Ejector Drilling vs. Gun Drilling: Which is Right for You?

Deep hole drilling is a machining process for creating precise, deep holes with high depth-to-diameter ratios, commonly used in industries like aerospace, automotive, and oil and gas. Two key methods, ejector drilling and gun drilling, each offer unique benefits depending on factors like accuracy, surface finish, and cost. 

What is Ejector Drilling?

Ejector drilling is a deep hole drilling method designed for holes with diameters typically ranging from 20mm to 200mm (about 0.8 to 8 inches). It is a modification of the BTA (Boring and Trepanning Association) system and is known for its ability to be used on conventional machine tools like lathes and machining centers, making it a more accessible option for some shops.

How Ejector Drilling Works

The ejector drilling process uses a unique dual-tube system. High-pressure coolant is pumped down the annular space between an outer and an inner tube. A portion of this coolant is diverted through holes in the drill head to lubricate and cool the cutting edge. The remaining coolant is forced through a venturi nozzle system within the inner tube, creating a vacuum effect.

This vacuum helps to draw the chips and used coolant away from the cutting face and evacuate them through the inner tube. This chip removal method is what sets it apart from other deep hole drilling systems. Because the system creates its own suction, it doesn’t require a high-pressure seal between the workpiece and the drill bushing, simplifying the setup.

Advantages of Ejector Drilling

• Adaptability: One of the biggest benefits is its ability to be retrofitted onto standard machine tools without extensive modifications.
• No High-Pressure Seal Needed: The self-contained coolant and chip evacuation system eliminates the need for a perfect seal at the workpiece, which can simplify setup and tooling.
• Effective Chip Removal: The venturi system provides reliable chip evacuation, which is critical for maintaining a clean cutting zone and preventing chip packing.

Disadvantages of Ejector Drilling

• Lower Coolant Pressure: The system operates at lower coolant pressures compared to gun drilling or a full BTA system. This can result in less efficient cooling and chip breaking, potentially affecting surface finish and tool life.
• Less Precision: While effective, ejector drilling generally offers less precision and straightness compared to gun drilling, especially in smaller diameter holes.
• Complex Tooling: The dual-tube design makes the tooling itself more complex and potentially more expensive than single-tube systems.

What is Gun Drilling?

Gun drilling is a deep hole drilling process renowned for its exceptional precision, straightness, and ability to produce excellent surface finishes. Originally developed for drilling gun barrels, this method is ideal for creating small to medium-diameter holes, typically from 1mm to 50mm (about 0.04 to 2 inches). It is the go-to method when tight tolerances and hole quality are the top priorities.

How Gun Drilling Works

The gun drilling process uses a long, fluted drill with a single cutting edge. A key feature is its internal coolant channel. High-pressure coolant is forced down this channel directly to the cutting tip. This high-pressure fluid serves three critical functions:

1. Cooling: It cools the cutting edge and the workpiece, preventing overheating.
2. Lubrication: It lubricates the tool-workpiece interface, reducing friction.
3. Chip Evacuation: It forcefully flushes chips away from the cutting zone and back out through the flute along the outside of the drill.

A guide bushing is used to ensure the drill starts precisely and maintains its path, which is crucial for achieving high levels of straightness. The combination of the specialized tool geometry and high-pressure coolant allows for a stable and controlled cutting process.

Advantages of Gun Drilling

• High Precision and Straightness: Gun drilling is capable of producing extremely straight holes with tight diameter control, often holding tolerances that other methods cannot achieve.
• Excellent Surface Finish: The burnishing action of the guide pads on the drill head results in a very smooth internal surface finish, often eliminating the need for secondary finishing operations.
• Versatility in Small Diameters: It is the most reliable method for drilling very small, deep holes.

Disadvantages of Gun Drilling

• Slower Feed Rates: Compared to BTA or ejector drilling, gun drilling typically operates at lower penetration rates, making it a slower process.
• Requires High-Pressure Coolant: The process is dependent on a high-pressure coolant system, which can be a significant investment if a shop is not already equipped for it.
• Limited to Smaller Diameters: While versatile, it is generally less efficient for holes larger than 50mm in diameter.

Ejector Drilling vs. Gun Drilling: A Direct Comparison

Choosing between these two methods depends on a clear understanding of your project’s specific requirements. Let’s compare them across the most important factors.

Precision and Hole Quality

• Gun Drilling: This is the clear winner for precision. It excels at producing holes with exceptional straightness, tight diameter tolerances, and superior surface finishes. If your application demands the highest level of accuracy, gun drilling is the preferred choice.
• Ejector Drilling: This method offers good accuracy but generally cannot match the precision of gun drilling. It is suitable for applications where tolerances are less critical.

Hole Diameter and Applications

• Gun Drilling: Best suited for small to medium diameters (1mm to 50mm). It is widely used in manufacturing molds, fuel injectors, medical devices, and components for the oil and gas industry, such as parts of a bottom hole assembly.
• Ejector Drilling: More effective for medium to large diameters (20mm to 200mm). It is often used for applications like manufacturing hydraulic cylinders, landing gear components, and large-scale industrial machinery parts where slightly wider tolerances are acceptable.

Cost and Efficiency

• Gun Drilling: The requirement for a dedicated high-pressure coolant system can increase the initial investment. While penetration rates are slower, the high-quality finish can reduce or eliminate the need for secondary processes like reaming or honing, saving costs in the long run.
• Ejector Drilling: Can be more cost-effective upfront, as it can be adapted to existing machinery without a dedicated high-pressure coolant unit. It generally offers higher feed rates than gun drilling in its applicable diameter range, making it a more efficient choice for larger holes.

Factors to Consider When Choosing

1. Hole Diameter and Depth: Is your hole small or large? Gun drilling excels at smaller diameters, while ejector drilling is built for larger ones.
2. Tolerance Requirements: How tight are your tolerances for straightness, diameter, and surface finish? For the highest precision, choose gun drilling.
3. Material: The type of material being drilled can influence tool life and performance. Both methods can handle a wide range of materials, but tooling and parameters must be adjusted accordingly.
4. Existing Equipment: Do you have a machine with a high-pressure coolant system? If not, the cost of acquiring one might make ejector drilling a more attractive option.
5. Production Volume: For high-volume production, the potential for faster cycle times with ejector drilling (in larger diameters) or the reduced secondary processing with gun drilling might be the deciding factor.

Conclusion

Gun drilling is best for small, precise holes with a high-quality finish, while ejector drilling excels at creating larger holes quickly and cost-effectively. Select the method based on your project’s needs.