13.2 Interference Detection

13.2 Interference Detection

Quick Answer: On the assembly, open Evaluate → Interference Detection, choose the components (or entire assembly), run the calculate, and inspect each interference volume and highlight. Optional Treat coincidence as interference flags face-to-face contact as well as true overlaps. Fix wrong mates/alignment or bad dimensions, then re-run. Use Clearance Detection when you care about gaps, not solid collisions.

You can pass a drag-test and still have a buried part: one component occupying the same volume as another. On CSWA that often means an Anti-Aligned face mate, a Distance flipped the wrong way, or a pin forced into a hole that is too small. Interference Detection makes those errors visible with numeric volumes before you trust assembly mass or CoM.

Why interference matters for exam points

• Mass may still look “plausible” when two steels overlap (volume is double-counted only in the sense of separate bodies still having their full volumes—each part’s mass is calculated from its own body; overlapping does not automatically Boolean-unite them). So total assembly mass can look fine while the pose is physically invalid and CoM is wrong relative to the intended non-penetrating layout.
• Graders and answer keys assume the intended non-interfering (or intentionally contacting) configuration from the drawing—not a random interpenetration that still solves mates mathematically.
• Interference is a fast debug signal when CoM choices make no sense but mates are all green.

Habit: After major mate groups—and again before final Mass Properties—run Interference Detection once. It costs seconds and can save a 30-point item.

Opening the tool

Typical path (wording varies slightly by version):

1. Activate the assembly window (not a part-only edit).
2. Evaluate tab → Interference Detection (also under Tools in some menus).
3. In the PropertyManager, set Selected components:
   • Default often includes the whole assembly when nothing special is pre-selected.
   • Or multi-select only the suspect components to speed large assemblies.
4. Set options (treat coincidence, ignore hidden, etc.—see below).
5. Click Calculate / OK to run.
6. Review the results list: each interference entry, volume, and graphical highlight.
7. Right-click or zoom-to options (when available) to inspect the overlapping region.

Component selection strategy

If you exclude the interfering pair from the selection set, the tool reports no interference even though the solids still clash. Selection mistakes create false confidence.

Interpreting results: volume of interference

Each reported interference typically includes:

• Components involved (which two bodies/instances).
• Interference volume (document volume units—mm³ under MMGS).
• Visual highlight of the overlapping region.

How to read volume magnitude

Do not chase zero interference if the design is a press-fit that truly overlaps slightly in CAD—CSWA associate assemblies usually expect clear fits or proper contact without deep burial. When in doubt, match the drawing isometric: parts should not ghost through each other.

Treat coincidence as interference

The option often labeled Treat coincidence as interference (or similar) changes what counts as a hit:

When to turn it on

• You want a strict report that any touching faces show up—useful for understanding contact sets.
• You are debugging whether two faces are truly coincident vs slightly separated.

When to leave it off

• Normal CSWA assemblies with many intentional Coincident face mates—turning the option on floods the results with “interferences” that are actually desired contacts.
• Final “is anything buried?” check: leave coincidence treatment off so only real penetrations dominate the list.

Exam tip: If results show dozens of tiny “interferences” after enabling treat-coincidence, do not panic-delete all coincident mates. Turn the option off and re-calculate to see true overlaps.

Common wrong-mate causes of interference

Map symptoms to Chapter 12–13.1 mistakes:

Mate green ≠ physically valid

Mates can be fully satisfied while solids intersect. Mate math constrains selected entities; it does not always enforce non-penetration of the entire bodies. That is exactly why Interference Detection exists as a separate Evaluate tool.

Workflow: detect → diagnose → fix → re-check

1. Calculate interference on the full assembly.
2. Sort or click the largest volume first—usually the smoking gun.
3. Isolate the two components (Hide others if needed).
4. Inspect mates on those components (Mate callouts / Mates folder).
5. Flip alignment or edit Distance/Angle/Tangent/Width as indicated.
6. Rebuild; Calculate interference again.
7. Confirm isometric matches the exam sheet.
8. Only then run Mass Properties (13.3).

Worked scenario

Assembly: Base (f), Cover, four pins. After mates, mass looks close to a choice but CoM is off.

1. Interference Detection → one interference: Cover ↔ Base, large volume.
2. Highlight shows cover sunk into base top.
3. Distance mate 2 mm was flipped into the base instead of standoff above.
4. Flip direction → gap appears → interference list empty.
5. CoM jumps to a sensible location; mass unchanged (pure pose fix).
6. Submit CoM with confidence.

Clearance Detection (brief contrast)

Clearance Detection (Evaluate) reports pairs of components whose minimum distance is less than a threshold you specify—or lists clearances between selected components.

Exam-relevant uses of clearance thinking

• Drawing requires minimum 1 mm air gap between two plates—Clearance Detection verifies the gap after a Distance mate.
• After modification, ensure a moving arm still clears a stop.

Do not run Clearance Detection expecting it to list solid interferences the same way. Overlap is interference territory; open air is clearance territory.

Related visualization aids

• Section View (assembly): cut through the stack to see burial that isometric shading hides.
• Isolate / Hide-Show: focus on two parts.
• Measure: confirm a gap distance numerically after clearance concerns.

Interference and materials / multibody notes

• Interference is a geometry check, not a density check. Fix pose first; materials still matter for mass (13.3 / Chapter 10).
• Multibody parts can interfere with other components even if bodies do not interfere with each other inside the part.
• Suppressed components are typically excluded—do not suppress a clashing part to “clear” the report if the stem requires it present.

Time-box for a 30-point assembly

Practice drills

1. Deliberately Anti-Align a cover into a base; run Interference Detection; note volume; flip; re-run to empty.
2. Enable Treat coincidence as interference on a flush assembly; observe many contact hits; disable and compare.
3. Set Distance to create a 3 mm gap; use Clearance Detection / Measure to confirm; then flip Distance to interfere and contrast tools.
4. Concentric pin with diameter larger than hole (edit practice part); observe interference that mates alone do not “refuse.”

Interference Detection is the quality gate between “mates solved” and “assembly is exam-valid.” Pair it with mate flips and DOF discipline, then move to assembly mass, coordinate systems, Measure, and modification re-checks in 13.3.