Deep Hole Drilling 101

Deep hole drilling is a specialized operation that creates more profound holes than their diameter. This technique is critical in industries where high accuracy and precision are paramount, such as aerospace, medical, and automotive manufacturing. By maintaining a focus on precision, deep hole drilling allows for the creation of deep, straight holes with smooth finishes essential for high-performance parts.

Now that you know what it’s all about, let’s dive deeper into why mastering this technique can significantly elevate your manufacturing capabilities.

What Is Deep Hole Drilling?

Deep hole drilling is a machining process used to create holes with a depth-to-diameter ratio of 10:1 or more, distinguishing it from standard drilling techniques where this ratio is much lower. The primary challenge in deep hole drilling is managing the accuracy and finish of the hole as its depth increases. This process is essential in creating cylindrical cavities deep within a substrate and is commonly employed in various critical engineering and manufacturing sectors.

The technique involves several specialized methods and tools to achieve the desired depth without compromising the integrity of the workpiece. Proper execution of deep hole drilling ensures that the holes are straight, have a good surface finish, and are dimensionally accurate, which are crucial in high-specification industries like aerospace, automotive, and heavy machinery. The process also requires specialized equipment to manage aspects like chip removal, lubrication, and cooling, which are vital to prevent tool wear and ensure smooth operation.

Advantages of Deep Hole Drilling

Precision and Accuracy: This method creates deep, straight holes with tight tolerances. The precision achieved with deep hole drilling is crucial for applications where components must fit accurately, such as in aerospace assemblies or intricate machinery parts.

Improved Surface Finish: Deep hole drilling can produce a smoother internal surface finish than conventional drilling methods. This is especially important for hydraulic systems and aerospace components, where surface irregularity could lead to failures or inefficiencies.

Enhanced Material Integrity: Deep hole drilling techniques minimize the stress imparted on the material, maintaining the structural integrity of the workpiece. This is essential when working with expensive or difficult-to-machine materials, such as titanium or hardened alloys.

Versatility in Applications: Whether it’s a gun barrel, a mold for plastic injection, or critical shafts in aerospace, the deep-hole drilling process is adaptable to various applications across multiple industries.

Efficient Material Removal: Modern deep hole drilling equipment is designed to efficiently remove chips and manage heat, allowing faster drilling with fewer stops to clear debris, reducing cycle times, and increasing productivity.

Complex Geometries: With advanced techniques like BTA and gun drilling, deep holes with complex geometries can be drilled in a single pass, reducing the need for multiple machining processes.

Types of Deep Hole Drilling Techniques

Gun Drilling

Gun drilling is one of the most widely used deep-hole drilling techniques, initially developed for making gun barrels. It utilizes a long, thin drill bit with a hole along its length for coolant to flow directly to the cutting edge and chips to be evacuated back through the same channel. 

This technique is ideal for drilling holes with diameters typically ranging from 1 mm to 50 mm and depth-to-diameter ratios of up to 300:1. Gun drilling is highly prized for its ability to produce accurate, straight, and clean holes in a variety of materials, including metals, plastics, and composites.

BTA (Boring and Trepanning Association) Drilling

BTA drilling is a more robust method than gun drilling and is used for larger diameters, typically from 20 mm upwards. This technique employs a drilling head and a tube, which serves both as a structural support for the head and as a conduit for coolant delivered to the cutting edge under pressure. The chips are evacuated through the space between the drilled hole and the tube, allowing for efficient removal and minimal contact with the drill bit. 

BTA drilling is particularly effective for deep-hole drilling under challenging materials and is often used in the oil and gas, power generation, and large-scale industrial manufacturing sectors.

Ejector Drilling

Ejector drilling, also known as double-tube system drilling, is an advancement over standard gun drilling, designed to handle more profound and broader holes. This technique utilizes two concentric tubes; the inner tube delivers cutting fluid to the drill bit, while the outer tube serves to evacuate the chips. The dual-tube setup enables a higher coolant flow rate and more efficient chip evacuation. It is ideal for bottomless holes ranging from 18 mm to 100 mm. 

Ejector drilling is particularly beneficial when heat and chip management are critical to maintaining precision and tool life.

T-Deep Drilling

T-Deep drilling is a specialized technique used for creating bottomless and accurate holes with large diameters. It combines BTA and gun drilling elements, adapting them to maximize efficiency and precision in extreme drilling conditions. 

This method is often reserved for tasks that require precise deep holes in aerospace, nuclear, and heavy machinery industries, where the hole’s depth and diameter exceed typical drilling capabilities. T-Deep drilling allows for enhanced control over the drilling process, resulting in better hole quality and surface finish, even at extreme depths.

Helical Drilling

Helical drilling is a technique used for creating deep holes with high aspect ratios, whereas traditional straight drilling methods might need help with tool deflection and hole accuracy. This method involves the tool moving in a helical path, gradually deepening the hole as it spirals. The spiral motion helps distribute the cutting forces more evenly, reducing stress on the tool and the workpiece.

Helical drilling is particularly effective when working with difficult-to-machine materials or at awkward angles or off-center positions. It provides better control over chip evacuation and coolant flow, which is critical in deep-hole drilling to prevent clogging and overheating. 

This technique is often used in the automotive and aerospace industries for critical applications such as creating cooling channels in molds or machining complex parts with irregular geometries. The process ensures a smoother finish and greater dimensional accuracy, making it a valuable method for challenging drilling tasks.

Material Considerations in Deep Hole Drilling

Workpiece Material

  • Machinability: Some materials are inherently easier to machine than others, affecting the choice of drilling technique and tools. Aluminum and brass, for example, are easier to drill than hard metals like stainless steel or titanium.
  • Hardness and Toughness: Harder materials require more robust drilling tools and possibly specialized drilling techniques to handle increased resistance without excessive wear.
  • Thermal Conductivity: Materials with high thermal conductivity, such as copper, help dissipate heat more efficiently during the drilling process, which can reduce tool wear and improve hole quality.
  • Chip Formation Characteristics: The material’s ability to form chips that break away cleanly from the workpiece is crucial. Materials that create long, stringy chips can clog the drill and interrupt the process, requiring materials and techniques that aid in effective chip control and evacuation.

Drill Material

  • High-Speed Steel (HSS): Commonly used for drilling most metals and plastics, HSS is versatile and cost-effective but may not be suitable for rigid materials or high-temperature applications.
  • Carbide: Provides excellent hardness and heat resistance, making it ideal for drilling harsh and abrasive materials. However, carbide drills are more brittle than HSS and can be more expensive.
  • Coatings: Tools are often coated with titanium nitride (TiN), titanium aluminum nitride (TiAlN), or diamond-like coatings to enhance their hardness, reduce friction, and increase lifespan. These coatings are especially beneficial when drilling hard or abrasive materials.

Coolant and Lubrication

  • Type: The choice of coolant or lubricant can significantly impact the drilling process. Oil-based coolants are generally better for more rigid materials, while water-soluble coolants may be sufficient for softer materials.
  • Delivery Method: Effective coolant delivery to the cutting edge is crucial, especially in deep-hole drilling. Techniques vary from internal channels in the drill bit (as in gun drilling) to external nozzles, depending on the drilling method and setup.

Tools and Equipment for Deep Hole Drilling

Drilling Machines

  • Dedicated Deep Hole Drilling Machines: These are specifically designed for deep-hole applications and typically feature robust construction and advanced control systems to handle the precision required for deep-hole drilling.
  • CNC Machining Centers: Many modern CNC machining centers have deep-hole drilling capabilities, offering high precision and programmability for complex and varied drilling tasks.

Drill Bits

  • Gun Drills: Characterized by a single flute and a hole for coolant passage, gun drills are ideal for creating deep, straight holes with diameters ranging from a few millimeters up to around 50 mm.
  • BTA Drills: BTA drilling tools are used for larger diameters and involve an external coolant supply and internal chip evacuation, which are suitable for intense drilling in challenging materials.
  • Ejector Drills: Similar to BTA drills but using a double-tube system for coolant delivery and chip evacuation, ejector drills are used for even deeper or larger diameter holes.

Tool Holders and Accessories

  • Tool Holders: These must provide precise alignment and stability to minimize vibrations, which can compromise hole quality in deep drilling.
  • Guide Bushings: To support the drill over extended lengths, guide bushings help maintain accuracy and prevent tool deflection.

Coolant Systems

  • High-Pressure Coolant Systems: Essential for flushing out chips from the hole and preventing overheating the tool and workpiece. These systems deliver coolant at high pressures directly to the cutting edge.
  • Filtration Systems: To maintain the coolant’s effectiveness and prevent clogging or damage to equipment, filtration systems are crucial.

Chip Management Systems

  • Chip Conveyors and Bins: Efficient chip removal is critical in deep hole drilling to prevent the re-cutting of chips, which can damage both the tool and the workpiece.
  • Chip Breakers: These may be used to ensure that chips are evacuated in manageable sizes, which is particularly important when working with challenging, ductile materials.

Measurement and Inspection Tools

  • Bore Gauges and Micrometers: Precision measurement tools are necessary to check the diameter and straightness of drilled holes to ensure they meet the stringent tolerances required.
  • Optical and Probe Inspection Systems: These systems can inspect the interior surface and geometry without disassembling the setup for deeper holes.


If you have questions about deep hole drilling or require assistance selecting the best methods and tools for your applications, do not hesitate to reach out. Our team of experts is ready to provide you with the insights and support needed to optimize your drilling processes and achieve superior results. Contact us today to learn more about how our advanced drilling solutions can benefit your manufacturing needs.