7.2 Draft Feature
Key Takeaways
- Draft angles faces relative to a direction of pull so molded or cast parts can release from a mold; CSWA may require draft on side faces of a boss or housing.
- Neutral plane draft uses a plane or face as the hinge reference: faces draft about the neutral intersection while you set angle and direction.
- Parting line draft uses a parting line (and often a pull direction) when the neutral plane method does not match the mold split—know the intro concept even if most associate items use neutral plane.
- Direction of pull must match mold open direction; reversing draft flips material add/remove on faces and changes mass and silhouette.
- Always examine draft in preview and section views; wrong faces or wrong angle produce tapered walls that fail visual and mass checks on modification steps.
7.2 Draft Feature
Quick Answer: Insert → Features → Draft tapers faces for mold release. For neutral plane draft, select a neutral plane, the faces to draft, a draft angle, and the direction of pull (which way faces taper). Preview the taper, confirm material is added or removed as the drawing intends, then OK. Wrong pull direction reverses the taper and corrupts mass.
Draft is a manufacturing-aware feature. Plastic and cast parts need a slight angle on walls parallel to the mold opening direction so the part can eject without scraping. CSWA does not grade you as a mold designer, but exam drawings and intermediate/advanced part stacks can include draft angles on bosses, ribs, or housing sides. When draft is required, treating walls as perfectly vertical leaves geometry that does not match the sheet—and mass that does not match the key.
Why draft exists (exam-level story)
| Concept | Meaning |
|---|---|
| Direction of pull | Direction the mold half opens / part ejects |
| Draft angle | Taper angle from the pull direction (often 1°–5° on real parts; use the drawing value) |
| Neutral plane | Reference plane where draft “hinges”; face width at the neutral intersection stays fixed |
| Parting line | Curve where mold halves meet; alternative draft definition |
If a face is parallel to the pull direction with zero draft, the mold wall drags on the entire face during open. A small draft creates clearance as soon as the mold moves.
CSWA implication: You apply the angle and faces shown, not a guessed manufacturing default. If the drawing says 3° draft on four side faces relative to the top, use 3°, not 1° “because plastics usually use 1°.”
Neutral plane draft (primary associate workflow)
Neutral plane draft is the method you should practice until it is automatic.
Steps
- Complete the solid so the faces to taper exist (extrude/revolve/cut as needed).
- Start Draft.
- Set type to Neutral Plane (default in many templates).
- Select the Neutral Plane (a plane or planar face—often the fixed face that should not move, such as the top of a boss or the parting face).
- Select Faces to draft (the walls that must taper).
- Enter Draft angle from the drawing.
- Set Direction of pull / reverse so the preview matches the intended taper (faces lean the correct way).
- Accept when preview shows correct undercut-free taper.
What the neutral plane does
At the intersection of each drafted face with the neutral plane, the edge generally stays put. Away from the neutral plane along the pull direction, the face rotates by the draft angle—either inward (removing material) or outward (adding material), depending on reverse/direction settings and face orientation.
| Setting | Effect |
|---|---|
| Neutral plane | Anchors the draft; choose the face/plane the drawing implies as fixed |
| Faces to draft | Only selected faces taper; unselected stay vertical |
| Angle | Magnitude of taper |
| Reverse direction | Flips whether faces open or close relative to pull |
Exam scenario: A rectangular boss on a base plate needs 2° draft on four sides with the top face of the boss remaining full size. Use the boss top (or a plane through that face) as neutral plane, select the four side faces, set 2°, and reverse if the preview shrinks the top instead of the bottom (or vice versa per the drawing).
Direction of pull: reverse until the preview matches
Pull direction is the single most common draft mistake.
| Symptom | Likely issue |
|---|---|
| Top becomes smaller when drawing shows full top | Direction reversed relative to intent |
| Draft adds material into a neighboring wall | Wrong faces or wrong reverse |
| Feature fails with undercut/draft errors | Faces cannot draft in that direction relative to neutral plane |
| Mass slightly wrong after draft | Angle OK but reverse wrong, or extra faces drafted |
Habit: Never OK a draft without rotating the view and watching the preview arrows and face motion. Section the model if needed. Draft of a few degrees on large faces changes volume measurably—enough to miss a two-decimal mass entry.
Parting line draft (intro)
Parting line draft uses a parting line (edges defining the mold split) plus a direction of pull, rather than a single neutral plane. Faces draft about the parting line definition. This appears when the split of the mold is not a simple planar neutral face—stepped parting lines, non-planar splits, or more advanced molded geometry.
| Draft type | Main references | CSWA emphasis |
|---|---|---|
| Neutral plane | Plane/face + faces + angle + direction | Primary—practice heavily |
| Parting line | Parting line edges + pull direction + angle | Intro—recognize name and when neutral plane is insufficient |
| Step draft | Related advanced option for stepped parting | Know it exists; not the first tool to reach for |
For associate-level timed work, if the drawing shows a clear planar top/bottom and side tapers, neutral plane is almost always the intended path. Do not invent a complex parting line workflow unless the geometry forces it.
Draft angle values and molded-part intent
| Drawing callout | Action |
|---|---|
DRAFT 3° or 3° DRAFT on sides | Draft feature 3° on those faces |
| Draft symbol in a view with angle | Match the angle; identify which faces |
| No draft shown | Do not add manufacturing draft “for realism” |
| Draft on bosses only, not base | Select only boss side faces |
Real molding guides often recommend 1°–2° for polished walls and more for textured surfaces. CSWA overrides all of that with the sheet. Adding unrequested draft changes mass and silhouette versus the answer key.
Examining draft on exam models
After creating draft—or when opening a partially built exam starter file—verify:
- Visual: Side faces lean; measure angle with measure tools or temporary section if unsure.
- FeatureManager: Draft feature lists correct face count and angle.
- Neighbor features: Fillets on drafted edges may need to be after draft; shell after draft offsets tapered walls.
- Modifications: “Change draft from 2° to 5°” → Edit Feature → angle → rebuild → mass.
- Mass Properties: Always re-measure after angle or reverse changes.
| Check | Pass criteria |
|---|---|
| Face set | Only faces called out are drafted |
| Angle | Exact drawing value |
| Neutral plane | Matches fixed face intent |
| Direction | Silhouette matches views |
| Rebuild | Green check; no undercut failure |
Section view trick: Cut through the boss with a section. Vertical walls show as vertical lines; drafted walls show as angled lines. Instant confirmation under time pressure.
Draft vs drafting the sketch (tapered extrude)
You can also apply draft while extruding (draft angle in Extruded Boss/Base or Cut PropertyManager). That tapers the extrude as it is born.
| Method | When |
|---|---|
| Draft in Extrude | Simple prismatic boss/cut should be tapered from the start; one feature |
| Draft feature | Faces already exist; multiple faces from different features; modification of angle independent of extrude depth |
| Loft/boundary | Complex non-constant draft—overkill for typical CSWA |
If the exam later changes only the draft angle, a separate Draft feature (or editable extrude draft) is easier than rebuilding profiles. If the first feature of the part is a drafted block and never changes angle independently, extrude draft is fine.
Interaction with shell, fillets, and cuts
| Order note | Why it matters |
|---|---|
| Draft → Shell | Shell offsets already-tapered outer faces; wall thickness follows drafted shape |
| Shell → Draft | Drafting thin walls can fail or create non-uniform thickness perception; often harder |
| Draft → Fillet | Fillets on tapered edges follow drafted corners |
| Fillet → Draft | Draft may fail on filleted face sets; prefer draft on planar faces then fillet |
| Draft → Hole Wizard | Holes on drafted faces need correct face selection; hole axis still typically normal to selected face |
Exam scenario: Housing with outer draft and 2 mm shell. Apply outer draft on the solid envelope first (or extrude with draft), then shell inward so openings and wall thickness stay controlled. Drafting after shell on thin faces is a common fail point.
Failures and fixes
| Failure | Cause | Fix |
|---|---|---|
| Cannot draft face | Face not adjacent properly to neutral plane; geometry locked | Reselect neutral plane; draft fewer faces; use parting line if appropriate |
| Undercut / invalid draft | Direction creates re-entrant geometry | Reverse direction; reduce angle; change face set |
| Adjacent face interference | Draft drives face into neighbor | Draft in stages; adjust model |
| Broken child fillet | Edges moved | Edit fillet selection after draft |
Draft checklist for CSWA
- Confirm draft is required by the drawing—do not invent it.
- Prefer neutral plane draft for planar fixed faces.
- Select exact face set; avoid tangent propagation surprises if options grab extra faces.
- Set angle from the sheet; reverse direction of pull until preview matches.
- Coordinate order with shell/fillets.
- Section-check taper; rebuild green.
- After angle mods, Mass Properties again.
Draft is a smaller mass lever than shell, but on large side faces a few degrees still shifts volume. More importantly, wrong draft fails visual design intent and can break downstream shell and fillet features—both fatal on a 15-point part question.
In neutral plane draft, what is the primary role of the neutral plane?
A CSWA drawing requires 3° draft on the four side faces of a boss so the top face keeps its full rectangular size and the base of the boss shrinks slightly. The preview shows the top shrinking instead. What should you do first?
How does parting line draft differ at an introductory level from neutral plane draft?
A drawing does not show any draft angle, but the candidate adds 2° draft on all vertical faces “because molded parts need draft.” What is the CSWA risk?