Buy-to-Fly Ratio: Additive vs Subtractive Manufacturing

Process

Material

Part Design

Lightweighting (web cutouts) 40%

Build / cut speed 1.0×

Live Readouts

Final part mass

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Raw material bought

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Buy-to-fly ratio

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Material waste

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Raw stock cost

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How it works

The buy-to-fly (BTF) ratio is the classic aerospace/manufacturing efficiency metric for a machined part:

BTF = mass of raw material purchased / mass of the finished part

Subtractive (CNC machining) starts from a solid rectangular billet that fully encloses the part, then cuts away everything that is not the part. You always buy the whole billet, so:

raw mass = billet volume × density
BTF = billet volume / part volume

As the part is "lightweighted" (more holes/cutouts removed from its web for strength-to-weight), the part volume shrinks but the billet you must buy does not — so BTF climbs. Real titanium aerospace brackets commonly run 10:1 to 20:1.

Additive (laser powder bed fusion / metal 3D printing) deposits material only where the part exists, layer by layer, from powder. You buy roughly the part's own volume plus a small support-structure/powder-loss margin (~12% here):

raw mass = part volume × 1.12 × density
BTF ≈ 1.1 – 1.3, almost independent of complexity

Process buttons — switch between carving a billet down (subtractive) and printing up from nothing (additive); the readouts and the animation both update.
Material — changes density and $/kg, which scale raw mass and cost but not the BTF ratio itself.
Lightweighting slider — enlarges the holes punched through the bracket's web, shrinking part volume. Watch subtractive BTF rise sharply while additive BTF barely moves.
Run process — replays the physical build/cut animation at the chosen speed.

This is why aerospace and medical-implant manufacturers increasingly print expensive alloys like titanium and Inconel instead of machining them from billet: less purchased metal, less energy-intensive re-melting of swarf, a smaller cradle-to-gate carbon footprint per part.

Subtractive · CNC machining

Finished part Billet stock / removed

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