Text-to-CAD Glossary: 20 Terms Engineers Should Know

This glossary covers the 20 terms that come up most often when evaluating or using a text-to-CAD tool — spanning file formats, tolerancing, and the manufacturing constraints that determine whether a generated part is actually usable. Each entry links to a deeper guide where one exists.

File formats and geometry representation

1. STEP (ISO 10303) — A neutral, vendor-independent CAD exchange format that stores exact B-Rep solid geometry, openable and editable across SolidWorks, Fusion, Inventor, CATIA, and more. See What Is a STEP File?

2. STL — A mesh format storing geometry as triangulated facets, with no exact dimensional data and no editability. Standard for 3D printing, not for machining or further CAD editing. See STEP vs. STL

3. B-Rep (boundary representation) — Geometry stored as exact mathematical surfaces, edges, and vertices rather than triangle approximations. The representation that makes a model editable and dimensionally exact. See B-Rep vs. Mesh

4. Mesh — Geometry approximated as a collection of flat triangles. Fine for visualization and 3D printing, not usable for precision machining or later editing.

5. Parametric model — A CAD model where dimensions are stored as named, editable parameters (not fixed coordinates), so changing one value regenerates dependent geometry automatically.

6. Solid geometry / manifold solid — A closed, watertight volume with no gaps, self-intersections, or open surfaces — the baseline requirement for a model to be manufacturable at all.

Tolerancing and dimensioning

7. Tolerance — The allowed range of variation around a nominal dimension, reflecting real manufacturing process capability. See Tolerances in CAD: what AI tools get wrong

8. GD&T (Geometric Dimensioning and Tolerancing) — A symbolic system (ASME Y14.5) for specifying allowed variation in a feature's form, orientation, and location, tied to function rather than just size. See GD&T basics for engineers

9. Datum — A theoretically exact reference plane, axis, or point, established from a real feature on the part, used as the basis for measuring orientation and location tolerances.

10. Fit (ISO 286) — A matched pair of hole and shaft tolerances (e.g., H7/g6) that together guarantee a specific clearance or interference range regardless of where each part lands within its individual tolerance. See Tolerances in CAD: what AI tools get wrong for how fit pairs work in practice.

11. Tolerance stack-up — The cumulative variation that results when several toleranced dimensions are chained together; the worst-case total is the sum of the individual tolerances, not any single one.

12. MMC / LMC (Maximum / Least Material Condition) — Modifiers on a GD&T callout that allow bonus tolerance as a feature departs from its most- or least-material size, applicable to features that mate with something else.

Manufacturing and design constraints

13. DFM (Design for Manufacturing) — The practice of designing parts to match the real constraints of the manufacturing process (wall thickness, draft angle, internal radii) rather than an idealized ignoring-the-process shape. See DFM 101

14. Draft angle — A slight taper applied to vertical walls in molded or cast parts, needed so the part can be removed from the tool without dragging or damaging the surface.

15. Wall thickness (minimum) — The thinnest a wall can be made while still filling reliably (injection molding) or printing/machining reliably, depending on process and material.

16. Property class (fastener grade) — A two-number code (e.g., 8.8, 10.9, 12.9) on metric bolts encoding minimum tensile strength and yield-to-tensile ratio. See ISO metric thread sizes explained

17. Module (gears) — The fundamental size parameter of a gear tooth, equal to pitch diameter divided by tooth count; two gears only mesh correctly if their modules match. See Generating fasteners, gears & bearings correctly

Text-to-CAD specific terms

18. Natural language to CAD — The general category of tools that convert a written (or visual) description of a part into a CAD model, as opposed to manual feature-by-feature modeling. See Natural language to CAD

19. Manufacturability check — An automated or manual verification step confirming a generated model doesn't just look right, but can actually be produced by the intended process within its real constraints.

20. Standard/catalog part — A component (bolt, bearing, gear) matched against a real, purchasable manufacturer part number rather than freely generated — the safer and more accurate approach for components that must interoperate with off-the-shelf hardware.

Terms that get confused with each other

A few pairs in this list are commonly (and incorrectly) treated as interchangeable:

• "Tolerance" vs. "GD&T" — tolerance is the general concept (allowed variation); GD&T is one specific, standardized system for expressing tolerance on form, orientation, and location. A part can have plain ± tolerances without any GD&T at all, and that's often the right choice — see the "when to use GD&T" section in our GD&T basics guide.
• "STEP file" vs. "CAD file" — STEP is one specific neutral format; "CAD file" could mean a STEP file, a native SolidWorks/Fusion file, or (loosely, and incorrectly) an STL mesh. When someone says "send me the CAD," they almost always mean STEP or a native file — never STL.
• "Parametric" vs. "editable" — a parametric model is editable by definition, but "editable" is a broader claim; a B-Rep model without named parameters is still editable (you can select a face and change its dimension directly), just less conveniently than a fully parametric one.
• "Manufacturable" vs. "valid geometry" — a model can be a perfectly valid, closed solid and still be unmanufacturable by a given process (wrong wall thickness for injection molding, no draft for casting). Valid geometry is necessary but not sufficient — see DFM 101 for the difference in practice.
• "Fit" vs. "tolerance" — a fit is a relationship between two toleranced dimensions (a hole and a shaft together); a tolerance on its own says nothing about how a part relates to whatever it's supposed to mate with.

How these terms connect

Most confusion in text-to-CAD evaluation comes from conflating adjacent-but-different terms: a "3D model" that's mesh, not B-Rep; a dimension with no stated tolerance, treated as if it were exact; a generated fastener that's dimensionally close but doesn't match a real standard part number. Knowing the precise term for each concept makes it much easier to ask the right question when evaluating any generation tool — "is this B-Rep or mesh," "what tolerance is this," "does this match a real catalog part" — rather than a vague "is this accurate."

Related reading: What is text-to-CAD and how does it work? · Is text-to-CAD accurate enough for manufacturing?