Declared external spur gear requirements

Draftwright's first gear surface supports one deliberately narrow class: a single metric, external, spur, cylindrical involute gear. It does not infer that class from a toothed-looking boundary. The author declares the manufacturing requirements; independent B-rep evidence may only confirm correspondence facts such as axis and repeat count.

Standards basis

Standard Role in the declaration
ISO 21771-1:2024 Cylindrical involute gear concepts and geometry
ISO 53:1998 Standard basic rack tooth profile
ISO 54:1996 Metric normal-module vocabulary
ISO 21771-2:2025 Tooth-thickness calculation and measurement vocabulary
ISO 1328-1:2013 Individual tooth-flank tolerance classification
ISO 2203:1973 Reviewed representation standard; conventional simplification is not yet claimed

ISO 21771-2 explicitly does not select the desired tooth thickness or its tolerance. Draftwright therefore requires both as authored values. It does not derive them from module, pressure angle, profile shift, or a mating-gear assumption.

Minimum complete set

Sheet.external_spur_gear(...) requires every item below; there are no optional normative fields:

  • target centre and axis;
  • integer tooth count z from 5 to 1,000;
  • metric module m from 0.5 mm to 70 mm;
  • pressure angle;
  • profile-shift coefficient x, including an explicit zero;
  • face width b from 4 mm to 1,200 mm;
  • tooth thickness at the reference cylinder and its signed lower/upper deviations;
  • ISO 1328-1 flank tolerance class 1–11.

The class and five requirement-standard editions are fixed by the typed feature, rather than repeated as free-form strings. Missing values, non-finite/non-positive sizes, invalid angles, inverted tooth- thickness limits, and out-of-range tooth/flank classes raise at the IR boundary. For this spur class, m × z is the reference diameter; it must also remain within ISO 1328-1's published 5 mm to 15,000 mm applicability range.

sheet.external_spur_gear(
    at=(0, 0, 5),
    axis="z",
    tooth_count=13,
    module=1.25,
    pressure_angle=20,
    profile_shift=0,
    face_width=10,
    tooth_thickness=1.9634954084936207,
    tooth_thickness_tolerance=(-0.03, 0.01),
    flank_tolerance_class=7,
)

The resulting gear-data table is late furniture placed through Drawing.add_table()'s shared free-space solve. The declaration exposes no raw page coordinate. Generated Sheet scripts use a lossless numeric spelling for these authored values and reproduce the same frozen IR record. Draftwright continues to project the source B-rep exactly; it does not yet replace that geometry with ISO 2203's conventional simplified representation, and the table makes no such claim.

Fail-closed correspondence

The data table proves what the author required; it does not prove which boundary is the gear. Completeness lint therefore distinguishes:

  • repeating-profile evidence with no declaration (gear_semantics_missing);
  • no unique centre/span correspondence (gear_correspondence_unverifiable);
  • a declared/evidenced axis disagreement (gear_axis_mismatch);
  • a tooth/repeat-count disagreement (gear_repeat_count_mismatch);
  • missing, ambiguous, or stale table output (gear_requirement_*).

The recognition run proves repeating-profile evidence from every edge of two opposed closed outer wires. It requires a bijective sector mapping and carries a traversal/phase-neutral curve signature; common-circle arcs alone cannot certify the profile. Lint then corresponds a declared target by centre and axial span before checking axis and count. Missing, ambiguous, incomplete, or unequal evidence stays explicitly unverifiable. A proved cyclic profile still retains unrecognised_defining_geometry: geometry evidence does not manufacture gear semantics or a complete manufacturing definition.

Out of scope

Internal gears, helical gears, racks, gear pairs, mesh/backlash design, materials, heat treatment, load capacity, process selection, and automatic semantic inference are unsupported. Supporting one of those requires its own complete typed class and standards evidence; it must not be represented by adding optional fields to this record.