Which Method is Widely Used for Aerospace Deep Hole Drilling?

The hole diameter and drilling length are essential aspects to consider in the deep hole drilling process. Deep hole drilling methods surpasses the field of drilling, and the method of choice depends on tool design and the mechanical process.

Tools with asymmetrical single-edged designs support different drilling methods, such as:

  • Single-lip deep hole drilling (SLD)
  • Single-tube system drilling (STS)
  • Double-tube system drilling (DTS)

Tools with symmetrical cutting edges use:

  • Twist drilling
  • Double-lip drilling

Twist Drilling

Due to the drilling tool’s symmetrical design, very little force is applied while twist drilling. Therefore, the method is most common and works particularly well with lower ranges of length-to-diameter ratio.

The downside to twist drilling is that at the slightest imbalance, the run-out error of the drilling tool is unavoidable. Hence a pilot hole is necessary for the machining centers and lathes when twist drilling. In addition, the pilot hole serves as a guide to provide tool precision and accuracy when drilling by aligning its diameter with that of the drilling tool.

The tool design for typical twist drilling comprises a flute, clamping sleeve, web, and helical coolant channels. The flute’s design is achieved by grinding it between the coolant supply bores. The specific design helps with the reliability of the flute in high chip evacuation, even when drilling deeper depths. On the other hand, the cylindrical clamping shank helps minimize the run-out error of the drilling tool.

Double-lip Drilling

Double-lip drilling method uses symmetrical tools like twist drilling and is commonly used with a drilling diameter range of d = 2.8 to 32 mm. During the double-lip drilling process, the cutting and passive forces oppose and compensate each other at the cutting edges. In addition, the double-lip drilling method requires no burnishing of the hole walls by the guide pads.

Depending on the workpiece material, double-lip deep hole drilling can reach higher length-to-diameter ratios than twist deep hole drilling. Double-lip deep hole drilling is used with machining short-chipping materials like aluminum cast alloys with high silicon content, grey cast iron, and free-cutting steels. However, double-lip deep hole drilling accounts for a small market share compared to twist and single-lip deep hole drilling.

The tool design of the flute is straight, which makes the geometry of the chip flutes a nominal rake angle of γ = 0°. The cooling lubricant passes inside the hollow shank through the two cylindrical channels. The chips and cooling lubricant are removed via the backlash. Concerning the tool diameter, the drilling head mounted onto the steel shank is made of solid cemented carbide.

Single-lip deep hole drilling (SLD)

Single-lip deep hole drilling is used in holes with a diameter range of d = 0.5 to 80 mm. Because of the asymmetrical tool design of the drilling tool, single-lip deep hole drilling takes place on particular machines, machining centers, or lathes.

The cooling lubricant is delivered via holes in the tool shank during the drilling process. The chips and cooling lubricant are removed through the longitudinal grooves along the flute. The hole surface decreases when the chip is in contact with the hole wall.

The drilling tool requires guidance by a pilot hole or a drilling bush at the beginning of the drilling process. The tool structure comprises the drill head, clamping sleeve, and tool shank. The drill head material is built with cemented carbide, and the tool shank is steel. In addition, both points are connected via solder joint by the straightening sleeve.

Special damping elements are inserted between the drilling spindle and the clamping sleeve to reduce the intensity of the vibrations released during the drilling process.

Single-tube system drilling (STS)

In the single-tube drilling method, two different drill heads are fitted, in contrast to single-lip, twist, and double-lip deep hole drilling. The difference is also because the cooling lubricant passes through the annular gap between the drill guide and drilling tube, not through the coolant pores.

Single-tube drilling method is also known as the Boring and Trepanning Association (BTA). A hollow drilling tool is inserted into the hole, and the cooling lubricant is released under high pressure into the outer part between the workpiece and the oil pressure head. As the cooling lubricant flows through the guide pads, it flushes out the chips inside the drilling tube.

The tool consists of a drill head, and a cylindrical drilling tube screwed onto the tube. Both have exchangeable cutting inserts and guide pads. Because of the tool’s design, the cutting force varies significantly at the begging of the deep hole drilling process and when the tool is stationary.

Double-tube system drilling (DTS)

The double tube system drilling method is applied when drilling with STS drills. The double tube system deep hole drilling method works perfectly with a diameter range between d = 18 mm to 65 mm. The unique deep hole drilling machines have an ejector drill design similar to the STS tool.

The cooling lubricant in the DTS drilling tool flows through the second tube into the annular gap and comes out via the bores around the cutting-edge area. The mixture of cooling lubricant and chips is removed through the ejector effect.

The double tube system drilling is standard in cases where the STS drilling is impossible, and high-pressure sealing is needed to drill between the oil pressure head and the workpiece.

Bottom Line

Twist drilling is the most common method because of its simplicity, easy chip evacuation, and low-cost application. However, it is unsuitable when working with unrealizable drilling depth in deep hole drilling.

Deep hole drilling is used in several fields like aerospace, borehole drilling, oil and gas, and other heavy industries. Therefore, it is essential to work with skilled personnel who understand the physics behind deep hole drilling and possess the expertise to execute the task.

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