Deep hole drilling is the mechanism by which engineers create holes ten times deeper than the actual hole diameter. It's a common practice in some fields, for instance, aerospace, borehole drilling, and oil and gas extraction. When performing deep hole drilling, a gun-drill tool with a kidney-like shaft utilizes a range of super metal alloys to create a hole with a high depth to diameter ratio. 

Deep hole drilling machines are superior, thanks to their multitasking machine techniques. The multitasking trend was introduced into the aerospace industry because essential components like the landing gear require deep hole machining. Traditionally, engineers used different machines to achieve a particular goal. However, deep hole drilling consolidates operations because a single device performs several tasks.

Explained below are the essentials of aerospace deep hole drilling:

Machinery and Drilling Tools

Countless investigations have been conducted on the mechanical and thermal load of the drilling tool. The drilling tool's design, character, and asymmetry determine its acting forces and torques during the drilling process.

There are several forces to be put into consideration when using deep hole drilling tools. The forces include:

Acting process forces
Acting feed forces
Inertia forces
Feed force (Ff)
Passive force (Fp)
Cutting forces (Fc)

The acting process and inertia forces applied on the head of the drilling tool impact the cooling lubricant flow and forces. In addition, the area of the tool handle or holder applies the acting feed force, drilling torque, and clamping force.

Apart from the mechanical load, the thermal load plays a significant role in deep hole drilling. Keep in mind there is friction between the guiding pad and the walls of the borehole. The heat produced at the cutting tip is not directly transferred to the cooling lubricant but is instead directed to the chips, workpiece, and the tool, then released for intensive cooling.

Cooling Lubricant

The cooling lubricant plays three major roles, which are:

Cooling- By getting rid of the heat generated from the tool and workpiece.
Lubrication- By reducing friction and the wear and tear from the cutting edges and guide pads.
Flushing- By continuously removing the chips.

Additional uses of the cooling lubricant include:

Protecting the metal alloys against corrosion and oxidation.
Reducing the deep hole drilling tool wear and tear, hence increasing durability.
Reduce instances of foaming tendencies and low oil mist formation.
It creates a damping effect on the boring bar by playing a part in sound absorption.

When drilling, adjustment of the viscosity rate and pressure of the cooling lubricant depends on the diameter of the drilled borehole. The commonly used cooling lubricants are water-immiscible minerals with ester oils mixed with special additives like extreme pressure additives (EP) and anti-wear additives (AW)

Process Dynamics

The design of the drilling tool and its dynamics significantly influence the deep hole drilling process. For example, a primary drilling tool consists of a drilling head and a boring bar. However, a tool like the single-lip drill consists of a drilling head and a fluted shank which tends to vibrate a lot due to the high length-to-diameter ratio.

The drilling tools have a low dynamical torsional and flexion rigidity which causes vibrations. The vibrations lead to the formation of radial chatter marks at the bottom of the borehole or helical surface structures on the walls of the borehole. In addition, the spiral pattern propagation on the borehole's wall is due to the spiraling effect.

The spiraling effect results from:

The inhomogeneity of the workpiece material.
The guide pad alignment of the drilling tool.
The poor alignment of the machine axes.

Quality Features

The quality characteristics of the drilling tool parts influence the outcome of the drilling process. For instance, the guide pads determine the final impact on the surface of the workpiece. In addition, the quality of the guide pads influences the feed grooves and the roughness of the surfaces during the deep hole drilling process.

The formation of feed marks relies on the number of overruns of the guide pads, forces at the guide pads, cutting speed, workpiece equipment, and the feed rate. If the normal forces are too high, there is an outbreak with the workpiece creating spellings. However, an open profile is generated if the normal forces are low.

Process Strategy

The deep hole drilling tool has an asymmetrical design. Hence, it requires guidance at the beginning of the drilling process. The initial direction is by a drilling brush or a boring brush. When drilling larger hole diameters, the drilling brush is ground and hardened, while in smaller hole diameters, the drilling brush is made of cemented carbide.

A resultant force forms due to the asymmetrical design of the drilling tool. The pressure then pushes them away from the axial position. In addition, the centrifugal forces are at play and enhance tool deviation from the axial position as it keeps rotating.

The start phase of the drilling process is an unstable stage. However, with a drilling brush, the drilling tool can withstand unstable situations at the beginning of the process. In addition, the start phase determines the durability of the deep hole drilling tool as the drilling head, circumferential chamber, and guide pad are heavily loaded.

Process Variants

The process variants differ depending on whether you are dealing with a through borehole or a blind borehole. The drilling process may involve solid drilling, counterboring, core drilling or trepanning, and form boring.

Solid drilling is the variant applied in most cases. The shape of the drilling head determines the form-boring variant as its design results from a specific borehole bottom. Counterboring and trepanning are also applicable in the deep hole drilling process.

Bottom Line

The deep hole drilling process is applicable in several fields requiring holes with a high length-to-diameter ratio. The fundamentals of the deep hole drilling process are machinery and drilling tools, cooling lubricant, process variants, process dynamics, quality characteristics, and process strategy.

If you require deep hole drilling services, it is best to consult with professionals to ensure the job is done correctly. In addition, years of experience and the availability of modern technology tools will get the job done in no time.