2.2 Document Units & MMGS Discipline

You can model a geometrically perfect part and still score zero on a CSWA mass question if the unit system or density is wrong. This section builds a hard habit: set and verify units first, apply materials with matching density units, and report mass to the precision the exam expects—typically two decimal places in MMGS.

Why Units Dominate CSWA Scoring

CSWA part and assembly questions frequently ask for:

• Mass (grams in MMGS)
• Sometimes volume or center of mass coordinates
• Dimensional modifications that cascade into new mass values

There is usually no partial credit. If the key expects 245.67 grams and you enter 0.54 (pounds mass from an IPS setup) or 245.7 with wrong rounding after a unit mismatch, the item is wrong.

Document Properties: Where Units Live

Set units in Options → Document Properties → Units (also reachable via the status-bar unit display in many SOLIDWORKS versions). Document Properties are per file, not global application defaults alone. Starting from the wrong template is a silent failure mode.

Before the first sketch on every CSWA part:

1. Open Document Properties → Units.
2. Select unit system MMGS (millimeter, gram, second) unless the problem explicitly states otherwise.
3. Check length unit = millimeters, mass unit = grams.
4. Note decimal display settings for length and mass.
5. Apply a material only after units are correct so density interprets properly.

If you inherit a sample file from the exam package, still verify units—do not assume the download matches your practice template.

MMGS vs IPS vs Other Systems

MMGS means:

• Sketch dimensions like 100 mean 100 mm, not 100 inches
• Mass Properties Mass reads in grams
• Density for materials is interpreted in a mass/length³ system consistent with g and mm (SOLIDWORKS stores density carefully—always confirm the material’s density display after unit changes)

The IPS Trap (Memorize This)

Students who practice in inches often leave IPS on. A 4-inch cube is easy to visualize; in MMGS that same physical size is about 101.6 mm on a side. Two failure modes appear on exam day:

1. You model in IPS thinking in inches, but the answer key is grams for an MMGS model of different intended millimeter sizes. Your mass will not match.
2. You switch the unit display to MMGS after modeling in inches without understanding whether dimensions convert or stay numeric. Depending on how you change units, numbers may convert correctly or you may re-enter mixed values. Always re-measure critical dimensions after any unit change.

Conversion anchors for emergency checks:

If your mass is roughly 450× too large or too small, suspect lb vs g. If linear sizes are 25× off, suspect in vs mm. If mass is 1000× off, suspect kg vs g or m vs mm volume scaling (volume scales with the cube of length—unit length errors devastate mass).

Length Error Cubes Into Mass Error

Mass = density × volume. Volume scales with length³. If every linear dimension is accidentally 25.4× too large (inch numbers treated as mm, or the reverse confusion):

• Volume factor ≈ 25.4³ ≈ 16,387
• Mass is wrong by the same huge factor (density held constant)

You will not “be close.” You will be orders of magnitude off. That is why unit discipline is more important than fillet aesthetics.

Decimal Precision on the Exam

Many CSWA numeric entries expect two decimal places for mass (for example, 128.45). Practices that help:

• Set mass decimal display in Document Properties / Mass Properties options so you can read two places cleanly
• Do not round intermediate sketch dimensions aggressively if the problem gives three-place inputs—use the given precision, then round the final Mass Properties output as instructed
• Watch for trailing precision traps: 10 vs 10.00 rarely matters for entry, but 128.4 vs 128.45 does if the key is two places and your display rounds early

If the problem statement says “enter mass to two decimal places,” treat that as a hard requirement.

Materials, Density, and Mass

Applying a material from the SOLIDWORKS Material dialog assigns density (and appearance). Mass Properties uses that density unless you override.

Critical interactions:

1. No material / density = 0 or default water-like values — mass will not match exam keys that specify Alloy Steel, ABS, etc.
2. Wrong material — same volume, wrong density → wrong mass; CoM location may still look plausible if the part is homogeneous.
3. Custom density — some problems specify density explicitly; enter it carefully in Document Properties or material override with correct unit labels.
4. Unit system change after material apply — re-open Mass Properties and confirm density units still make sense; re-apply material if density looks absurd (for example, steel showing as a tiny decimal that implies unit mismatch).

Exam sequence (safe order):

1. Set MMGS (or stated system).
2. Create geometry from given dimensions in those units.
3. Apply specified material (or density).
4. Mass Properties → read mass / CoM.
5. Perform required modifications.
6. Mass Properties again → enter answer.

Never type a mass from memory of a previous attempt; always re-evaluate after the last rebuild.

Status Bar and Dual-Dimension Awareness

The status bar often shows the current unit system. Clicking it is a fast way to open unit settings. Dual dimensions (showing inches and mm together) can help practice, but on the exam they can also confuse which number you typed into a PropertyManager field. Prefer a single clear unit system matching the problem.

PropertyManager Numeric Fields and Unit Labels

When extruding, the depth box shows a number and an implied unit from the document. If the document is MMGS and the problem says “extrude 2 inches,” you must convert: 2 × 25.4 = 50.8 mm, then type 50.8—not 2.

Conversely, if everything is given in millimeters, type millimeters. Do not “helpfully” convert to inches because your home template was IPS.

Assembly Mass Properties and Units

In assemblies, Mass Properties combines component masses. Each part should use consistent materials and the assembly document unit system should match how you report the answer. Mixed-unit parts (one part modeled as if inches, another as mm) produce garbage totals. For CSWA assembly questions:

• Verify units on the assembly document
• Open critical parts and confirm their Document Properties if mass is wildly unexpected
• After mate or dimension modifications driven by assembly-level changes, re-run Mass Properties at assembly level as required

Common CSWA Unit Failure Scenarios

Pre-Flight Unit Checklist (Use Every Question)

1. Document unit system = MMGS (unless stated otherwise).
2. Problem dimensions interpreted in mm (or convert explicitly).
3. Material / density applied and density magnitude looks realistic for the material class.
4. Mass Properties mass unit = grams.
5. Answer entered to required decimals (commonly two).
6. After any modification, rebuild + Mass Properties refresh.

Unit errors are invisible in a shaded isometric view. The part “looks fine” while the score does not. Make Document Properties the first click of every CSWA modeling problem, and MMGS discipline becomes free points.