13.3 Assembly Mass Properties & Key Parameter Mods

13.3 Assembly Mass Properties & Key Parameter Mods

Quick Answer: With the assembly fully mated, interference-clean, and materials applied, run Evaluate → Mass Properties at assembly level. Use a custom Coordinate System when the exam triad is not the default origin. Apply key parameter modifications (Distance/Angle values or allowed part dimensions), rebuild, re-check, and recalculate. Report mass in MMGS grams to two decimals (unless the stem says otherwise). Stay within ~1% of the keyed choice; no partial credit.

Assembly Creation is 4 × 30 = 120 points—half the commercial CSWA. Nearly every assembly item ends in a numeric mass and/or center of mass after mates and often after a modification. This section is the scoring endgame: measure correctly, modify correctly, re-measure correctly.

Assembly Mass Properties vs part Mass Properties

Always confirm the active document is the assembly when the stem says assembly mass/CoM. Open Mass Properties from the assembly window.

Prerequisites before you click Mass Properties

1. All required components inserted; no extras.
2. Base Fixed; others fully constrained to the drawing pose (drag-test).
3. No red mate errors; interference empty (or only understood contacts).
4. Material applied on every component that needs density (Chapter 10)—assembly mass is the sum of component masses.
5. Document units MMGS (typical) with mass in grams.
6. Correct configuration active if multiple exist.

Missing material on one downloaded part is a silent full-item fail: pose perfect, density wrong, mass wrong.

Reading assembly mass, volume, and CoM

Open Evaluate → Mass Properties:

MMGS and two decimal places

Prep and exam practice strongly favor:

• Length: millimeters
• Mass: grams, reported to two decimal places (for example 845.37 g)
• CoM coordinates: millimeters, typically to two decimals unless the interface specifies otherwise

Re-read the question blank: “grams,” “kg,” or “lb” changes everything. Do not paste a gram value into a kilogram blank.

The ~1% multiple-choice rule

When answers are multiple choice, distractors are spaced so that:

• Correct geometry + materials + pose lands on one option.
• Small errors (wrong mate flip, missed part, wrong density) land on another option or in the gaps.

Industry prep discussion treats remaining within about 1% of the keyed mass as the credit band for choice items. Practically:

• If you are within a fraction of a percent of one choice → select it after a quick sanity check.
• If you sit halfway between two choices → do not average—re-audit materials, missing components, and mods.
• Numeric entry may require still tighter agreement; trust recalculated Mass Properties display discipline.

No partial credit: 29/30 modeling correctness with a wrong final mass still scores zero for that 30-point assembly question.

Coordinate system for assembly CoM

Default CoM is relative to the assembly origin and default planes—the same frame where you Fixed the base. If the exam shows a triad at a corner, hole, or fixture point:

1. Insert → Reference Geometry → Coordinate System (in the assembly).
2. Pick origin (vertex, sketch point, or reference point on the grounded geometry).
3. Define X and Y directions to match the exam arrows; Z follows the right-hand rule.
4. Name it clearly (CS_Exam).
5. Mass Properties → set Output coordinate system / coordinate system selector to CS_Exam.
6. Read X, Y, Z in that frame.

Sign trap: Same as parts (Chapter 10.3)—wrong axis direction flips signs while mass stays perfect. Show Origins/Coordinate Systems and compare to the sheet before typing.

Measure tool between components

Evaluate → Measure helps verify the assembly without replacing Mass Properties:

Rules:

• Use Measure to validate setup and modification values.
• Use Mass Properties for the graded mass/CoM.
• After Measure confirms a 12.00 mm gap, still recalculate mass if the stem asks for mass—do not invent mass from gap geometry by hand under time pressure.

Key parameter modifications (the second half of many items)

Typical multi-step assembly question:

1. Mate as drawn → Mass1 / CoM1.
2. Modify a key parameter → rebuild → Mass2 / CoM2.

Mate-value modifications

Efficient path: Expand Mates → find the named/obvious Distance or Angle → edit value → green rebuild → drag-test → interference check → Mass Properties.

Do not delete the entire mate stack to change one number.

Component dimension modifications

Some stems allow or require editing a part dimension (hole diameter, plate thickness, extrude depth) while the assembly is open:

1. Right-click component → Edit Part (or open the part).
2. Change the driving dimension per the stem.
3. Return to assembly; rebuild all.
4. Confirm mates still solve (hole/shaft fits may break if diameters cross).
5. Interference Detection if fits tightened.
6. Mass Properties again.

If mass changes after a pure angle edit, you edited geometry or material by accident—or measured the wrong document.

Re-check protocol after every modification

Use this every time—do not skip under the clock:

1. Rebuild (resolve errors).
2. Drag-test critical components (no new free motion).
3. Interference Detection (flipped distance after edit is common).
4. Confirm materials still applied (editing parts can distract you into forgetting).
5. Confirm coordinate system still selected if CoM is relative to custom CS.
6. Mass Properties → Recalculate / re-open so you do not read a stale mental number.
7. Enter Mass2/CoM2—not Mass1.

Stale-value trap

Candidates measure once, modify, then type the first mass from memory. Train a physical habit: after mod, the Mass Properties dialog must be refreshed before the answer is committed.

Worked end-to-end assembly scoring sequence

Stem (compressed): Zip with Base, Arm, Pin. MMGS. Materials listed. Fix base; mate pin concentric/coincident; arm with Distance 25 mm and Angle 0° as drawn. Report assembly mass (g). Then set Distance to 40 mm and Angle to 30°; report CoM relative to the triad at Base corner A.

1. Insert/fix; apply materials; MMGS.
2. Standard mates + Distance 25 + Angle 0; drag-test; interference clear.
3. Mass Properties → mass m₁ to two decimals → answer A.
4. Edit Distance → 40; Angle → 30; rebuild.
5. Drag-test; interference; Measure gap/angle sanity.
6. Create/select CS_A at corner triad.
7. Mass Properties with CS_A → CoM X,Y,Z → answer B.
8. Optional: mass after mod should equal m₁ if only mates changed—use that as a consistency check.

Debug matrix when assembly mass/CoM fails choices

Pre-submit assembly answer checklist

1. Assembly document active; all parts present.
2. Mates clean; drag-test passed; interference clean.
3. Materials + MMGS; no mass overrides on parts.
4. Modification applied exactly once as specified.
5. Custom CS selected if required.
6. Mass Properties recalculated after final rebuild.
7. Mass to two decimals in the asked unit; CoM signs match triad.
8. Values within ~1% of a single MC option—or exact numeric entry discipline.
9. You are not submitting the pre-modification numbers.

Linking the assembly chapters

• 12.1–12.4: Insert, standard mates, numeric/tangent mates, DOF diagnostics.
• 13.1: Advanced helpers (Width, Symmetric, limits, light Path).
• 13.2: Interference (and clearance contrast) as geometric validation.
• 13.3: Mass/CoM, CS, Measure, mods, 1% MC / two-decimal MMGS scoring.

When this chain is automatic, each 30-point assembly item becomes a pipeline—not a mystery. Drafting competencies (Chapter 14) then reuse the same spatial judgment in 2D views, but assembly analysis is where half the exam is won or lost.