Deep hole drilling is one of machining’s most demanding operations. When hole depth exceeds 5–10 times the diameter, standard drilling methods often fail due to chip evacuation, coolant delivery, and tool deflection. At Juize Machinery, we have mastered deep hole drilling techniques that produce straight, dimensionally accurate holes even at extreme depths. As a Gold Verified Supplier on Alibaba, our machining capabilities include holes up to 40× diameter and beyond.
What Defines a “Deep Hole”?
A deep hole typically has a length-to-diameter (L/D) ratio greater than 10:1. At this ratio:
Standard twist drills cannot evacuate chips effectively
Coolant cannot reach the cutting edge
The drill tends to wander, producing non-straight holes
Common Challenges
Chip Packing: Long, stringy chips clog flutes, causing tool breakage
Coolant Starvation: Reaching the cutting edge becomes impossible with through-spindle coolant
Deflection: Long, slender tools bend, producing oversized or tapered holes
Surface Finish: Poor chip evacuation leaves torn surfaces
Straightness: Holes drift off-axis as depth increases
Deep Hole Drilling Methods We Use
- Gundrilling
The industry standard for deep, precise holes (up to 100× diameter). A single-flute tool with high-pressure coolant delivered through the tool. Coolant exits at the cutting face, forcing chips out through the V-shaped flute. Best for straightness and surface finish. Typical applications: fuel injectors, hydraulic cylinders, barrel bores. - BTA / STS Drilling (Boring and Trepanning Association)
For the largest diameters (20–300+ mm). Coolant is delivered through the annular space between the drill tube and the hole wall; chips exit through the drill tube interior. Suitable for very deep holes (often >100× diameter) in production quantities. Common in heavy equipment and large hydraulic components. - Ejector Drilling
A derivative of BTA using a dual-tube system. Easier setup for smaller diameters. Chips and coolant return through the inner tube. - Peck Drilling with Carbide Drills
For moderately deep holes (up to 15× diameter in favorable materials). The tool repeatedly retracts to break and clear chips. Slower but requires less specialized equipment.
Critical Parameters We Control
Coolant Pressure and Flow: Gundrilling may require 1000–2000 psi to clear chips from deep holes
Coolant Filtration: Microscopic particles recirculating can damage tool edges; we maintain filtration to 10–20 microns
Entry and Exit Bushings: Guide tools for straight starts and prevent breakage on exit
Tool Support: Steady rests and guide pads prevent vibration
Common Materials We Drill
| Material | Difficulty | Recommended Method |
| Low-carbon steel | Moderate | Gundrill or peck with carbide |
| Stainless steel (304, 316) | High (work-hardening) | Gundrill with rigid support |
| Aluminum | Low | Any method, but chip evacuation critical |
| Tool steels (H13, D2) | High | BTA for diameters >20mm, gundrill for smaller |
| Inconel / Titanium | Very high | Gundrill with high-pressure coolant, slow feeds |
Hole Quality Inspection
We verify deep hole quality using:
Air gaging for fast, accurate diameter measurement
Coordinate measuring machine (CMM) with extended probes
Sectioning for destructive verification of straightness and surface finish (sample parts)
Video borescope for internal surface inspection
Designing for Deep Holes
Our engineering team can advise on:
Minimum practical diameter for depth required
Specifying surface finish and straightness realistically
Avoiding interrupted cuts (cross holes intersecting deep bores)
Providing through holes instead of blind holes where possible
