Deep Hole Drilling vs EDM vs Laser vs AM: Selection Guide

A cooling hole in an Inconel turbine blade, an oil passage in a 300 mm hydraulic cylinder, and a conformal cooling channel in an injection mould tool all require holes. But the optimal process for each is completely different — and choosing wrong can multiply cost by 10× or reduce part life by 100×.

Engineers specifying deep or precision holes have more options than ever: conventional gun drilling and BTA drilling, EDM drilling, laser drilling, and additive manufacturing. Each process has strengths and limitations that make it optimal for specific combinations of hole geometry, material, production volume, and quality requirements.

This article provides a structured comparison of these five processes, with quantitative data on aspect ratio capability, tolerance, surface finish, material constraints, cost, and cycle time.

Process Overview ​

Deep Hole Drilling (Gun Drilling and BTA) ​

Deep hole drilling is a mechanical cutting process using specialised tools with internal coolant delivery. Gun drilling uses a single-lip tool with coolant through the tool and chips exiting externally. BTA drilling uses a multi-lip tool with coolant delivered through the annulus and chips exiting through the centre of the drill.

Best for: High-volume production of deep, straight holes in machinable metals. Hydraulic cylinders, fuel injectors, landing gear, medical implants.

Limitations: Round holes only. Requires through-coolant. Tool wear in hard materials. Chip evacuation challenges at extreme depths.

EDM Drilling ​

EDM drilling uses electrical discharges between a rotating tubular electrode and the workpiece, with dielectric fluid flushed through the electrode to remove debris.

Best for: Hardened steels (60+ HRC), carbides, superalloys (Inconel, Hastelloy, titanium), any conductive material. Turbine blade cooling holes, fuel injector nozzles, dies and moulds.

Limitations: Conductive materials only. Recast layer present (may need post-processing). Slow material removal rate. Electrode wear requires compensation. Aspect ratio limited by debris evacuation.

Laser Drilling ​

Laser drilling uses focused laser pulses to ablate material through vaporisation and melt ejection.

Best for: High-speed micro-hole drilling in thin materials. Any material (metals, ceramics, polymers, glass). Non-contact processing. Hundreds of holes per second.

Limitations: Aspect ratio limited in metals (typically <10:1 for industrial reliability). Hole taper common. Recast layer and spatter. High capital cost. Deep holes in metals >2 mm are very challenging.

Additive Manufacturing (Built-In Holes) ​

AM creates holes as integral features during the layer-by-layer build process rather than as a post-processing operation.

Best for: Complex internal cooling channels (conformal cooling), lattice structures, holes in geometries that cannot be post-machined, low-volume custom parts.

Limitations: As-built holes lack precision and surface finish. Support removal from internal holes is difficult. Minimum hole size limited by powder removal. Requires secondary machining for functional tolerances.

Tip: Additive manufacturing and deep hole drilling are complementary, not competing. The optimal strategy for many parts is to AM the near-net shape with rough channels, then gun drill or ream the critical bore sections to final tolerance. This combines the geometric freedom of AM with the precision and surface finish of machining.

Comparative Matrix ​

Decision Framework ​

Step 1: Aspect Ratio ​

The aspect ratio (depth ÷ diameter) is the most important single factor in process selection.

Deep hole drilling dominates for aspect ratios above 30:1. EDM can reach 120:1 with specialised insulated electrodes, but with significantly longer cycle times.

Step 2: Material ​

Step 3: Tolerance and Surface Finish ​

Step 4: Geometric Constraints ​

Step 5: Production Volume ​

Step 6: Thermal Damage Sensitivity ​

Warning: Recast layer from EDM drilling and heat-affected zone from nanosecond laser drilling can reduce fatigue life by 50–90% in superalloys. If the hole is in a fatigue-critical location, specify ECM, gun drilling with coolant, or femtosecond laser, and verify with metallurgical sectioning.

Process-Specific Guidance ​

When Deep Hole Drilling Is the Best Choice ​

Deep hole drilling (gun drilling or BTA) should be the default process when:

• Aspect ratio exceeds 30:1
• Material is machinable (steels, aluminium, titanium, most alloys)
• Hole diameter exceeds 1 mm
• Production volume exceeds 100 parts
• Surface finish Ra < 1.6 µm is required
• Straightness is critical (gun drilling achieves 0.1–0.5 mm/m)

When EDM Drilling Is the Best Choice ​

EDM drilling should be chosen when:

• Material hardness exceeds 50 HRC or material is a superalloy
• Hole diameter is below 0.5 mm with aspect ratio above 10:1
• Non-round hole shapes are required
• Tight tolerances (±0.005 mm) are needed
• The part cannot withstand cutting forces (thin walls)
• The material is already heat-treated and gun drilling would require annealing

When Laser Drilling Is the Best Choice ​

Laser drilling should be chosen when:

• Material is non-conductive (ceramics, glass, polymers)
• Hole diameter is below 0.1 mm
• Drilling speed per hole must be very high (500+/minute)
• The material is thin (<2 mm)
• Non-contact processing is required (fragile parts)
• Angled holes on curved surfaces (turbine blades)

When Additive Manufacturing Is the Best Choice ​

AM should be chosen for hole features when:

• Cooling channels must follow a curved or conformal path
• Multiple channels branch or intersect
• The hole is in a location that cannot be accessed by a drill or EDM electrode
• Combined part consolidation (replacing multi-part assemblies)
• The hole can be post-machined to final tolerance

Hybrid Process Strategies ​

The most effective approach often combines multiple processes:

FAQ ​

What is the maximum aspect ratio achievable with each hole-making process? ​

Gun drilling achieves the highest aspect ratio at up to 300:1. BTA drilling reaches 100:1. EDM drilling with standard electrodes reaches 30:1, and up to 120:1 with specialised insulated electrodes. Laser drilling in metals is limited to approximately 10–20:1 (up to 50:1 with femtosecond lasers in specific materials). Additive manufacturing has no inherent aspect ratio limit but struggles with powder removal from deep, small-diameter channels below 3 mm diameter.

Which process produces the best surface finish in a deep hole? ​

ECM (electrochemical machining) produces the best surface finish at Ra 0.2–0.8 µm with no thermal damage. Gun drilling achieves Ra 0.4–1.6 µm with no recast layer. EDM finishing can reach Ra 0.2 µm but leaves a recast layer that may require removal for fatigue-critical applications.

Is EDM drilling faster than gun drilling for small holes? ​

No, gun drilling is significantly faster for holes above 0.5 mm diameter in machinable materials. EDM drilling is slower due to the erosion process. However, for very small holes (<0.5 mm) in hard materials (>50 HRC), EDM may be faster when considering the total process time including any pre-annealing and post-heat-treatment steps that gun drilling would require.

Can additive manufacturing replace deep hole drilling for cooling channels? ​

Partially. AM excels at conformal cooling channels with complex 3D paths that cannot be drilled. However, as-built AM channels have rough surfaces (Ra 6–15+ µm) and lower dimensional accuracy compared to gun-drilled holes. For hydraulic or sealing applications, AM channels typically require post-machining. The optimal approach often combines AM for channel routing with gun drilling for critical bore sections.

What is the cost comparison between gun drilling and EDM for a typical deep hole? ​

Gun drilling typically costs $0.10–$1.00 per hole at production volumes above 1,000 parts, while EDM drilling costs $1–$10 per hole for comparable geometries. The ratio widens at higher aspect ratios, where EDM cycle times increase non-linearly. However, EDM becomes cost-competitive when drilling pre-hardened materials (eliminating annealing and re-hardening costs) or when the material is too hard for economical gun drilling.

Which process should I choose for turbine blade cooling holes? ​

Laser drilling (nanosecond or femtosecond) is the standard for turbine blade cooling holes in production due to its speed, non-contact processing, and ability to drill at shallow angles on curved surfaces. Femtosecond lasers produce minimal recast layer for fatigue-critical blades. For the highest quality requirements, a hybrid laser + EDM or laser + ECM approach removes the recast layer left by laser drilling.

What is the smallest hole each process can produce? ​

Laser drilling (femtosecond) can produce holes as small as 1–5 µm. Micro-EDM reaches 5–10 µm. ECM reaches approximately 100 µm. Gun drilling is limited to approximately 0.5 mm. Additive manufacturing is limited to approximately 0.3 mm (limited by powder particle size and removal capability).

When are secondary operations required? ​

EDM drilling leaves a recast layer (typically 2–20 µm thick) that requires removal for fatigue-critical applications. Laser drilling (nanosecond) leaves a heat-affected zone and spatter that may require post-processing. Additive manufacturing produces as-built holes that typically require reaming or drilling for functional tolerances. Gun drilling and ECM rarely require secondary operations — one of their key advantages.

Summary ​