Parametric CAD

Parametric CAD builds models from fully defined sketches (constraints + dimensions) and a feature history (extrude, cut, fillet, pattern). For 3D printing, the big win is repeatable iteration: if you name the dimensions that matter for fit (hole_d, clearance, wall, thickness), you can tune a print for different hardware or printers by changing a few numbers and letting the model rebuild predictably.

TL;DR

Create named parameters (hole_d, clearance, wall) and fully define your sketches; then build features in a stable order so you can tune fit by changing a few numbers instead of remodeling.

Parametric CADTopic-specific diagram for the concept, checks, and tradeoffs in this lesson.SketchConstraintFeatureUpdate dimension
A quick visual map of the main decisions behind parametric CAD.

What “parametric” means in practice

In parametric CAD you sketch 2D shapes, lock them down with constraints and dimensions, and then turn them into 3D with features (extrude, cut, fillet, pattern). The CAD system stores those steps as a history. When you edit a dimension like hole_d = 5.2, every feature that depends on it updates, so you can iterate without rebuilding the part from scratch.

Core building blocks (and what they control)

  • Sketch: the 2D profiles that drive geometry
  • Constraints: control shape and relationships (parallel, concentric, equal, symmetric)
  • Dimensions: control size and position (the numbers you’ll edit later)
  • Features: create or remove volume (extrude, revolve, cut)
  • Edge features: improve strength/handling (fillet, chamfer)
  • Patterns/mirrors: repeat details consistently
  • Parameters/variables: named dimensions shared across sketches/features

A history order that stays editable

A stable model usually follows this order: (1) primary shape and overall size, (2) locating datums like centerlines and symmetry, (3) functional features like holes/slots, (4) reinforcement like ribs or bosses, (5) edge treatments like fillets/chamfers. This reduces rebuild errors because downstream features are less likely to lose their references when the base shape changes.

10-minute exercise: a tunable two-hole plate

  1. Create parameters: thickness, plate_w, plate_h, hole_d, hole_spacing, edge_margin, fillet_r.
  2. Sketch a center-rectangle; constrain it to the origin (or symmetric about centerlines) and dimension plate_w and plate_h.
  3. Extrude to thickness.
  4. On the top face, sketch two hole centers: use a centerline, make the points symmetric, and dimension hole_spacing and edge_margin.
  5. Cut the holes using hole_d.
  6. Fillet the outer edges with fillet_r.
  7. Edit hole_d and thickness (and optionally hole_spacing) to verify the model rebuilds cleanly with no failed features.

Common parametric CAD failure modes

Edits cause features to fail or jump to wrong faces

Likely cause: Unstable references (selecting edges/faces that change when dimensions change)

Fix: Reference sketches/planes/origin; use symmetry and construction geometry; avoid edge-based selections when possible

Sketch keeps moving or changing shape after later edits

Likely cause: Under-defined sketch (missing constraints/dimensions)

Fix: Add constraints first (coincident, horizontal/vertical, equal, symmetric), then add dimensions only for what still moves

Changing one dimension breaks downstream geometry

Likely cause: Too much happening in one sketch/feature; tangled dependencies

Fix: Split into multiple sketches/features; keep each feature’s job simple and easy to rebuild

Holes/slots don’t print to size even though the CAD is correct

Likely cause: Printer/material needs a clearance adjustment (process variation), not a redesign

Fix: Drive hole_d/clearance from parameters and tune from a small test print; keep the model parametric so the fix is a number change