GD&T for Additive Manufacturing: Practical Guidelines

Geometric dimensioning and tolerancing (GD&T) is the universal language for defining part requirements on engineering drawings. But GD&T conventions were developed for subtractive and formative manufacturing processes—machining, casting, forging, sheet metal—where the designer can assume certain manufacturing behaviors. Additive manufacturing breaks many of those assumptions, creating gaps between what the drawing specifies and what the AM process can practically achieve.

This guide provides practical guidelines for applying GD&T to additively manufactured metal parts in aerospace and defense applications. The audience is design engineers defining tolerances, quality engineers interpreting them, and procurement professionals evaluating whether a supplier's AM capability can meet the drawing requirements.

Why GD&T for AM Requires Special Attention

Conventional GD&T assumes that the manufacturing process produces uniform material properties, predictable surface textures, and repeatable feature geometry. AM introduces variability that conventional processes do not:

Build-direction dependence. AM parts have different dimensional accuracy, surface roughness, and mechanical properties depending on feature orientation relative to the build direction. A hole printed vertically (parallel to build direction) has different roundness and surface finish than the same hole printed horizontally. This anisotropy must be accounted for in tolerancing.

As-built vs. machined surfaces. Many AM parts have a mix of as-built surfaces (rough, with layer staircase effects) and machined surfaces (smooth, dimensionally precise). The drawing must clearly distinguish which surfaces are controlled in the as-built condition and which will be machined to final tolerance.

Distortion and residual stress. Thermal cycling during the build creates residual stresses that cause distortion during and after stress relief. Parts may warp, twist, or spring when removed from the build plate. Datum features must be robust enough to survive this distortion, and tolerance stack-ups must account for it.

Internal features. AM can produce internal channels, cavities, and lattice structures that cannot be accessed by conventional measurement tools. The drawing must specify how these features are dimensioned, what measurement method is acceptable (CT scanning, flow testing, borescope), and what the acceptance criteria are.

Datum Strategy for AM Parts

A well-chosen datum scheme is the foundation of a manufacturable, inspectable GD&T definition. For AM parts, the datum strategy must bridge the gap between the as-built geometry and the final machined part.

Establish primary datums on machined surfaces. Whenever possible, primary datum features (A, B, C) should be on surfaces that will be machined after the AM build, stress relief, HIP, and heat treatment. This ensures the datum surfaces are dimensionally precise and not affected by as-built roughness or thermal distortion.

Include sacrificial datum targets or tooling features in the AM build. For parts that must be inspected or fixtured in the as-built condition (before machining), design datum targets or tooling tabs into the AM geometry that provide repeatable locating surfaces. These features are removed during machining but serve as interim datums for CMM inspection, CT scanning alignment, and machining fixture setup.

Consider the inspection sequence. If the part undergoes multiple post-processing steps (HIP, heat treat, rough machining, finish machining), the drawing should define which datum features are established at each stage. A datum progression from as-built tooling features to rough-machined datums to finish-machined datums prevents accumulation of positional error through the processing chain.

Avoid datums on as-built overhanging surfaces. Overhanging (downskin) surfaces have the worst dimensional accuracy and roughness in AM. If a datum must be placed on a non-machined surface, choose an upskin or vertical surface where dimensional control is best.

Practical Tolerance Guidelines by Feature Type

The following guidelines reflect achievable tolerances for LPBF (laser powder bed fusion) metal parts in production conditions with typical aerospace alloys (Ti-6Al-4V, Inconel 718, 316L stainless). EBM tolerances are generally wider.

As-built external dimensions (not machined): ±0.1–0.3 mm (±0.004–0.012”) for features under 100 mm, with wider tolerances for larger features. Profile tolerances of 0.2–0.5 mm are typical for as-built surfaces.

As-built hole diameters: ±0.05–0.15 mm for holes printed vertically (parallel to build direction). Holes printed horizontally typically require 0.15–0.3 mm tolerance due to overhang effects at the crown. Holes requiring tighter tolerances should be designed undersized and reamed or machined to final size.

Machined features: Same tolerances as conventional CNC machining—typically ±0.025 mm (±0.001”) or better on critical features. The AM near-net shape must provide sufficient machining stock (minimum 0.5–1.0 mm per side recommended) to allow the machinist to clean up the as-built surface and establish the datum.

Position tolerance (machined holes/features): Standard aerospace positional tolerances apply to machined features. For as-built features, positional tolerances should be widened to account for build-direction-dependent accuracy and thermal distortion.

Surface finish (Ra): As-built vertical walls: Ra 5–12 μm. As-built upskin: Ra 3–8 μm. As-built downskin (overhangs): Ra 15–30 μm. Machined surfaces: Ra 0.8–3.2 μm (same as conventional). Specify finish requirements only on surfaces where they are functionally required—do not over-specify as-built surfaces unless they are sealing or mating surfaces.

Flatness and straightness: As-built flatness on large surfaces (50–200 mm span) is typically 0.1–0.5 mm, highly dependent on geometry, build orientation, and thermal management. For critical flatness requirements (<0.05 mm), plan to machine the surface.

Drawing Practices for AM Parts

Several drawing practices improve clarity and reduce misinterpretation for AM parts:

Clearly identify machined vs. as-built surfaces. Use the machining symbol (material removal required) on surfaces that will be machined, and explicitly note surfaces that are "as-built" or "as-additively-manufactured." This prevents suppliers from quoting unnecessary machining or, worse, shipping parts with unmachined critical surfaces.

Specify machining stock requirements. Note the minimum machining stock on surfaces that will be machined from the as-built condition. This is critical for additive manufacturing because the supplier must account for this stock in the build geometry, and insufficient stock leads to parts that cannot be cleaned up during machining.

Define internal feature inspection methods. For internal channels, lattice structures, and enclosed cavities, specify the acceptable inspection method (CT scanning with defined voxel size, flow testing with defined acceptance criteria, pressure/leak testing with defined parameters) directly on the drawing or in a referenced inspection plan.

Include build orientation requirements if critical. If part performance depends on build orientation (e.g., fatigue life is orientation-dependent for the qualified AM process), specify the required build orientation or reference the qualification plan that defines it. This prevents suppliers from re-orienting the part for manufacturing convenience and invalidating the qualification basis.

Reference AM-specific material and process specifications. Callout AM material specifications (e.g., AMS 7000 for Ti-6Al-4V LPBF, customer-specific AM specs) and process specifications (HIP, heat treatment) on the drawing or in the technical data package. Do not rely on generic material specifications written for wrought or cast product forms.

How Metal Powder Supply Supports Your AM Design and Qualification

Achieving tight GD&T requirements on AM parts starts with consistent feedstock quality. Metal Powder Supply provides titanium, tungsten, molybdenum, tantalum, and niobium powders with controlled chemistry, particle size distribution, and morphology—the powder characteristics that directly influence dimensional accuracy, surface roughness, and material properties in the finished part.

As a DFARS-compliant, AS9100D-certified, ITAR-registered supplier, we provide the certified feedstock and documentation your AM qualification program requires.

Request a quote or contact our technical team to discuss your AM powder and qualification needs.

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