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 (this simplified 10×5×10 voxel bracket climbs from about 1.06:1 to 2.2:1 across the slider's range — same trend, smaller box).
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:
raw mass = part volume × (1 + support fraction) × density
support fraction = 0.12 + 0.08 × lightweighting
Note on this margin: the source 3D model this page is paired with used a fixed 12% margin at every lightweighting setting, which its own write-up then described as producing a BTF "range of 1.1–1.3, almost independent of complexity" — a fixed fraction can only ever give an exact constant (1.12:1) at every setting, never a range, so the two statements contradicted each other (confirmed numerically). Physically a fixed margin is also the less realistic choice: more web cutouts mean more overhung, unsupported surfaces, and LPBF support structures scale with overhang area — so here the support fraction rises mildly with the lightweighting slider (12%→20%), which both fixes the inconsistency and matches the real trend, while staying far flatter than subtractive's climb.
- Process buttons — switch between carving a billet down (subtractive) and printing up from nothing (additive); the isometric view, chart and readouts all update.
- Material — changes density and $/kg, which scale raw mass and cost but not the BTF ratio itself (mass cancels out of the ratio).
- Lightweighting slider — enlarges the holes punched through the bracket's web, shrinking part volume. Watch the BTF chart: the subtractive curve climbs sharply while the additive curve barely tilts.
- Run process — replays the physical build/cut animation, voxel by voxel, at the chosen speed.
- Drag on the main view — orbit the bracket (horizontal drag rotates it, vertical drag tilts the camera); scroll/pinch to zoom.
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.