Aerospace Hardware: AM vs Machining

Aerospace fasteners and small hardware—lugs, brackets, sensor mounts, clips, line clamps, pins, bushings, threaded fittings, and custom retainers—make up a huge share of the parts list in any defense or aerospace assembly. These parts rarely make headlines, but they can dominate the bill of materials, drive assembly labor, and create disproportionate quality and supply chain risk. When a part is small and “simple,” the natural instinct is to machine it. But with metal additive manufacturing (AM) maturing rapidly, engineering and procurement teams need a clear framework for deciding when it makes sense to print small hardware instead of machining it.

This article compares additive manufacturing (AM)—primarily powder bed fusion (PBF) such as DMLS/SLM—against CNC machining (including 5-axis) for fasteners and small aerospace hardware. The focus is on practical tradeoffs: geometry, tolerance, material, cost, lead time, inspection, and qualification—all in the context of regulated environments (AS9100, NADCAP, ITAR, DFARS).

Geometry

Most small hardware parts have simple geometry that is well-suited to machining. Standard fasteners, pins, spacers, and simple brackets can often be turned or milled from bar stock in a single setup. Machining excels when the geometry is axially symmetric, prismatic, or otherwise accessible to standard tooling. For these shapes, machining is fast, predictable, and hard to beat.

AM becomes geometrically advantageous when small parts have features that are difficult or impossible to machine. Examples include: integrated internal channels (for cooling, sensing, or fluid routing), complex topology-optimized lightweight brackets, parts with lattice or porous structures (e.g., for crush/energy absorption or thermal management), and multi-feature parts that would otherwise require multiple operations, setups, or assembly.

Part consolidation is a specific geometry advantage worth calling out. If your assembly currently uses several machined pieces fastened, brazed, or welded together, AM can sometimes consolidate them into a single printed part—eliminating fastener holes, sealing challenges, and stack-up tolerance risk. For small assemblies (manifold blocks, fluid connectors, multi-port housings), this can be a genuine win—but only if the consolidated part can still be inspected and qualified.

Design-for-AM is not optional. Small parts printed without attention to support strategy, minimum wall thickness, and orientation often have issues: trapped supports in threaded features, surface quality problems, and features that can’t be post-processed or inspected. If the design is a direct copy of a machined part, AM rarely offers a geometric advantage.

Tolerances and finishes

Machining dominates when tight tolerances and smooth finishes are required. Screw threads, bearing bores, sealing surfaces, and precise mating faces are most reliably achieved by turning, milling, or grinding. Modern CNC processes hold tight tolerances with stable Cp/Cpk on production hardware, and surface finishes can be controlled to very low Ra values.

As-built AM surfaces are rougher and less controlled than machined surfaces. PBF parts typically have surface roughness significantly higher than machined parts, with partially fused particles and stair-stepping on angled/curved surfaces. For fasteners and hardware that interface with other parts—threads, bearing fits, sealing grooves, locating diameters—this is usually insufficient.

The practical solution is hybrid: print for shape, machine for precision. For AM-produced small hardware, the standard approach is:

1) Print with machining stock on all critical interfaces.
2) Stress relieve and remove supports.
3) Machine critical features (threads, bores, seals, datums) using CNC.
4) Inspect machined features using CMM, go/no-go gages, or optical metrology.

This hybrid route adds cost and setup time, so it only makes sense if AM provides a genuine advantage (geometry, consolidation, lead time, or material savings). If the entire part needs tight tolerances, machining alone is almost always the better choice.

Thread-specific note: Printed threads are generally not recommended for functional fastening in aerospace. Machine them. If the thread is in a non-structural or low-load application (e.g., sensor cover, access panel), evaluate case-by-case, but default to machined threads for anything load-bearing.

Material options and limitations

Machining offers the broadest material selection for small hardware. Virtually any wrought or forged alloy (stainless steels, nickel superalloys, titanium alloys, aluminum alloys, tool steels, copper alloys, and specialty alloys) can be machined with standard processes. Material certifications, mill test reports, and heat lot traceability are well-established.

AM material selection is narrower but covers most aerospace-relevant alloys. Common PBF alloys include 17-4 PH and 316L stainless, Inconel 625 and 718, Ti-6Al-4V, AlSi10Mg, CoCr, and maraging steels. New alloys are being qualified continuously, but if your hardware requires a specific alloy not yet available in powder form, AM may not be an option—or may require custom powder procurement and process qualification.

AM material properties are process-dependent. Unlike wrought materials with decades of handbook data, AM properties depend on machine parameters, powder quality, build orientation, and heat treatment. For small hardware in regulated programs, procurement should require:

Powder lot traceability (chemistry, size distribution, storage, reuse controls).
Documented build parameters (locked and revision-controlled).
Mechanical testing (tensile, and fatigue where applicable) from qualification builds and/or periodic production witness coupons.
Heat treatment and HIP records (if applicable; HIP is often specified for fatigue-critical AM parts).

DFARS specialty metals clause applies to many aerospace metals. For DFARS-compliant programs, confirm that the AM powder’s melt/heat source qualifies. For machined parts, this traces back to the mill cert for the bar/plate/forging stock. For AM, it traces to the atomizer and powder supplier’s melt certification.

Quantities and cost

Machining is usually the most cost-effective route for small, simple hardware in moderate-to-high volumes. Setup costs are low (especially for parts that can run on Swiss-type or multi-spindle machines), cycle times are short, and material waste is manageable for small parts.

AM becomes cost-competitive in specific scenarios:

Very low volumes (1–50 parts) where programming, fixturing, and setup for machining dominate the cost.
Parts that consolidate assemblies, eliminating downstream operations (welding, brazing, fastening, inspection of joints).
Geometries that require multiple setups, EDM, or complex fixturing to machine—driving machining cost up.
On-demand/spares scenarios where maintaining inventory or re-ordering minimum quantities from a machine shop is impractical.
Material cost savings: For expensive alloys (e.g., titanium, nickel), machining may waste significant material, especially for parts with low buy-to-fly. AM can reduce starting stock, though powder cost and post-processing partially offset this.

Important cost caveat: AM cost for small hardware must include the full post-processing and inspection chain, not just print time. This includes: stress relief, support removal, HIP (if required), heat treatment, CNC machining of critical features, surface finishing (bead blasting, shot peening, tumbling), and final inspection with documentation. Skipping any of these for production/flight hardware is a non-starter in regulated programs.

Batch efficiency matters. AM machines have a fixed cost per build (gas, setup, cool-down, depowdering). Filling the build plate with multiple parts (or multiple part numbers) amortizes that cost. A single small part on a large build plate is expensive per piece; a full plate of mixed hardware from the same alloy and parameter set can be surprisingly efficient.

Post-processing requirements

Machined small hardware typically requires minimal post-processing: deburring, passivation or anodizing, sometimes shot peening or plating, and packaging per spec. These are well-established, predictable, and fast.

AM small hardware has a longer post-processing chain. A representative chain for PBF aerospace hardware:

1) Stress relief (to reduce residual stress and distortion risk before removing from build plate).
2) Part removal and support removal (manual or wire EDM; trapped supports in small features can be labor-intensive).
3) Heat treatment (to achieve target mechanical properties; alloy-specific).
4) HIP (if specified; closes internal porosity, improves fatigue; adds cost and lead time).
5) CNC machining of critical features (threads, bores, seals, datums).
6) Surface finishing (bead blast, shot peen, tumble, etc.).
7) Cleaning (especially if internal channels are present; cleanliness verification may be required).

Each step adds handling, queue time, and documentation. For a single simple fastener, this chain can make AM uneconomical. For a consolidated part that replaces an assembly, the total post-processing chain may still be shorter than what the assembly would have required.

Fixturing is a practical challenge for post-processing small AM parts. Holding a small, organic-shaped AM part for CNC machining often requires custom fixtures. Design-for-AM should include consideration of datum targets and workholding features—even if those features are sacrificial (removed after machining). The AM supplier can guarantee clean-up after HIP and stress relief.

2) Thread and bore strategy: For small AM hardware with threaded features, the recommended approach is: print with a pilot hole or boss, then machine the thread and bore to spec. Direct printing of threads is not reliable at aerospace tolerance levels, and the surface condition of as-printed threads creates risk for galling, fatigue initiation, and dimensional non-conformance.

Inspection and qualification

Machined small hardware benefits from mature, efficient inspection workflows. CMM, go/no-go gages, surface roughness measurement, and visual inspection under controlled lighting are standard. For production hardware, sampling plans (e.g., C=0 per AQL tables) can be applied once the process is established.

AM inspection requires more deliberate planning, especially for internal features. For small parts, external features can be inspected the same way as machined parts (after CNC finishing). Internal features—channels, lattices, enclosed voids—require non-traditional methods:

CT scanning (computed tomography): gold standard for verifying internal geometry, porosity, and trapped powder/support. For small hardware, CT resolution is generally good, and scan times are manageable.
Borescope: useful for accessible channels but limited reach and resolution.
Mass/flow checks: for fluid-path parts, a calibrated mass or flow test can verify internal cleanliness and open passages without CT.

Qualification considerations for regulated programs (AS9100/NADCAP):

First article inspection (FAI): per AS9102 when required; for AM, the FAI should reference the specific machine, parameter set, build orientation, and post-processing route—since changing any of these is a process change.
Witness coupons: for mechanical property verification, especially if the hardware is fatigue- or fracture-critical. Define coupon placement and testing frequency in the quality plan.
Process control: for production AM hardware, the build file, parameter set, and powder management should be treated as controlled manufacturing documents, analogous to CNC programs and tooling offsets for machined parts.
Traceability: serial-level or lot-level traceability from powder heat/lot to final part, through all post-processing steps, linked to CoC and inspection reports.

ITAR and DFARS considerations: For controlled programs, confirm that the AM supplier can handle ITAR-controlled technical data (build files, CAD models, process parameters) under their export control program. For DFARS, verify specialty metals traceability. These requirements apply regardless of part size.

Decision checklist

Use this checklist to decide between AM and CNC machining for a specific fastener or small hardware part. The goal is not to default to either process but to match the process to the part’s actual requirements.

1) Does the geometry require AM? If the part has internal channels, lattice features, or consolidates multiple pieces, AM may be the only practical route. If the geometry is standard (round, prismatic, simple pockets), machine it.

2) What tolerances and finishes are required? If the part is dominated by tight-tolerance features (threads, bores, sealing surfaces), machining is the primary value-add. If only a few features are tight and the rest is complex geometry, print and machine the critical interfaces.

3) What material is specified? Confirm the alloy is available in powder form with acceptable property data. If the alloy is exotic or the program requires well-characterized wrought material data, machining from bar/forging may be lower risk.

4) What is the volume and demand pattern? For steady, high-volume demand, machining wins on cost and throughput. For sporadic, low-volume, or multi-variant demand, AM avoids inventory and setup penalties.

5) What is the full post-processing and inspection chain? Map the complete routing (stress relief → support removal → HIP → heat treat → machining → finishing → inspection → documentation) and compare to the machined routing. If the AM chain is longer and costlier without a corresponding geometry or consolidation benefit, machining is the better call.

6) Can the part be inspected? If internal features exist, define the inspection method (CT, flow test, borescope) and acceptance criteria before committing to AM. If the part cannot be verified to the program’s satisfaction, the design may need to change regardless of process.

7) What documentation and traceability are required? Define CoC content, material traceability, FAI expectations, and flowdowns (ITAR, DFARS, AS9100, NADCAP) upfront. Both AM and machined suppliers should be held to the same documentation standard; differences in process control maturity may influence source selection.

8) Is there a lead time driver? If the part is urgent (spares, redesign, prototype), AM may deliver faster because it avoids material procurement lead time (if powder is stocked) and eliminates setup/fixturing for complex shapes. If the schedule allows, machining from in-stock bar is often the simplest path.

Bottom line: For most standard aerospace fasteners and simple hardware, CNC machining is the default—and for good reason: cost, tolerance, finish, qualification maturity, and throughput all favor machining. AM earns its place for small hardware when it enables geometry, consolidation, or supply chain flexibility that machining can’t match, provided the post-processing and inspection chain is fully planned and the supplier can deliver with regulated-level documentation.

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