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What is Deep Hole Drilling?

A deep hole is defined by its depth-to-diameter ratio (D:d), and typically holes greater than 10:1 are considered deep holes. Deep hole drilling into metal has a range of applications across several industries, with its origins tracing back to the need for straighter, more accurate gun barrels, and expanding as other industries integrated deep hole drilling processes to improve their own applications.

Deep hole drilling consists of BTA drilling and gun drilling, with additional processes designed for specific tolerance objectives and generally performed on BTA-style deep hole drilling machines. Deep hole drilling is used in a variety of materials from aluminum to super-alloys, and is capable of achieving tight diameter control, straightness, and superior surface finish into workpieces.

Deep hole drilling processes work by using special tools and setups to deliver high pressure coolant, evacuate chips cleanly, and achieve depth-to-diameter holes into metal beyond what a common CNC machine can reach. This allows manufacturers to achieve their manufacturing tolerances and production requirements reliably, accurately, and efficiently.

Deep hole drilling is generally performed on dedicated deep hole drilling machines which are manufactured and assembled to optimize the processes for straightness and efficiency. Advances in technology allow CNC machining centers, equipped with high pressure, through-spindle coolant, to be capable of gundrilling up to a limited depth-to-diameter ratio. Profimach Profidrill produces under OEM highly capable BTA and gundrilling machines, with applications exceeding 400:1 D:d ratios.

Deep Hole Drilling Tools

Gundrill Tooling

  • Gundrilling is used for smaller hole drilling, typically 1 – 50 mm in diameter
  • High-pressure coolant is introduced through the spindle and center of the gundrill
  • Chips are discharged through a v-shaped groove in the length of the tool
  • Gundrill tips are ground to achieve drilling goals and extend tool life

BTA Tooling

  • BTA deep hole drilling is used for larger hole drilling, typically 20 – 250 mm in diameter
  • High-pressure coolant is introduced around the outside of the tool through the pressure head assembly
  • Chips are discharged through the tool center, through the drill tube and machine spindle
  • BTA drilling typically performs 5-7 times faster than gundrilling, and requires higher power
  • Additional extended diameters can be drilled on BTA machines with secondary deep hole operations, such as counter-boring

Deep Hole Drilling Process

Rotating Tool
  • Typically used for non-symmetrical components, or round parts with off-center holes
  • Cutting speed is determined by tool spindle speed
  • Drill drift can be significant when compared to rotating workpiece, or counter-rotating process
Rotating Workpiece
  • Typically used for round parts with a deep, on-center hole
  • Cutting speed is determined by part, balanced to allow high rotating speeds
  • Drill drift is reduced compared to rotating tool only
Counter-Rotating Tool and Workpiece
  • Ideal process for round parts with a deep, on-center hole
  • Cutting speed is determined by a combination of tool and workpiece rotation
  • Provides optimal hole straightness and concentricity

What is Gun Drilling?

Deep Hole Drilling Overview

Gundrilling is a deep hole drilling process that uses a long, thin cutting tool to produce holes in metal at high depth-to-diameter ratios. Gun drilling is effective in diameters from 1 – 50. A gundrill tool differs from a conventional twist drill by its unique head geometry; a standard gundrill has a single effective cutting edge that removes chips as it advances into a metal workpiece.

The gundrilling process is able to drill deep holes beyond what is possible with conventional machinery and tooling such as twist drills, by using high pressure coolant for clean chip exhaust, even at extreme depths.

Gun drilling was initially developed for the manufacture of gun barrels, where both straightness and barrel durability were essential to properly functioning barrels. The gundrilling process and tooling have been refined for maximum performance and use on modern, dedicated machinery. Firearms continue to be a common application for gundrilling. Other industries include diesel fuel components, medical tooling, plastic injection molds, and more.

Gun drilling can be done, with limited performance, on common CNC machines. Holes with a depth-to-diameter ratio of 20:1 or greater generally require dedicated equipment to achieve the highest productivity and process reliability, and can reach extreme ratios of 400:1 or greater.

Gun Drilling vs BTA Drilling

The gun drilling process is ideal for smaller hole diameters, up to 50 mm. The gun drilling process differs from BTA drilling due to coolant entry and chip removal; gundrills introduce coolant internally through a small hole within the tool, and chips are removed by coolant through a groove outside the length of the tool. BTA drilling machines introduce coolant externally, through an assembly around the tool, while chips are evacuated through the drill itself.