Drilling is a relatively common process, however, deep hole drilling is a very challenging task.
Typically, a deep hole is defined by the the ratio of diameter to depth, initially referring to a hole with a depth-to-diameter ratio of more than five times, meaning that a hole with a ratio of 5:1 or greater between the depth of the hole and the diameter of the drilling tool is considered a deep hole.
To this day, deep hole drilling has been studied as a special technology in the field of metalworking, with proven solutions from machine tools, drilling tools, to drilling processes.
Key issues for deep hole milling
A good knowledge and understanding of the drilling process will make it easier to guide you in choosing the most effective machining technique.
Deep hole drilling requires addressing three key issues: chip breaking and evacuation, cooling, and reducing drilling cycle time.
Other important factors include machining accuracy, repetitively and surface finish.
chip breaking and exhaust
Chips must be small enough to be evacuated smoothly. Long, coarse chips will damage the surface finish and also cause premature tool wear.
cooling and heat dissipation
The coolant must reach the tip of the drilling tool to keep both the tool and the workpiece sufficiently cooled and to force the chips out of the hole.
reducing drilling cycle time
The scheme of drilling cycle should be optimized so that the total number of drilling cycles and the corresponding canned cycles time are as minimal as possible.
To achieve the desired accuracy, repetitively and surface finish, a rigid machine with good damping characteristics and low spindle runout is also required.
Of course, the correct drill geometry will make deep hole drilling more efficient.
Common types for Deep Hole Drilling
There are a few methods for deep hole drilling, which are mainly classified as peck drilling, gun drilling, ejector drilling and BAT drilling.
In the following, we will describe each of these drilling methods.
Peck Drilling
The peck drilling is an intermittent feed cycle drilling method.
The advantage of peck drilling is improved chip evacuation, but the disadvantage is that it takes longer to complete each hole.
Two common options for peck drilling are listed below:
- G73 (chip breaking cycle):
- G83 (deep hole cycle):
Gun Drilling
Gun drilling tools are generally long and thin.
Compared to conventional drilling, it uses high-pressure coolant for chips removal and is typically used to produce holes with diameters of 1-50 mm [~0.04″-2.00″].
The different types of gun drilling are listed below.
- Single Flute Solid Carbide
- Two Flute
- Indexable
Ejector Drilling
The ejector drilling is a variant of the BTA deep hole drilling which is a dual tube drilling system and use internal and external dual coolant and internal chips removal.
The system press coolant between the inner and outer of the drill tube while chips are exhausted with the high-pressure coolant from the chip box.
In contrast, ejector drilling does not perform as well as BTA deep hole drilling in terms of drilling efficiency and chip removal.
BTA Drilling
As a more efficient and precise deep hole drilling method, BTA drilling is a single tube drilling system in which high-pressure coolant is introduced to the outside of the drill tube, and chips are exhausted out of the inside of the drill tube with the coolant.
The BTA drilling is typically used for hole drilling processes from 20 to 630 mm [0.80″ – 24.0″] in diameter and performs very well even when drilling holes with extreme depth-to-diameter ratios.
This drilling process is usually intended for precision tooling manufacturing, aerospace and medical, etc.
Conclusion
When deep hole drilling is required, if equipped with a through spindle coolant (TSC) system, the choice of drilling strategy can be made according to the coolant supply and chip transport.
Such as: gun drilling systems, ejector drilling systems (double tube systems) and BTA drilling systems (single tube systems).
These systems allow a more friendly drilling of holes deeper than 100 times the diameter.
Otherwise, peck drilling is a very useful method for deep hole drilling when not equipped with a TSC.