Deep Hole Drilling

Complex machine parts, such as fuel injectors, Fuel Rails for diesel engines, heat exchanger tube sheets, aircraft landing gear, etc., need deep hole drilling machines. Deep hole drilling technology makes precision cuts considering the material types and thickness. Thus, most of the manufacturing industries are developing this specific deep hole drilling machine for more accuracy.

As long as we use metals, we must consider advancing the CNC machinery in the manufacturing area. However, this content will demonstrate a basic introduction to deep hole drilling, the different deep hole drilling technologies, applications, and upsides. In addition, we will also talk about some common issues you may encounter while using a deep hole drilling machine.

Introduction to Deep Hole Drilling

Deep hole drilling is a sort of drilling technology that can be manual or computer operated. A basic definition of Deep Hole Drilling would be its depth-to-diameter ratio (D:d). In this situation, you may term deep holes if the holes are larger than 10:1.

We have previously stated that DHD into metal offers a variety of applications across many sectors. Why is DHD valuable? Well, DHD enhances the many metals uses with its foundations going back to the requirement for straighter, more accurate gun barrels and extending as other industries adopted DHD techniques to improve their applications and quality.

Deep hole drilling methods operate by utilizing specific drilling tools and setups to supply high-pressure coolant, expel chips cleanly, and create depth-to-diameter holes into metal beyond what a typical CNC machine can reach. This process enables manufacturers to meet their manufacturing tolerances, and production needs reliably, precisely, and efficiently.

Deep hole drilling is usually done on specialized deep hole drilling equipment designed and constructed to optimize the procedures for straightness and efficiency.

However, DHD advancements in technology enable CNC machining centres, equipped with high pressure, through-spindle coolant, to allow different DHD Drilling up to a restricted depth-to-diameter ratio. There are a large number of machine vendors manufacture intelligent DHD machines improving the various application.


  • The deep hole drilling method is significantly quicker than other traditional procedures, thus a shorter cycle time.
  • Due to excellent precision, it makes very little rejection.
  • You can create bigger Diameter Holes.
  • You can also further enlarge the hole diameter by counter boring operation.
  • Higher penetration rates.

Why is cooling lubricant essential in DHD Process?

Deep hole drilling also varies from conventional drilling. Depending on the drilling procedure and the drilling diameter, one must supply cooling lubricants to the cutting blades in massive amounts and under high pressure.

This cooling lubricant provides excellent cooling and, at the same time, good lubrication of the contact regions between the workpiece and the cutting edge of the tool on the one hand and the workpiece and guiding pads of the device on the other. In addition, the cooling lubricant leads to continuous removal of chips from the cutting zone, which makes surface-damaging and time-consuming chip removal strokes unnecessary and thus enhances the quality of the borehole and the productivity of the operations.

Applications of Deep Hole Drilling

Deep holes exist in various applications, spanning almost every sector, each with its own set of stringent criteria and unique difficulties, from tight tolerances to rigid materials to high production objectives.

  • Fuel Injector Bodies: High quality, high volume components need numerous holes at extreme tolerances, accomplished via a 3-spindle gundrilling center with robotic and conveyor automation.
  • Fuel Rails for Diesel Engines: Gasoline rails need tiny straight holes that won’t fail while under fuel pressure. Gundrilling is capable of producing holes drilled to these tight standards.
  • Heat Exchanger tube sheet: Thousands of precise holes are attainable rapidly and effectively using a multi-spindle tube sheet drilling machine.
  • Aircraft landing Gear: Drill and shape landing gear actuating cylinders while maintaining an exceptional straightness tolerance in high-strength metals.
  • Fluid Assembly Ends: A set of holes is bored in a steel block for hydraulic fracturing equipment. A hefty workpiece and tight tolerances need competent machinery and tools.
  • Hydraulic cylinder inner bores: Finish an existing bore to mirror-like surface finish standards for better hydraulic cylinders.

Deep Hole Drilling Tools and Technologies

DHD technology may accomplish the creation of deep holes using several different Deep Hole Drilling methods. Shops must achieve precision, repeatability, and excellent surface polish while preserving predictable tool life to produce a high-quality end product and keep costs under control.

However, Selecting the most suitable instrument for the task and applying the proper operating methods may enhance this challenging process, making it productive and lucrative.

In the following, we’ll discuss five deep hole drilling equipment and technologies: gun drilling, BTA drilling, ejector drilling, bottle boring tooling, and counterboring tooling.

Gun Drilling

Gundrilling is a deep hole drilling technique that utilizes a long, thin cutting instrument to create holes at high depth-to-diameter ratios. Gundrilling is usually successful in sizes from 1 – 50 mm [~0.04 – 2.00 in].

A gun drill varies from a typical twist drill by its unique head geometry; a standard gun drill has a single effective cutting edge that eliminates chips as it progresses into the workpiece.

Gun drills are renowned for producing long, straight, tight tolerance holes with excellent surface quality at a high penetration rate. A gun drill diverges from a conventional twist drill by its unique head geometry. A typical gun drill has a single effective cutting edge that eliminates chips as it progresses into the workpiece.

The gun drilling method can drill deep holes beyond what is feasible with traditional equipment and tools like twist drills using high-pressure coolant for clean chip removal, even at extreme depths.

These deep hole drilling tools maintain a position to exact tolerances, produce burr-free holes. You can resize them for precise requirements. Additionally, this process has unique, repeatable holes with excellent surface finishes, which avoids the necessity for one or more secondary or finishing operations.

Gun drills are readily connected with CNC machining centres, lathes, and milling machines for a relatively modest investment, making these tools cheap for small or big businesses with varied production needs.

There are various kinds of gun drills, Single Flute Solid Carbide, Two Flute, Indexable.

The gun drilling method can drill deep holes beyond what is feasible with traditional equipment and tools, such as twist drills, by utilizing high-pressure coolant for clean chip exhaust, even at extreme depths.

BTA Drilling

deep hole drilling
deep hole drilling with BTA system

This drilling process is a deep hole drilling technique that utilizes a specialized drilling tool on a long drill tube to create deep holes in metal, from holes with a diameter of 20 mm [0.80 in] and bigger, up to depth-to-diameter ratios of 400:1. BTA drilling is the most successful way of drilling deep holes. It is a cleaner, more dependable, and competent operation than traditional twist drills and can reach bigger diameters and higher feed rates than the alternative gun drilling.

The drilling tool heads are threaded or fixed onto long drill tubes and utilize numerous cutting surfaces on a single tool to remove chips effectively, exhausting them using high-pressure coolant via holes in the tool head, then out the drill tube and into the machining spindle. Manufacturers offer BTA tooling in brazed or inserted carbide versions.

BTA stands for Boring and Trepanning Association. It is also frequently referred to as STS (single-tube system) drilling since it utilizes one single drill tube for the BTA tool instead of other procedures like ejector drilling, which requires two.

Ejector Drilling

Ejector Drilling is a version of the BTA drilling technology. People primarily utilize this DHD technique on traditional machine tools and machining centres.

However, except for BTA deep-hole drilling, there is no sealing to restrict the escape of the metal-working fluid at the workpiece. Therefore, you may utilize this drilling method if it is not feasible to close the metal-working fluid circuit, e.g., for oblique boring or discontinuous workpieces. Drilling depths of up to 100 times D are achievable with this technique.

The machine supplies the metal-working fluid via an annular gap between the drill tube and an inner tube (double-tube system) (double-tube system). The fluid emerges from the side of the drilling head, runs over it, and then flows back into the inner tube together with the chips. It injects some of the fluid into the internal tube ring nozzle. The resultant under pressure at the chip mouth allows backflow (the ejector effect) (the ejector effect). An ejector cooling-lubricant feed system is needed to produce the ejector effect.

Bottle Boring Tooling

This DHD process is a tool that is used to create an internal profile inside the length of a bore utilizing a cutting insert, which is extended and retracted using CNC capabilities on a BTA-equipped deep hole drilling machine.

Bottle boring tools as a unique tool solution for customized design adaption to your requirements, Large feed-out stroke ratio, and maximum stability.

Counter Boring Tooling

This DHD method is a tool that is used to implement complete bores by removing stock. This kind of tooling is usually utilized on BTA-equipped deep hole drilling machines.

Advantages of multi-edged counterboring tools include increased feed rates and better workpiece quality. Constant wall thickness is frequently a desirable outcome, which may be accomplished utilizing tool guiding in the tube.

Common Issues with Deep Hole Drilling

In the operation of deep hole drilling, issues such as dimension accuracy, surface quality, and tool life frequently arise. How to minimize or perhaps prevent these issues is a significant problem to be addressed. After deep research, we have collected the most common points that you will find. We will try to mention them as straightforward as possible. Consecutively, we will also tell you how to prevent these issues.

The hinged interior hole is not round

This problem may arise due to the following situation:

  • The reamer is too big, stiff enough, and vibrates while reaming.
  • The primary deviation angle of the reamer is too short.
  • The cutting edge of the reamer is thin; the reamer allowance is too big.
  • The face of the interior hole contains notches and crossing holes
  • The surface of the cavity contains sand holes and air holes
  • The bearing of the primary shaft is loose, there is no guiding sleeve, or the space between the reamer and the guide sleeve is too broad, and the clamping of the thin-walled workpiece is too stiff.

Solve: The reamers with inadequate stiffness may adopt reamers with uneven tooth spacing, and the installation of reamers should adopt a rigid connection to enhance the primary deviation angle.

Surface Roughness is high at an inner hole

This issue may develop due to the following situation:

  • Excessive cutting speed.
  • Improper choice of cutting fluid.
  • Too big primary deviation angle of the reamer, reaming edge, not on the same circumference.
  • Too big reaming allowance; uneven or too little reaming allowance, local surface not reamed.
  • It may happen due to abnormal swing difference of reamer cutting portion, not a sharp edge, bumpy surface.
  • Too broad reaming edge; no chip removal during reaming hole Excessive wear and tear of the reamer.
  • Reamer bruises with burrs or broken edges.
  • Edges with debris tumors and material interactions are inappropriate for reamers with zero or negative rake angles.

Solve: You can efficiently address this problem by taking care of the circumstances above, such as decreasing the cutting speed, selecting cutting fluid according to the processing material, etc.

Broken Hinged handle

This issue may arise due to excessive reaming allowed, improper distribution of rough and fine reaming allowance and selection of cutting parameters during reaming taper holes, tiny chip space of reamer teeth, and chip blockage.

Solve: You may address this issue by modifying the pre-machined aperture size and the allowance allocation. Also choosing the cutting parameters appropriately, decrease the number of reamer teeth, increase the chip space or grind the clearance of the cutter teeth will help you solve this issue.

Curve centerline of the reaming rear hole

This straightness issue may arise due to the following reason:

  • Drilling hole deviation before reaming, mainly when the aperture is tiny, can not repair the initial bending because of the weak stiffness of the reamer.
  • The primary deviation angle of the reamer is too big; the inadequate guidance causes the reamer to stray from the direction in reaming easily; the chamfer of the cutting portion is too large.
  • The reamer displaces at the gap in the center of the discontinuous hole; the reamer is employed in one direction while reaming a hand hole. Too much power causes the reamer to deflect to one end, which destroys the verticality of the reamer.

Solve: Increasing the reaming or drilling process corrective hole and decreasing the primary deviation angle may address this problem. Furthermore, you may also try adjusting the suitable reamer. And after that replace the reamer with the guiding part or extend the cutting portion. Always pay attention to the proper functioning.


DHD or Deep Hole Drilling is a famous drilling technology in the modern manufacturing industry. Generally, the drilling ratio between the length and diameter of the hole should be more than 10:1 to perform DHD.

Gundrilling and BTA drilling is the most common DHD procedure in metal drilling technology. There is a specific limitation for these two drilling processes. During operating these machines, you may encounter few common problems. We have stated them above.

However, We have gone through deep research to bring you the most valuable content on this topic. We have mentioned the references below the pictures. We hope this article will help you in further study.