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. UNISIG produces highly capable BTA and gundrilling machines, with applications exceeding 400:1 D:d ratios.
Deep Hole Drilling Process
- 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
- 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
Deep Hole Drilling Tools
- Gundrilling is used for smaller hole drilling, typically 1 – 50 mm [0.04 – 2.00 in] 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 deep hole drilling is used for larger hole drilling, typically 20 – 200 mm [0.80 – 8.00 in] 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 Machining and Other Processes
- After drilling, additional machining methods can further improve deep hole concentricity, straightness, surface finish, and add complex features. These methods machine the inside surface of the hole, much like other external machining processes, and most can be performed on BTA deep hole drilling equipment.
- Processes are typically performed on a workpiece with an existing bore, due to prior drilling or centrifugal casting. Many of these deep hole machining processes are optimized for specific applications, such as skiving machines for the hydraulic cylinder industry, or bottle boring for landing gear actuators.