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How to machine long and accurate holes in the current demanding environment

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article image There are a number of different methods to carry out deep hole drilling depending on your metalworking needs.

The origin of deep hole drilling dates back to early day gun barrel workshops.

From this martial beginning, deep hole machining (DHM) technology has evolved into a sophisticated family of productive machining methods for applications such as aircraft landing gear and power plant turbines.
The defining characteristics of DHM include high material-removal rates and accuracy, including hole straightness, dimensional tolerances and surface finish.

“Deep” refers to a high length-to-diameter ratio of the hole, ranging from 5 to 150 x D.
Process security is a critical concern in most DHM operations.

Conventional CNC lathes, multi-task machines and machining centres with powerful coolant systems are also increasingly being used for machining deep holes, with a goal of minimising the number of setups.

DHM methods may even be applied for shorter holes, due to the need for the high productivity.

​Meth​ods
The most common method of deep hole drilling is solid drilling, in which a hole is simply drilled in solid material in a single operation.

Counterboring is mainly used to enlarge an existing hole and to improve the surface finish and straightness.

Pull boring is a variant where the tool is fed backwards through the workpiece.

Trepanning forms a core in the middle of the hole and gives the benefit of lower power consumption.

The cores are often used for testing, particularly when working with exotic materials, and also as raw material for other production processes.

A specialised method, core cropping, is used to remove the core after trepanning.

The more specialised methods generally aim at very close tolerances and/or particular shapes.

Skiving and roller burnishing essentially involve a tool combination to deliver extreme surface finish, dimensional and roundness tolerances.

In chamber boring, also called bottle boring, a long hole is opened up inside the workpiece with smaller-diameter entry and/or exit holes. A particular benefit here is the ability to machine complex internal shapes, enabling the manufacturer to reduce the weight of critical components without compromising surface integrity.

Form tools, or tools for blind holes, are mainly used to machine surfaces between differing diameters in applications with high requirements for surface integrity.

 

Tool systems
The most commonly used deep hole machining system, gun drilling, uses a solid tool with a coolant duct and a V-shaped flute for chip removal.

It is increasingly popular in conventional CNC machine tools equipped with applicable high-pressure coolant systems.

Gun drills, which are highly accurate tools, lend themselves to tight surface finish requirements and diameters down to one millimetre, but the penetration rate is low due to the slenderness of the shank.

The maximum drilling depth is 100 x D.

The single tube system (STS) can be compared to a reversed gun drill: Coolant flows to the drill head between the hole wall and a cylindrical drill tube and flushes the chips into the tube.

This requires a pressure head to seal the entry side of the workpiece.

The STS is optimal for extremely deep holes and, due to high coolant velocity, difficult materials such as low-carbon steel, stainless steel and titanium.

The STS has a standard diameter range of 15.60–130 millimetres and is the only alternative for diameters larger than 200 millimetres.

In an ejector or double tube system the coolant flows between an outer and an inner drill tube.

A major advantage is the possibility of adapting this system for conventional machine tools such as lathes.

No sealing is needed between the workpiece and the drill bushing.
The standard diameter range is 18.40–65 millimetres; the maximum diameter is 200 millimetres.

Conclusion
Deep hole machining is a long-standing yet advanced and productive solution for applications requiring high material-removal rates and accuracy. Process security is often a critical concern.
Various methods and tool systems offer operational flexibility and wide-ranging opportunities for application/ specific optimisation.

 

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