Deep Hole Drilling Services

Deep Hole Drilling – Fundamentals and Methods

Deep‑hole drilling is an advanced machining process used when conventional drills can no longer maintain straightness, tolerances and surface quality at extreme depths. According to the VDI 3210 standard, deep‑hole drilling processes are used for diameters between 0.2 mm and 2 000 mm when the drilling depth exceeds three times the diameter. For small diameters, length‑to‑diameter (L/D) ratios of 100 : 1—and in special cases up to 900 : 1—are achievable, while large diameters are limited by the machine’s bed length. Since these holes are so deep, the coolant must be pumped to the cutting edges at high pressure to lubricate, cool and continuously remove chips. These fundamental demands distinguish deep‑hole drilling from conventional drilling and explain why dedicated machines, special tools and expertise are required.

Where deep‑hole drilling came from and why it matters

The technology emerged during the First World War when manufacturers needed precise and straight bores for gun barrels. A century later, deep‑hole drilling is used across aerospace, automotive, medical, oil & gas, firearms and hydraulic industries. Modern machines can produce holes up to 200× longer than their diameter and the technology has branched out into more than 15 industries. Deep holes are generally defined as having a depth‑to‑diameter ratio greater than 10 : 1. This ratio matters because normal twist drills rarely maintain straightness at these depths; they struggle with chip evacuation, tool wear and surface finish. Deep‑hole drilling machines use high‑pressure coolant, specialised tool guidance and self‑piloting inserts so that chips are continually flushed out, the tool remains centred and the bore is burnished for a smooth finish.

Understanding the main methods

Two families of tools dominate deep‑hole drilling: asymmetrical single‑lip tools (gun drills, BTA drills and ejector systems) and symmetrical multi‑lip drills. Symmetrical tools (e.g. spiral deep‑hole drills) operate in a small diameter range of roughly 0.2 – 32 mm and achieve L/D ratios up to 85 : 1. Asymmetrical tools support diameters from 0.5 mm to 2 000 mm and can achieve much higher L/D ratios because guide pads on the tool head transfer cutting forces to the bore wall and stabilise the tool. The guide pads also smooth the bore wall; surface roughness can improve by about 70 %, which often eliminates additional finishing steps.

Gun drilling

Originally developed for rifle barrels, gun drilling produces long, straight holes with diameters typically between 1 mm and 35 mm. A gun drill has a single straight flute and a hollow body. High‑pressure coolant is pumped through the tube to the cutting edge, and the chips are evacuated along a V‑shaped flute. Gundrills can handle depth‑to‑diameter ratios of 400 : 1 on dedicated machines and even 300 : 1 on some systems. Modern advancements include brazed‑shank and solid‑carbide drills for small diameters and indexable‑insert drills capable of machining holes as small as 13 mm.

BTA (Single‑Tube System)

When holes exceed 12 mm (½ in) in diameter, BTA drilling—developed by the Boring & Trepanning Association over 80 years ago—becomes more efficient. A BTA drill uses a larger cutting head mounted on a tube. Coolant is pumped outside the tube along the bore, and chips are evacuated inside the tube. This reverse flow prevents chips from damaging the bore surface and enables faster material removal. BTA drilling commonly handles diameters from 12 mm to 250 mm and is popular for holes >25 mm. The system also supports complex operations like counter‑boring, step bores and trepanning.

Ejector and double‑tube systems

The ejector system uses two concentric tubes: coolant flows between the tubes to the cutting head, and a venturi effect evacuates chips and coolant through the inner tube. This method suits medium diameters (around 16 mm and above) and is often used on standard CNC machines where high‑pressure coolant delivery is available but BTA equipment is impractical.

Other deep‑hole machining techniques

Parabolic flute drills and peck drilling cycles extend the capability of twist drills. Parabolic flutes allow chip extraction for depths up to 20× diameter, while peck drilling cycles periodically retract the drill to break and evacuate chips. Most tooling manufacturers consider depths more than 3 – 4× diameter to be “deep” for twist drills. However, for extremely deep holes (over 10× diameter), dedicated gun or BTA drills with through‑spindle coolant are essential.

Choosing the right method – quick reference

Applications and benefits across industries

Deep‑hole drilling’s precision and ability to maintain straightness over long lengths has led to widespread adoption:

• Aerospace: Landing‑gear components, engine shafts and hydraulic systems require accurate, long bores. The high strength and fatigue resistance demanded in aerospace make the process indispensable.

• Automotive: Engine blocks and transmission shafts often have intricate internal channels that benefit from deep‑hole drilling.

• Medical equipment: Orthopaedic rods and surgical instruments need precise, smooth bores to ensure safety and reliability.

• Oil & gas: Down‑hole tools and drilling equipment depend on deep‑hole drilling for robust, straight channels.

• Hydraulic cylinders and heavy machinery: Long, straight bores ensure proper sealing and efficient operation.

• Firearms manufacturing: The process remains a cornerstone for producing accurate barrels.

Beyond manufacturing, the Deep‑Hole Drilling (DHD) residual‑stress measurement technique is used for stress analysis in thick components. DHD is a semi‑invasive method where a small reference hole is drilled into a component, its diameter is measured to 0.5 µm accuracy, and a cylinder of material containing the hole is trepanned. The reference hole’s diameter is re‑measured; differences between pre‑ and post‑trepanning measurements are used to calculate residual stresses. The technique measures stresses up to depths of 750 mm and is suitable for various materials.

Key challenges and best practices

While deep‑hole drilling delivers exceptional precision, it comes with challenges:

• Chip evacuation: Removing chips becomes harder as the hole gets deeper. High‑pressure coolant and specialised drill geometries (e.g., parabolic flutes or internal coolant channels) are crucial. Peck drilling cycles and custom G‑code strategies can prevent chip clogging and tool breakage.

• Maintaining straightness: Long tools can deflect, producing tapered or off‑centre holes. Guide pads in gun and BTA drills support the tool against the bore wall, reducing straightness deviation.

• Heat and tool wear: Deep holes generate significant heat. Effective coolant delivery and controlled feed rates help manage thermal expansion and prolong tool life.

• Choosing the right coolant: Using the correct metal‑working fluid reduces tool wear and improves chip evacuation. Studies have shown that selecting appropriate neat oils or water‑miscible coolants can reduce machining costs by up to 45 % and tooling costs by 18 %.

BACH INDUSTRY AG – your partner in deep‑hole drilling

BACH INDUSTRY AG supplies high‑precision deep‑hole drilling services. Our portfolio includes CNC deep‑hole drilling machines, vertical and horizontal deep‑hole drilling machines, BTA deep‑hole drilling machines and attachments for lathes. We also provide deep‑hole drilling tools, including deep‑hole drill bits, carbide drills, twist drills, boring bars and jigs, ensuring you can handle both gun drilling and BTA operations with confidence.

Comprehensive services

We go beyond selling machinery. Our deep‑hole drilling services support customers who need one‑off jobs or lack the capacity in‑house. Whether you need deep‑hole gun drilling, job‑work or pull‑boring, our specialists ensure tight tolerances and high surface finish. For customers searching for deep‑hole drilling suppliers or deep‑hole boring companies, we offer consultation, machine selection, tooling, programming support and after‑sales service. Thanks to our network, we can serve customers near you in Switzerland, Germany, Austria, Liechtenstein, Belgium and even across Africa. We also export equipment to India, the United Kingdom and other regions through our distributor network.

Focus on quality and value

Customers appreciate that our deep‑hole drilling machine price structure is transparent and competitive. By leveraging long‑term relationships with top manufacturers and investing in local stock, we offer machines at attractive costs without compromising quality. Our team helps you calculate total cost of ownership—covering purchase price, operating cost, tooling and coolant consumption—so you know exactly what you are buying. For clients in Switzerland, Germany, Austria, Liechtenstein, Belgium and Africa, we can advise on import duties, finance options and maintenance plans. This makes it easy to buy the right equipment and stay productive for years.

Leading technologies and partners

We partner with world‑class brands such as UNISIG, TBT, Botek, Precihole, Sandvik Coromant and Walter to deliver proven solutions. For example, BTA deep‑hole drilling machines from our portfolio draw on more than 80 years of Boring & Trepanning Association expertise. CNC deep‑hole drilling machines with real‑time monitoring, servo‑controlled axes and sophisticated chip‑management ensure accurate results even when machining tough super‑alloys. We also supply gun‑drill tool systems, ejector assemblies and custom fixtures, enabling high‑volume production or flexible job‑shop operations.

Expertise you can trust

Selecting the correct feed rates, spindle speeds and coolant pressures is vital. Our engineers draw on industry best practices: high‑pressure coolant delivered directly to the cutting zone optimises chip evacuation; pilot holes and steady‑rest supports reduce tool deflection; and custom CNC cycles adjust peck intervals as depth increases. Combined with our training and technical support, this knowledge ensures that your team can safely produce deep, accurate bores in materials ranging from steel and aluminium to Inconel and titanium.

Conclusion

Deep‑hole drilling is both a science and an art. Its origins in gun‑barrel manufacturing have evolved into a versatile technology that produces holes 200× longer than their diameter and delivers high‑precision bores for aerospace, medical, automotive and energy industries. The process relies on specialised tools—gun drills, BTA systems and ejector tubes—each optimised for specific diameters, depths and applications. Understanding the fundamentals, selecting the right method and following best practices ensures success.

As a leading supplier and service provider, BACH INDUSTRY AG offers state‑of‑the‑art deep‑hole drilling services. Our commitment to quality, innovation and customer satisfaction has made us a trusted partner across Switzerland, Germany, Austria, Liechtenstein, Belgium and Africa.