always used and installed in the compression head of the compression assembly.
The role of coolant in the drilling system is to support and lubricate pads, extend tool life, dissipate heat and remove chips. Coolant is an essential part of the chip breaking and evacuation process in any drilling operation. In small diameter drills, coolant pressure is important, but in STS or injectors, drilling flow is important.
Gallons per minute (or GPM) is important to assist in carrying the chips out of the drill pipe, and then time in the tank to cool before returning to the cutting edge. During this process, the coolant should be filtered through 10–20 μm, which helps to increase tool life and provide a better surface finish.
When high coolant pressure is applied to a long drill, it can cause the drill to become unstable, which can cause chatter, and using too high a coolant pressure can also lead to chip buildup. For most twist drill applications, Walter recommends a pressure between 300 - 600 psi.
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