What is a deep-hole drilling machine? It is a highly specialised CNC system designed specifically for machining deep holes with a diameter-to-length ratio of over 20:1. It features exceptional axial precision and can achieve hole depths of up to 400:1.

By now, achieving IT7 tolerances (0.01–0.02 mm) in deep hole drilling will require more than just torque. It will also require integrated high-pressure cooling systems (>70 bar) and advanced vibration damping technologies.

This guide breaks down the key parameters of deep hole machining, compares BTA and gun drilling processes, and helps you to optimise production efficiency.

Core Technologies: BTA vs. Gun Drilling

Selecting the right deep hole drilling machines depends primarily on the required hole diameter and the necessary chip evacuation method.
 

Small Diameter Precision (<20mm)

Gun drilling uses single-edged tools with internal coolant holes and external V-shaped grooves for chip evacuation. For workpieces made of typical AISI 1045 steel, maintaining a cutting speed (V_c) of 80 m/min and a feed rate (f) of 0.02 mm/rev ensures proper chip curling for efficient evacuation. This setup is as specialized as choosing the best bit for drilling aluminumwhen handling high-silicon or gummy non-ferrous alloys. However, if the coolant pressure falls below 50 bar during the drilling of a 10 mm deep hole, chip clogging occurs, resulting in immediate tool failure and surface scoring.

Heavy-Duty Bore Production (>20mm)

The BTA (Boring and Tapping Association) system is the industry standard for large diameters and high metal removal rates. Unlike gun drilling processes, the BTA system forces coolant flow through the gap between the drill tube and the hole wall, expelling chips from the centre of the drill tube.
 
This internal chip evacuation mechanism enables an D:L ratio of up to 100:1, eliminating the clogging risks associated with external chip evacuation grooves. Induction-hardened BTA drills can withstand feed rates of 0.15 mm/rev while maintaining a straightness of 0.1 mm per metre for 4140 alloy steel.

Critical Machine Components for "Deep in All Holes"

The stability of deep hole drilling machines is governed by the synergy between the guide bushing and the vibration dampening system.

Vibration Dampening Systems

In boreholes exceeding 1,000 millimetres in length, resonance phenomena may cause 'rifling-like' patterns to form on the inner diameter. Installing hydraulic stabilising brackets (shock absorbers) at 800–1000 mm intervals effectively suppresses these vibration frequencies. Applying a clamping pressure of 15 to 20 bar to the shock pads prevents drill pipe whipping, which is the main cause of borehole deviation.

High-Pressure Coolant Units

During deep hole drilling, coolant serves as both a lubricant and a delivery medium. A multi-stage filtration system is essential, as particles larger than 20 microns can cause abrasive wear and lead to the consumption of drill guide shims. For technical selection, following a detailed carbide guide is vital to match guide shim hardness with workpiece abrasiveness. When drilling 316L stainless steel, using synthetic oil at a concentration of 8% to 10% significantly reduces heat build-up at the cutting edge, extending tool life by 30% compared to standard emulsions.

Advanced Processes: Skiving & Roller Burnishing

The precision of deep hole drilling machines often extends beyond the initial hole. For applications such as hydraulic cylinders or high-end oil tools, achieving a mirror-like finish in a single pass is the efficiency benchmark.

Surface Integrity

Integrating the scraping and roll finishing (SRB) processes into the production line eliminates the need for secondary honing operations. During the scraping process, 0.2–0.4 mm of material is removed from the inner diameter, while the finishing rollers perform cold working on the surface. For standard cold-drawn tubes, this process reduces surface roughness from Ra 3.2 μm to Ra 0.4 μm at a feed rate of 1,200 mm/min. The spindle speed is set at 450 rpm to ensure that the rollers apply sufficient pressure to flatten surface peaks, but not so much that work-hardened brittle zones form.

Material Hardness Factors

Tool geometry is critical when deep-hole machining materials with a hardness of HRC 30–40. Increasing the rake angle of the scraping insert to 15° reduces cutting forces. If the hardness of the material varies by more than three grades along the length of the workpiece, the polishing pressure must be automatically compensated for by hydraulic feedback to maintain a diameter tolerance of 0.005 mm.

Industry Pitfalls & Efficiency Metrics

Coolant Pressure Drops

A sudden drop in drill bit port pressure is the most reliable 'early warning system'. A 10% decrease in pressure typically indicates partial chip blockage or the initial stage of drill pipe cracking. Digital flow meters must be monitored; for example, a flow rate of 150 litres per minute is required for a 20-millimetre BTA drill bit to maintain the kinetic energy needed to push chips back through the return pipe.

Tool Life Management

Relying on 'expected service life' is a common mistake. Instead, monitor the spindle load current. An increase in torque by 15% indicates that the carbide coating has been depleted. Replacing the insert at this point saves 50% in costs, which is far less than the cost of rebuilding the entire tooling set required after a catastrophic failure during deep hole drilling.

FAQ

Q: How do you ensure straightness in deep hole drilling?

To prevent deviation during drilling, the workpiece and deep-hole drill are made to rotate in opposite directions, a process known as reverse rotation. This configuration compensates for minor deviations in tool geometry. When the speed ratio exceeds 100:1, the guide sleeve must be calibrated by a laser alignment system to within 0.005 millimetres of the spindle axis. If deviation occurs, inspect the wear on the carbide guide shims, as even a 0.02 mm difference can cause a 1 mm deviation in the position of the hole over a 2-metre drilling stroke.

Q: What are the limits of "holed deep" applications?

Physical limitations are usually determined by the length of the machine tool bed and the capacity for chip removal. Current high-end deep-hole drilling machines can achieve machining depths of up to 15,000 millimetres during boring operations. However, the process is often limited by coolant temperature: if friction causes oil temperatures to exceed 50°C, the thermal expansion of the drill rod will affect the stability of the hole diameter. Integrated cooling systems must be installed for drilling depths exceeding 5,000 mm.

Q: Can deep drill machines handle hardened steel?

Yes, but the strategy shifts from high-speed steel to indexable carbide or cermet tips. For hardened steel (HRC 50+), reduce the feed rate by 40% and increase the coolant concentration to 12% to enhance the extreme-pressure (EP) film between the tool and the workpiece. This prevents the "built-up edge" (BUE) that typically destroys surface finish in hardened alloys.

Conclusion

When selecting deep hole drilling machines, balancing machining precision with long-term operating costs is a core competitive advantage. Overlooking minor losses in high-pressure filtration systems or guide components often leads to the scrapping of high-value workpieces. By precisely optimizing BTA and gun drilling parameters alongside real-time load monitoring, linearity and surface consistency in deep hole machining can be significantly enhanced.

Optimize Your Deep Hole Precision Today

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