What is Deep Hole Drilling?

Deep Hole Drilling Process | Gun-drilling | BTA drilling

When the Length to Diameter ratio (L/D) is greater than 10:1, in practice - such holes are typically considered as 'deep holes'.

Gun drilling & BTA drilling - together make up two elementary types of Deep Hole Drilling processes, and are used on a variety of materials from Aluminium to Super-alloys.
Due to the way in which the process takes place, compared to traditional twist drills - deep hole drilling is capable of achieving tighter diameter control, hole straightness, and superior surface finish into workpieces. Hence, relative to CNC Machining Centers, deep hole drilling achieves pretty large L/D ratios (as high as 400:1 or more!) owing to the use of specialized tool geometry with high pressure through coolant to evacuate chips on real-time basis.

This operational schema enables manufacturers to achieve pretty good manufacturing tolerance bands and production volumes quite reliably, accurately, and efficiently.

Deep hole drilling machines are different from general drilling machines and machining centers - as in being designed, manufactured & assembled in a way that optimizes the drilling process for straightness and operational efficiency. But then of course - advances in attained in CNC machining centers equipped with high spindle-through coolant pressure - enable gun-drilling albeit with limited L/D ratios.

Insight's iDrill DH Series line-up is a class of machines that exude performance driven design from quarters of machine building - setting these machines in a class of their own.

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Define Deep Hole Drilling

A quick ground-up glossary first-
Hole: A circular cut-out in a work-piece, with two defining dimensions - (i) Diameter of the hole (D)  (ii) Length of the hole (L).
Drilling: The process of making a hole in a work-piece, using a rotating tool that advances into the work-piece in axial direction.
Deep hole: When length of the hole is greater than 10 times the diameter i.e., L >= 10 x D, such a hole is considered a deep hole.

So, Deep Hole Drilling is the process of drilling holes with lengths of greater than 10 times the diameter.

The practice of making deep holes - began with the manufacturing of gun barrels but later expanded into a multitude of applications from making long holes in injection molds, automotive parts such as crank-shaft, cam-shaft, fuel injector body, fuel rail, aerospace parts such as landing gear, general engineering components such as hydraulic cylinders, a host of parts in oil & gas industry, and so many others.

Both the types of deep hole drilling i.e., BTA drilling and Gun drilling - a type of coolant to help evacuate chips during the cutting process - even at extreme depths owing to high pressure of the coolant. Result is then a specialized hole making process that can complete the jobs that other types of drills cannot.

Historical Development of Deep Hole Drilling

History of Gun Drilling
Gundrilling although unbeknownst to the contemporary engineers, began its journey in the late 19th and early 20th century. Early gun barrels were manufactured by either by roll-forging or “Damascus” spiral welding. As the propellants became more powerful with innovations - the need for stronger, more dependable steel barrels grew. Alloyed steel had become quite widely used by then due to its superior mechanical properties. And hence, drilling of alloyed steel bars became necessary to extract better performance outcomes.
For long holes, twist drill-bits were welded to long shafts. That was a task in itself - laborious, time consuming and prone to a lot of rework & rejections. On top of it, the drill would drift and wander as it progressed into the deeper hole depths.

But then, wood-working industry had quite well-established methods long hole drilling methods that employed long, straight grooved or D-section augers. Such tools, ground with eccentric / off-center cutting edges, had been found to provide pretty straight, controlled hole geometry. Drawing from this idea and implementation into metal cutting - greatly improved the hole quality. But, it was still an expensive and laborious affair.

The final step to that brought to form the modern-day gun-drill took place in 1930s with the idea of affixing the cutting-bit to a formed tubular shank through which the coolant could be passed, which would then flow through a hole in the cutting-bit - to lubricate cutting action, cool the work-piece & tool, and flush the chips out.

History of BTA Drilling
While Gun drilling proved to be a robust operation, and continues to be the preferred operation for deep hole drilling, better surface quality of the holes became necessary for some applications. The fact that the generated chips that are evacuated through the gun-drill - remain in contact with the work-piece while being forced out by the high pressure coolant, was causing micro disturbances to the surface roughness of the holes. While this was not important for most applications, the need for a different type of process that creates sufficiently nice surface finish, but not as fine as honing, became relevant.

As a result - a new deep hole drilling method was developed around 1940 by the "Boring and Trepanning Association" - which was dominated by the now liquidated company Gebrüder Heller in Bremen Germany.

As the developments continued - the grooved tubular shank used until then - was replaced by a circular tube with a closed cross-section - which was more torsionally rigid, and coolant flow was reversed i.e., coolant came from outside and took the chips into the tubular shank - effectively eliminating rubbing of the chips with the drilled hole. And the new process was named 'BTA deep hole drilling' by around 1950s.

Deep Hole Drilling Kinematics

Three kinds of kinematic approaches are employed in Deep Hole Drilling process. Each one is apt for a specific set of applications.

1) Rotating Tool
▸ Generally employed for non-symmetric work-pieces for cylindrical round with off-center holes.
▸ Cutting speed is a function of speed of tool spindle
▸ Hole center shift caused by drill-drift is higher relative to other two approaches

2) Rotating Workpiece
▸ Typically employed for cylindrical work-pieces with co-axial hole
▸ Cutting speed is a function of work-piece rotational speed; For smaller holes, dynamic balance of work-piece rotation becomes important.
▸ Hole drift is relatively lesser compared to rotating tool approach

3) Rotating Tool with Counter-rotating Workpiece
▸ Ideal approach for cylindrical work-pieces with co-axial hole
▸ Cutting speed is function of combined rotational speed of tool and workpiece
▸ Hole drift or the hole axis straightness is best of all approaches

Deep Hole Drilling Kinematics

Deep Hole Drilling 'Kinematic Approaches'

Characteristics of Gun Drilling

▸ Gun drilling is most effective & cost-efficient from ⌀1mm to ⌀50mm
▸ A gundrill differs from traditional twist drill by its cutting edge geometry which allows it to use the hole it has drilled as a guide for advancement
▸ Standard gundrills have a single cutting edge that cuts through the work-piece (produces chips) as it advances into the workpiece.
▸ Gundrilling can produce deep holes of order of 400:1 L/D, which is otherwise not possible with traditional twist-drills.
▸ Gun drilling is possible to perform on typical CNC machines - albeit to a limited extent, say of order of 10:1 to 20:1 - when sufficient coolant pressure is available.
▸ Coolant enters the cutting area from inside of the tool and exits from the area between tubular shank & work-piece hole.
▸ Deep holes with L/D of 20:1 or higher - require proper a Gun-drilling Machine to justify manufacturing in terms of productivity, process reliability & cost per piece.

Characteristics of BTA Drilling

▸ BTA drilling can produce deep holes typically from ⌀20mm or greater and can reach L/D of 400:1.
▸ BTA drilling is a far superior process compared to conventional twist drills. And relative to gundrilling - it can produce deep holes with much larger diameters and faster feed rates.
▸ BTA drilling tools are mounted on long tubular shank by threads provided on the tool-tips.
▸ Tool-tips use multiple cutting edges to remove chips effectively, while evacuating the chips by using high pressure through coolant.
▸ Coolant enters the cutting area from outside the tool and carries the chips into the tool shank, and then out through the tool spindle.
▸ Tool-tips are available in brazed or inserted carbide configs.

Gun Drilling vs BTA Drilling

▸ The gundrilling is ideal for smaller hole of up to ⌀50mm. BTA drilling become viable and effective for holes sizes from ⌀20mm to ⌀200mm. Simplistically speaking - Gundrilling is efficient for smaller diameters and BTA for larger ones.
▸ Coolant entry and chip removal are fundamentally different; Gundrilling supplies coolant through a small hole within the tool, and chips are evacuated through a groove on the OD of the tubular shank. BTA drilling supplies coolant from outside the tool, through a pressure head assembly, and the chips are removed through hole in the tubular shank of the tool itself.
▸ BTA drilling can attain typically 4-7 times faster feed-rates compared to gundrilling at the same diameter, owing to the design of tool shank which offers better torsional rigidity.
▸ Gundrilling on the other hand, is a lot more cost efficient process compared to BTA. Simpler machine design of gun-drilling also makes it easy on initial investment.
▸ A balanced trade off must be made from running cost, hole quality and productivity points of view.

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Gun Drilling Process Ranges

Diameter Ranges (in mm):
⌀1 to ⌀3: Possible with right machine and build
⌀3 to ⌀50: Quite common
⌀50 to ⌀75: Possible to do, but becomes less efficient compared to BTA drilling

L/D Ratios (L:D):
Upto 5       â–¸ Traditional twist drills suffice; on conventional machines
5 to 10       â–¸ High performance twist drills with through coolant supply are necessary
10 to 20     â–¸ Special deep-hole-drills with high pressure through coolant supply, can be done on CNC machining centers.
20 to 100   â–¸ Standard Gun-drilling Machine suffices.
100 to 200 â–¸ High performance Gun-drilling Machine becomes necessary.
200 to 400 â–¸ Specialist Machine design, made to purpose, with comprehensive engineering,

BTA Drilling Process Ranges

BTA drilling is the ideal process for a wide range of deep hole diameters & hole depths. BTA drilling is capable of drilling quite extreme L/D ratios with quite tight tolerances.

Diameter Range (in mm):
⌀8 to ⌀65.   : Brazed type perishable tool
⌀10 to ⌀114 : Spade drill type
⌀16 to ⌀28. : Indexable type single Insert
⌀ 25 mm +  : Indexable type with multiple Inserts

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