Dimensioning, GD&T, and Section Views for Manufactured Parts

Dimensioning, GD&T, and Section Views for Manufactured Parts

Accurate dimensioning is the language that connects a designer's intent to the machinist's hands. When dimensions are incomplete, redundant, or ambiguous, parts are made wrong and money is lost. This lesson applies ASME Y14.5-2018—the governing standard for dimensioning and tolerancing in the United States—to a real machined part inside SolidWorks.

Why ASME Y14.5 Matters

ASME Y14.5-2018 defines every symbol, rule, and interpretation used on engineering drawings for manufactured parts. Employers in manufacturing and marine fabrication require compliance with this standard because a drawing that ships to a job shop in another state—or another country—must communicate without ambiguity. The CSWA exam tests your ability to create and interpret drawing views correctly, and ASME Y14.5 knowledge is assumed.

Two foundational rules from the standard:

• Rule 1 (Envelope Principle): When only a size tolerance is given, the surface of the part must not violate a perfect-form envelope at maximum material condition (MMC). No separate straightness or flatness callout is needed to control overall form at MMC unless the drawing explicitly releases Rule 1.
• Rule 2: Each GD&T control applies independently unless modifiers such as the circle-M (MMC) or circle-L (LMC) symbol are added.

The Feature Control Frame

Every GD&T callout lives inside a feature control frame—a rectangular box divided into compartments read left to right:

1. Geometric characteristic symbol (e.g., the flatness symbol ⏥)

2. Tolerance value, with modifiers if needed

3. Primary datum reference (if required)

4. Secondary and tertiary datum references (if required)

Some controls—flatness and straightness among them—are form controls that reference no datum. Others—perpendicularity, true position, runout—always reference at least one datum.

Four GD&T Controls You Will Apply Today

Flatness controls how much a surface may deviate from a perfect plane. It requires no datum. A flatness callout of 0.05 mm means the entire surface must lie between two parallel planes 0.05 mm apart. Apply flatness to the flange face when the sealing surface must be smooth enough to hold a gasket.

Perpendicularity controls the orientation of a surface or axis relative to a datum. A perpendicularity callout on a bore axis relative to datum A (the flange face) ensures the bore does not lean away from true 90°. The tolerance zone is a cylinder or pair of parallel planes, depending on the feature.

True Position locates the center of a hole—or a pattern of holes—relative to datums. It is almost always applied with the MMC modifier on bolt-circle holes because that allows bonus tolerance as holes depart from MMC, making assembly easier without scrapping good parts. The tolerance zone is a cylinder whose axis must contain the actual feature axis.

Runout (Circular Runout) controls the wobble of a surface around a datum axis during one full revolution. It is the correct control for the bore of a rotating bushing where concentricity with the outer diameter matters. Circular runout is measured at a single cross-section; total runout sweeps the full length.

Full Section Views

A full section view is created by passing a cutting plane completely through the object and removing the front half. In SolidWorks Drawing mode:

1. Open or create the front view.

2. Click Insert → Drawing View → Section View, then draw a straight cutting-plane line through the center of the part.

3. Project the view to one side; SolidWorks hatches the cut material automatically.

For an offset section, the cutting plane jogs (offsets) to pass through features that do not all lie in one plane—common on flanged parts where bolt holes are staggered. The offset is shown by bends in the cutting-plane line; each bend is 90° and not labeled separately.

Hatch lines follow ASME Y14.2 conventions: thin, evenly spaced lines at 45° for general materials, with different angles used when adjacent parts are shown. Always confirm that hidden lines inside the section view are removed unless needed for clarity.

Dimensioning Rules to Follow

• Every feature must have a size and a location dimension—no guessing.
• Do not create redundant (reference) dimensions unless enclosed in parentheses and labeled REF. Redundant driven dimensions cause conflict and confuse the machinist.
• Dimension to visible edges, not hidden lines.
• Place dimensions outside the view boundary when possible; leaders and witness lines must not cross dimension lines unnecessarily.
• Specify surface finish (Ra), material, and general tolerances in the title block or a general note.

Safety in the CAD Lab

The CAD lab is low-hazard compared to a machine shop, but these points apply:

• Ergonomics: Adjust your chair and monitor height before starting a long drawing session. Eye strain and repetitive-strain injuries are real risks.
• Electrical safety: Do not run power cables across walkways; report damaged cords immediately.
• Printed drawing handling: Engineering scales are rigid and can cause puncture injuries if carried improperly—carry them point-down or in a case.
• When drawings will later be used to guide physical machining, remember that an incorrect GD&T callout is an indirect safety hazard: a mis-machined bushing in a marine propulsion system can fail under load.

What You Will Produce

By the end of this lab you will have a fully dimensioned SolidWorks drawing of the flanged bushing—complete with a full section view, flatness on the flange face, true position on the bolt-circle holes, and runout on the bore—plus a written correction memo identifying GD&T errors on a seeded drawing. Both deliverables are graded against the rubric; you need 75% to pass.