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Additive manufacturing and deep hole drilling are complementary rather than competing processes. AM excels at producing complex near-net shapes with internal cavities, curved cooling channels, and organically optimised geometries. Deep hole drilling remains the most economical method for producing straight, deep, precision bores — features that AM cannot produce efficiently. The intersection of these technologies — post-processing AM parts with deep hole drilling, hybrid additive-subtractive machines, and conformal cooling channel finishing — represents a growing application area for deep hole drilling technology.
Overview
The relationship between deep hole drilling and additive manufacturing spans three distinct application areas:
Post-processing of AM metal parts — additively manufactured components that require deep, straight bores for fasteners, fluid passages, hydraulic ports, or bearing seats. The AM material behaves differently from wrought stock, requiring adjusted drilling parameters.
Hybrid manufacturing — combined additive-subtractive machines that integrate laser deposition, machining, and deep hole drilling in a single setup. These systems can produce a near-net shape additively, then machine features including deep holes without transferring the workpiece.
Conformal cooling channel finishing — additively manufactured mould inserts with conformal cooling channels use deep hole drilling for the inlet and outlet straight sections that connect the curved AM channels to the external coolant supply.
Each area has distinct technical considerations. This article covers all three with emphasis on the deep hole drilling aspects.
Material Differences: AM vs Wrought
Additively manufactured metals have microstructural characteristics that differ significantly from wrought or cast materials. These differences directly affect drilling performance.
Key Microstructural Differences
| Property | Wrought Material | LPBF Material | DED Material |
|---|---|---|---|
| Grain structure | Equiaxed, elongated in rolling direction | Fine columnar grains along build direction | Columnar grains with epitaxial growth |
| Grain size | 10–100 µm | 0.5–5 µm (very fine) | 5–50 µm |
| Anisotropy | Moderate (rolling direction) | Strong (build direction dependent) | Moderate to strong |
| Surface condition | Homogeneous | As-built roughness Ra 5–20 µm | As-built roughness Ra 3–15 µm |
| Internal porosity | Minimal | < 0.5% (optimised) | < 1% (may contain lack-of-fusion defects) |
| Hardness (as-built) | Reference | 10–30% harder than wrought | Similar to wrought |
| Hardness (heat-treated) | Reference | Comparable to wrought | Comparable to wrought |
Cutting Force Comparison
Research comparing AISI 316 produced by casting, LPBF, and WAAM found that AM-produced parts generate higher cutting forces during drilling than conventionally produced parts:
| Manufacturing Method | Relative Thrust Force | Chip Form | Burr Formation |
|---|---|---|---|
| Casting | Reference (lowest) | Continuous | Moderate |
| LPBF | 15–30% higher | Segmented | Larger burrs |
| WAAM | 10–25% higher | Mixed | Moderate |
The higher forces are attributed to the finer grain structure (Hall-Petch strengthening) and the presence of internal porosity that creates intermittent cutting conditions.
Anisotropy Effects
Build direction significantly affects drilling behaviour. Research on LPBF maraging steel M300 shows:
- Maximum cutting temperature in conventionally produced parts is approximately 8% higher than horizontally-built LPBF parts and 6% higher than vertically-built parts
- Maximum von Mises stress in horizontally-built parts is about 2% higher than conventionally produced parts and 3.5% higher than vertically-built parts
- Surface finish after drilling varies by build orientation, with vertical builds (drilling parallel to build layers) producing the best finish
Recommendation: When drilling holes in AM parts, note the build direction relative to the hole axis. Drilling parallel to the build direction (through layers) produces more uniform cutting conditions than drilling perpendicular to layers (across layer boundaries).
Drilling AM Metal Parts: Parameters and Strategies
General Parameter Adjustments
When deep hole drilling additively manufactured metals, the following adjustments relative to wrought material are recommended:
| Parameter | Adjustment vs Wrought | Reason |
|---|---|---|
| Cutting speed | Reduce by 10–20% | Higher as-built hardness increases tool wear |
| Feed rate | Reduce by 10–15% | Higher cutting forces reduce tool life |
| Coolant pressure | Increase by 20–30% | Porosity creates interrupted cut; chip evacuation more critical |
| Tool material | Same (carbide or CBN) | AM materials do not require different tool materials |
| Edge preparation | Honed edge preferred | Interrupted cut from porosity increases chipping risk |
Material-Specific Parameters
The following table provides starting parameters for deep hole drilling of common AM materials. These assume carbide tooling with AlTiN or TiAlN coating, coolant pressure 50–100 bar.
| AM Material | Condition | Hardness | v_c (m/min) | f (mm/rev) | Notes |
|---|---|---|---|---|---|
| Maraging steel M300 (1.2709) | As-built | 32–38 HRC | 25–40 | 0.02–0.08 | High cutting forces; reduce feed in as-built condition |
| Maraging steel M300 | Aged (490°C) | 52–57 HRC | 15–25 | 0.02–0.05 | Similar to aged wrought; use CBN for production volumes |
| Ti6Al4V (ELI) | As-built | 36–42 HRC | 15–30 | 0.02–0.06 | 20% lower speed than wrought; chip evacuation critical |
| Ti6Al4V | Stress relieved | 34–38 HRC | 20–35 | 0.02–0.07 | Adjust after heat treatment |
| Inconel 718 | As-built | 40–48 HRC | 8–15 | 0.02–0.05 | Most challenging; use high-pressure coolant |
| Inconel 718 | Aged | 44–52 HRC | 6–12 | 0.01–0.04 | CBN tooling recommended for production |
| AlSi10Mg | As-built | 100–120 HB | 80–120 | 0.05–0.15 | Relatively easy; similar to cast A356 |
| 316L stainless | As-built | 85–95 HRB | 30–50 | 0.03–0.10 | Higher forces than wrought 316L |
| 17-4 PH (H900) | Aged | 40–47 HRC | 15–25 | 0.02–0.06 | Similar to wrought condition |
Vibration-Assisted Drilling
Research from IFT and FOTEC demonstrates that vibration-assisted drilling (VAD) can reduce cutting forces by up to 30% when drilling LPBF martensitic steel compared to conventional drilling. The ultrasonic vibration (20–80 kHz) reduces friction at the cutting interface and improves chip fragmentation.
For AM parts with challenging microstructures, VAD offers particular benefits:
- Reduced force fluctuations from porosity interaction
- Improved chip breaking in materials prone to snarled chip formation
- Lower temperature at the cutting edge
Hybrid Manufacturing: Integrated AM and Deep Hole Drilling
Available Hybrid Systems
Several machine tool manufacturers now offer hybrid systems that combine additive deposition with subtractive machining including deep hole drilling:
| System | AM Method | Subtractive Capabilities | Deep Hole Drilling |
|---|---|---|---|
| AMBIT Series 7 (Hybrid Mfg Tech) | DED (laser powder/wire) | Milling, drilling, turning | Via tool changer |
| Mazak Integrex i-200S AM | Laser cladding | Turning, milling, drilling | Standard drilling cycles |
| Okuma MU-8000V Laser X | Laser metal deposition | 5-axis milling, drilling, boring | Deep hole boring cycles |
| Mitsui Seiki Vertex 55X-H | DED (powder feed) | Vertical machining, drilling | Planned laser drilling add-on |
These systems enable a workflow where:
- Near-net shape is built additively on a base substrate
- Critical features are machined including deep hole drilling
- Additional material can be deposited over machined surfaces
- Final machining achieves tolerances
Advantages for Deep Hole Drilling
The primary advantage of hybrid systems for deep hole drilling is the elimination of fixturing issues. When a deep hole must be drilled in an AM part with complex external geometry, fixturing in a standalone deep hole drilling machine may be difficult or impossible due to the irregular shape. On a hybrid machine, the part remains in the same setup throughout additive and subtractive operations, ensuring that deep hole features are referenced to the same datum as the AM build.
Limitations
Hybrid machines have limitations for deep hole drilling:
- Maximum L/D ratio is limited by tool changer capacity — standard machining centre tool holders limit drill length to approximately 300–400 mm
- Coolant pressure on hybrid machines is typically 20–40 bar, insufficient for high-pressure gun drilling (80–200 bar)
- Access — the additive head and subtractive spindle share the same workspace; very long drills may interfere with the machine enclosure
For deep holes exceeding the capability of hybrid machines, transfer to a dedicated deep hole drilling machine is required.
Conformal Cooling Channels and Deep Hole Drilling
Conformal cooling channels (CCCs) in additively manufactured mould inserts follow curved paths that follow the mould cavity shape — geometry that cannot be produced by drilling. However, deep hole drilling remains relevant for:
- Inlet and outlet bores — straight channels connecting the curved conformal section to the external coolant supply
- Plug holes — holes for sealing plugs at the ends of printed channel sections
- Sensor ports — straight bores for thermocouples or pressure transducers
- Back-up conventional channels — hybrid designs that combine AM conformal sections with conventionally drilled straight sections
Typical Process Flow
- Design — conformal cooling channel is designed as a 3D curved path with straight inlet/outlet extensions
- AM build — the mould insert is printed with the curved channel and a near-net shape
- Heat treatment — stress relief and aging to final hardness
- Deep hole drilling — straight inlet/outlet bores are gun-drilled or BTA-drilled to connect the AM channel to the external surface
- Finish machining — parting line surfaces, ejector pin holes, and cavity surfaces are machined
- Polishing — cavity surface is polished to the required finish
Channel Connection Considerations
When connecting a drilled straight bore to an AM curved channel:
| Issue | Recommendation |
|---|---|
| Diameter match | Drilled hole diameter should match or be 0.5–1.0 mm smaller than the AM channel |
| Alignment tolerance | ±0.5 mm at the junction point is typically acceptable for coolant flow |
| Intersection angle | Keep drilled hole within 30° of the AM channel axis at the junction |
| Burr at junction | Use coolant flow deburring or abrasive flow machining to remove burrs at the intersection |
| Pressure test | Test the completed channel assembly at 1.5× operating pressure |
Support Structure Removal and Internal Channel Post-Processing
Additively manufactured parts with internal cavities or overhanging features require support structures that must be removed during post-processing. This is a particular challenge for internal channels where mechanical access is limited.
Support Removal Methods for Internal Features
| Method | Best For | Limitations |
|---|---|---|
| Chemical dissolution (acid) | Ti6Al4V internal channels | Material loss (~0.1 mm), safety, waste disposal |
| Pressurised fluid spray | Accessible internal cavities | Limited reach in deep channels |
| Ultrasonic cavitation | Small complex parts | Equipment capacity limits |
| Abrasive flow machining | Channel polishing + support removal | Multiple passes required |
| Mechanical break-out | Large channels with thick walls | Risk of part damage |
| Thermal / burnout | Polymer supports only | Not applicable to metal AM |
Selectively Weakened Structures
A research approach from the University of Stuttgart (Maucher, Teich, Möhring) addresses the support removal problem by designing the AM build with selectively weakened structures that facilitate drilling as a post-process:
By increasing the hatch distance during LPBF of maraging steel M300 from 75 µm (standard) to 200–300 µm, porous structures are created that act as built-in "weak spots":
| Sample | Density (g/cm³) | Axial Force Fz (N) | Moment Mz (Ncm) |
|---|---|---|---|
| Fully dense AM | 8.1 | 5,750 | 2,990 |
| Weakened W2 (200 µm hatch) | 7.6 | 4,850 (−16%) | 2,650 (−11%) |
| Weakened W3 (300 µm hatch) | 7.1 | 3,980 (−31%) | 2,420 (−19%) |
This approach enables the drilling operation to remove the weakened support structure as part of the post-process, eliminating the need for a separate support removal step.
Standard Support Structures and Drilling Interference
Standard AM support structures ("line" and "rhombus" patterns) cause significant problems when drilling through them:
- Cutting force fluctuations exceeding ±50%
- Induced vibration damaging the drill edge
- Long snarled ribbon chips that clog flutes
- Poor surface finish on the drilled bore
Recommendation: When deep hole drilling through AM parts with internal support structures, either (a) use selectively weakened structures designed for drilling, (b) pre-finish bore the hole after rough drilling to remove damaged surface material, or (c) use abrasive flow machining as a secondary finishing process.
Process Planning for AM Parts with Deep Hole Features
Workflow Decision Matrix
| Part Type | Recommended Workflow | Deep Hole Drilling Step |
|---|---|---|
| AM part with straight through-bore | Design bore as through-hole in AM, drill in post-process | After heat treatment, on gun drilling machine |
| AM part with blind deep hole | Add drilling start point to AM design, drill in post-process | After stress relief, before aging |
| Hybrid mould insert | AM curved channels + printed substrate, drill connections | After AM build, before finish machining |
| Part < 300 mm on hybrid machine | Full workflow on hybrid AM/subtractive machine | In hybrid machine setup |
| Part > 300 mm requiring deep hole | AM near-net shape + transfer to DHD machine | On dedicated deep hole drilling machine |
Recommended Post-Processing Sequence
- Stress relief (while part is still on build plate) — reduces residual stress before machining
- Wire EDM — remove part from build plate
- Heat treatment — age or anneal to final material condition
- Deep hole drilling — gun drilling or BTA drilling of through-bores, cooling channels, and connection bores
- Finish machining — milled surfaces, tapped holes, and other precision features
- Surface finishing — polishing, abrasive flow machining, or post-processing as required
- Inspection — dimensional verification including deep hole straightness and diameter
Summary
| Application Area | Deep Hole Drilling Role | Key Considerations |
|---|---|---|
| AM post-processing | Produce straight precision bores in AM parts | 10–20% speed reduction, 10–15% feed reduction vs wrought |
| Hybrid manufacturing | Integrated DHD in hybrid AM/subtractive machines | Limited L/D, coolant pressure, and tool length |
| Conformal cooling | Inlet/outlet connections for AM curved channels | Alignment ±0.5 mm, diameter match, burr removal |
| Support removal | Drill through weakened support structures | Use selectively weakened hatch patterns for predictable forces |
| Tooling | Cooling channels and connection bores in AM mould inserts | Coordinate with AM channel design, pressure test after drilling |
FAQ
Is drilling additively manufactured metal more difficult than drilling wrought metal?
Generally yes. LPBF and DED metals have finer grain structures and higher as-built hardness than wrought equivalents, resulting in 10–30% higher cutting forces. Cutting speed should be reduced by 10–20% and feed rate by 10–15% compared to wrought material parameters. After heat treatment, the machinability is closer to wrought material.
What cutting parameters should I use for drilling LPBF Ti6Al4V?
For LPBF Ti6Al4V in the as-built condition (36–42 HRC), start with cutting speed 15–30 m/min and feed 0.02–0.06 mm/rev using AlTiN-coated carbide tooling with high-pressure coolant (70+ bar). This is approximately 20% lower speed than recommended for wrought Ti6Al4V. Stress-relieved material allows slightly higher parameters.
Can gun drills be manufactured by additive manufacturing?
Yes. A Chinese patent (CN112719823A) describes a method using arc 3D printing for the drill handle and stem combined with supersonic flame 3D printing for the carbide bit, with a cobalt transition layer. This approach avoids brazed joint issues and achieves bit hardness ≥ 89 HRC.
What hybrid machines support deep hole drilling?
The AMBIT Series 7 (Hybrid Manufacturing Technologies), Mazak Integrex i-200S AM, and Okuma MU-8000V Laser X all support drilling operations including deep hole boring cycles. However, the maximum drill length is limited by the tool changer (typically 300–400 mm) and coolant pressure is lower than dedicated deep hole drilling machines (20–40 bar vs 80–200 bar).
How do I connect a straight drilled hole to an AM curved cooling channel?
Design the AM channel with a straight extension section at the connection point. Drill the inlet/outlet bore to match within 0.5 mm diameter. Allow ±0.5 mm alignment tolerance at the junction. After drilling, use coolant flow deburring or abrasive flow machining to remove burrs at the internal intersection.
Can deep hole drilling remove AM support structures?
Yes, but only if the support structure is designed for the drilling process. Standard line or rhombus supports cause force fluctuations exceeding ±50% and poor surface finish. Selectively weakened structures (increased hatch distance to 200–300 µm) or cylindrical pre-built bores provide more uniform cutting conditions and are recommended when drilling through support zones.
What is the best post-processing sequence for an AM part with deep holes?
The recommended sequence is: (1) stress relief on build plate, (2) wire EDM to remove from plate, (3) heat treatment to final condition, (4) deep hole drilling (gun drilling or BTA), (5) finish machining, (6) surface finishing, (7) inspection.
Do I need different tooling for drilling AM metals?
Standard carbide tooling (AlTiN or TiAlN coated) suitable for the equivalent wrought material works for AM metals. No special tooling is required. However, a honed edge preparation is recommended to reduce chipping risk from intermittent cutting through internal porosity. CBN tooling is recommended for production volumes in aged maraging steel or aged Inconel 718.
How does build orientation affect drilling of AM parts?
Build orientation affects cutting forces, temperature, and surface finish. Drilling parallel to the build direction (through layers) produces more uniform cutting conditions. Drilling perpendicular to layers creates intermittent cutting as the drill crosses layer boundaries, generating higher force fluctuations and potentially poorer surface finish.
What is the maximum depth achievable with hybrid AM-subtractive machines?
Hybrid machines are limited by tool changer capacity — standard holders support drills up to 300–400 mm length. For deeper holes, the part must be transferred to a dedicated deep hole drilling machine. The practical maximum L/D on a hybrid machine is approximately 30:1, compared to 200:1 for dedicated gun drilling machines.