6.4 Fillet Features
Key Takeaways
- Constant-size Fillet rounds selected edges (or faces) with one radius; it is the default edge treatment on CSWA part drawings that show R callouts on solid edges.
- You can fillet multiple edges in one feature when they share the same radius; group by radius to keep modification steps simple when the exam changes R values.
- Feature order matters: fillets after shell (or before) change which faces exist and can cause fillet or shell failures—rebuild order is a common advanced-part trap.
- Fillets fail when the radius is too large for a short edge, when adjacent geometry cannot blend, or when parent edges disappear after an edit—reduce radius, select fewer edges, or change order.
- Full Round Fillet replaces a face set with a tangent round between three face groups; variable-size fillets exist but constant-size covers most associate-level items.
6.4 Fillet Features
Quick Answer: Use Insert → Features → Fillet/Round to apply a constant-size radius to one or more model edges (or faces). Match R callouts on the drawing, fillet after the solid edges exist, watch order vs Shell, and if a fillet fails on a short edge, reduce radius, split the feature, or fix parent geometry. Full Round is a special three-face blend for when the exam wants a face replaced by a full round.
Fillets are among the most common finishing features on CSWA basic and intermediate parts. Drawings show rounded corners with R (radius) values on the solid, not only sketch fillets. Feature fillets create true 3D blends along model edges, change volume slightly, and therefore change mass and sometimes center of mass. Treating them as optional cosmetics is how candidates lose 15-point items by 0.2 g.
Constant-size fillet (default CSWA tool)
Constant size fillet applies one radius to every edge (or edge set) included in that feature.
Typical workflow:
- Complete boss/base, cuts, and major holes so the edges you need actually exist.
- Start Fillet.
- Set Fillet Type to Constant size (default in most templates).
- Select edges (or use face selection / edge network options carefully).
- Enter the radius from the drawing (for example, R3 → 3 mm in MMGS).
- Preview; resolve any red failure callouts before accepting.
- Rebuild and optionally check Mass Properties if you are near a numeric answer.
| Option you will see | Exam relevance |
|---|---|
| Radius | Must match drawing R; primary modification target |
| Multiple radius items in one feature (items list) | Different radii in one fillet feature; advanced organization |
| Tangent propagation | Auto-selects tangent edge chains—great when intended, dangerous if it grabs extra edges |
| Full preview | Catch failures before commit |
| Select through faces | Pick hidden back edges on thick parts |
Tangent propagation trap: On a cast-like bracket, enabling propagation can fillet an entire loop you did not mean to round, removing more material (or adding blend volume differently) than the drawing shows. If mass is wrong after fillets, click the fillet feature, review the edge list, and turn off unwanted propagation.
Multiple edges, one radius vs many features
Strategy A — one feature, many edges (same R): Fast, clean FeatureManager, one place to edit when the exam says “change all R5 fillets to R6.”
Strategy B — separate features per edge group: Use when radii differ (R2 on small internal corners, R8 on outer frame) or when you need to suppress one group during troubleshooting.
| Situation | Prefer |
|---|---|
| All visible outer corners R3 | One constant-size fillet, multi-edge |
| Outer R10, inner pocket R2 | Two fillet features (or one feature with multiple radius items) |
| Exam modifies only “the four base corners” | Isolated feature containing only those four edges |
| Fillet fails as a large set | Split into smaller features to isolate the bad edge |
CSWA modification language often changes one radius. If that radius lives in a dedicated feature or a clearly named item, you edit once and rebuild. If you buried mixed radii with wrong edges, you waste minutes hunting.
Full round fillet (intro)
Full Round Fillet is not “constant size with a big R.” It blends three face sets: center face set plus two side face sets, replacing the center face with a rounded surface tangent to the sides. Think of rounding a rectangular bar’s end face into a full bullnose between top and bottom.
| Fillet type | What you select | Mental model |
|---|---|---|
| Constant size | Edges (or faces to edge-convert) + one R | Round the sharp edge |
| Variable size | Edge + different radii at vertices | Radius changes along the edge |
| Face fillet | Face sets to blend between | Hold faces, solve blend |
| Full round | Three face groups | Replace middle face with full round |
Associate-level drawings that simply show R on edges expect constant size. Full round appears when the geometry clearly removes an entire face in favor of a continuous round. Variable size is more CSWP-flavored; know the name so multiple-choice distractors do not trick you, but practice constant size heavily.
Fillet order vs Shell (and other features)
Feature order in the FeatureManager is design intent.
Fillet then Shell
If you fillet outer edges first, then Shell removes material and can leave wall thickness that includes the rounded outer shape. Internal corners after shell may need separate fillets.
Shell then Fillet
Shelling first creates thin walls and new inner edges. Fillets applied afterward act on the shelled body. A radius larger than remaining wall geometry or edge length will fail.
| Order | Risk |
|---|---|
| Large fillets → Shell with small thickness | Shell may fail or produce zero-thickness faces near rounds |
| Shell → large fillets on short inner edges | Fillet fail: radius too large for edge |
| Cuts that remove filleted edges → child fillet errors | Edit or reorder; dangling edge references |
| Fillets before a cut that should have sharp edges through the round | Cut may not match drawing; reorder cut before fillet |
Exam scenario: Advanced part says shell 2 mm, then round internal corners R3. If any inner edge is shorter than needed for R3, the fillet fails. Solutions: smaller radius if the drawing allows, select only viable edges, or adjust shell thickness/order only if the drawing supports it—never invent geometry; if the drawing shows R3, your model must achieve R3, so fix selection/order rather than silently using R1.
Why fillets fail (and how to fix them under time pressure)
| Failure symptom | Likely cause | Fix |
|---|---|---|
| Feature red; “unable to create fillet” | Radius larger than adjacent face/edge can support | Reduce R only if drawing wrong—usually split edges, change selection, or fix underlying short edge |
| Works on three edges, fails on fourth | Fourth edge too short or non-manifold after prior feature | Fillet the three; handle fourth separately or with smaller R if sheet shows different R |
| Was fine, broke after dimension mod | Parent edge shortened or removed | Edit parent sketch/feature; reselect edges |
| Fillet on edge that no longer exists | Cut or delete face removed the edge | Move fillet later/earlier or reselect new edges |
| Preview looks twisted | Competing adjacent fillets or tight corners | Change order; fillet large edges first sometimes helps |
Short edges: A 2 mm edge cannot accept an R5 constant fillet in any meaningful way. Zoom the failure highlight. Measure the edge. If the drawing truly shows a large R on a tiny step, re-check whether the radius belongs on a different edge (outer vs inner) or whether a sketch fillet was intended on the profile before extrude.
Sketch fillet vs feature fillet
| Sketch Fillet | Feature Fillet | |
|---|---|---|
| When | Inside an active 2D sketch | On 3D model edges after features |
| Result | Trims sketch entities; adds arc to profile | Rounds solid edge; creates fillet faces |
| Exam cue | Radius drawn on the 2D profile before boss | R callout on the finished solid views |
| Mass impact | Changes extruded profile area | Changes solid volume along edges |
| Best for | Profile shape that must include the radius in the sketch intent | Standard edge breaks after prismatic extrude/revolve |
If you sketch-fillet a rectangle and extrude, the vertical edges of the solid may already be rounded in one direction—feature fillets on the “same” corners can be redundant or fail. Match the drawing: profile radii in the sketch view → sketch fillet; corner breaks only on the solid iso view → feature fillet.
Radius changes in modification questions
CSWA loves a two-step part:
- Build model, record mass.
- “Change all R4 outside fillets to R6” or “change the fillet on Edge A to R2,” then report new mass/CoM.
Discipline:
- Find the fillet feature(s) in the FeatureManager (expand to see radius).
- Edit Feature, change radius, do not delete and recreate unless necessary.
- Rebuild; resolve any new failures from the larger radius.
- Confirm material still applied.
- Mass Properties → enter answer with required decimal places.
Larger fillets remove more material on external convex edges (generally decrease mass for external rounds) and can add or remove differently on concave internal corners—do not assume direction without measuring. Always re-run Mass Properties; never hand-estimate the mass delta.
Practical CSWA fillet checklist
- Identify every R on the drawing; list edges and radii before clicking.
- Prefer feature fillets for solid edge breaks; sketch fillets only when the profile itself shows the arc.
- Group same radii; keep different radii editable.
- Place fillets after the edges exist; coordinate with shell/cut order.
- Disable unwanted tangent propagation.
- On failure, isolate edges; do not randomly suppress unrelated features.
- After any radius modification, rebuild + Mass Properties.
Fillets are small tools with outsized scoring impact: they are fast to apply and fast to get slightly wrong. Treat radius, selection set, and feature order with the same seriousness as an extrude depth.
A CSWA drawing shows four outer corners of a rectangular plate with R5 and a deep pocket with internal corners R2. What is the best FeatureManager organization?
After a 2 mm Shell, a constant-size R4 fillet fails on several short internal edges. Which response is most appropriate on the exam?
What is the primary difference between a sketch fillet and a feature fillet on CSWA parts?
An exam modification says: change the constant-size fillet radius from 3 mm to 5 mm, then report mass. What should you do first?