Understanding Engineering Drawings — Bench

Watched Understanding Engineering Drawings by The Efficient Engineer. A drawing is the contract between designer and manufacturer — it must be complete, unambiguous, and readable by anyone trained in the conventions.

Types of Drawings

Assembly Drawing — shows how parts fit together as a complete unit. Includes a bill of materials (BOM) with numbered callouts. Does not dimension individual parts.

Detail Drawing — defines a single part completely. All dimensions, tolerances, material, and finish live here. A machinist should be able to make the part from this drawing alone.

Layout Drawing — establishes overall geometry and spatial envelope early in design. A design tool, not a manufacturing document.

Interface Control Drawing (ICD) — defines the boundary between two systems or suppliers: connector positions, mounting hole patterns, envelope dimensions. Neither side owns the other’s internals; both own the interface.

Projection: How 3D Becomes 2D

Orthographic Projected Views

Orthographic projection places the object inside an imaginary glass box. Each face of the box is a projection plane. The object’s geometry is projected straight outward — no perspective — onto each plane. The box is then unfolded flat to produce the drawing.

The standard view set: Front, Top, and Right Side (or Left Side). Additional views added as needed for clarity.

Third-Angle Projection (ASME — USA)

The projection plane sits between the viewer and the object. Think of looking through a glass pane at the object — the image forms on the near side.

Layout: Front view center, Top view directly above it, Right Side view to the right of the Front.

First-Angle Projection (ISO — Europe)

The object sits between the viewer and the projection plane. The image forms on the far side — as if the object casts a shadow.

Layout: Front view center, Top view directly below it, Right Side view to the left of the Front — the opposite of Third Angle.

Taper Cone Symbol

The title block always includes a small symbol indicating which projection convention the drawing uses: a truncated cone shown in two views. In Third Angle the circle (small end) appears on the left; in First Angle the circle appears on the right. Always check this before reading a drawing.

View Types

Isometric View — a 3D pictorial view at a standard 30° angle. Not used for dimensioning; used for clarity and context, often placed alongside orthographic views or in assembly drawings so the reader can quickly orient themselves.

Exploded View — an isometric or pictorial view with components separated along assembly axes, showing how parts relate before assembly. Common in assembly drawings and instruction manuals.

Detail View — a magnified view of a small or complex area, labeled (e.g. “DETAIL A”) and shown at a larger scale. The source area is circled and labeled on the parent view. Used when a feature is too small to dimension clearly at drawing scale.

Sectional View — the object is cut along a cutting plane and the interior is shown. The cut surface is indicated with hatching (parallel lines at 45°). The cutting plane is shown on another view as a line with arrows indicating the viewing direction (e.g. “A–A”). Used when internal geometry is too complex to communicate with hidden lines alone.

Dimensioning

Callout Dimensions

A callout is a dimension or note that points to a specific feature using a leader line. Used for:

Hole sizes and depths
Radii and chamfers
Surface finish symbols
Special notes (e.g. “DRILL AND TAP M4×0.7 THRU”)

Auxiliary Dimensions

Reference dimensions — shown in parentheses, e.g. (25). They are derived from other dimensions already on the drawing and provided for convenience only. They are not controlled or inspected. You can’t change a part and only update the auxiliary dimension — the controlling dimensions must change.

Dimensioning Best Practices

Each dimension appears once — never duplicate
Dimension to the feature, not to construction geometry
Use baseline dimensioning (all from one datum) to prevent tolerance accumulation across a chain
Avoid chain dimensioning for tight-tolerance features — errors stack
Dimension in the view where the feature is most clearly shown
Keep dimension lines outside the part outline whenever possible
Group related dimensions together; don’t scatter them across views

Hole Callouts: Counterbore, Countersink, Through

Symbol	Name	Meaning
⌀	Diameter	Hole diameter
↓	Depth	Feature depth
⌴	Counterbore	Flat-bottomed recess (for socket head cap screws)
∨	Countersink	Conical recess (for flat-head screws)
THRU	Through	Hole goes all the way through

Example callout: ⌀4.5 THRU / ⌴⌀8.5 ↓ 4.5 — a 4.5mm through hole with an 8.5mm counterbore 4.5mm deep.

Tolerancing

Tolerancing Best Practices

Every dimension that matters functionally needs an explicit tolerance
General tolerances (in the title block) cover unspecified dimensions — typically ±0.1mm for machined parts
Specify only as tight as the function requires — tighter tolerances cost more to manufacture and inspect
Tolerances should reflect what your manufacturing process can achieve; specifying tighter than the process capability is meaningless

Fits

For mating features (shaft in hole), tolerances are defined as pairs:

Clearance fit — shaft always smaller than hole; they slide freely
Interference fit — shaft always larger; pressed together, held by friction permanently
Transition fit — could be either depending on where each lands in its tolerance band

ISO fits use letter-number codes: H7/g6 = H7-tolerance hole, g6-tolerance shaft — a standard close sliding fit.

Geometric Dimensioning and Tolerancing (GD&T)

Standard linear tolerances (±0.1mm) control size but not shape — a shaft could be within diameter tolerance but bent, tapered, or oval. GD&T controls the geometry of features: their form, orientation, location, and runout.

Feature Control Frame

The feature control frame is the core of GD&T — a rectangular box divided into compartments read left to right:

| geometric characteristic symbol | tolerance value | datum references |

Example: | ⊙ | 0.05 | A | B | Means: the position of this feature must be within a 0.05mm diameter tolerance zone, relative to datums A and B.

Common GD&T Symbols

Symbol	Characteristic	Controls
—	Flatness	How flat a surface is
○	Circularity	How round a cross-section is
⌭	Cylindricity	Roundness + straightness of a cylinder
∥	Parallelism	Orientation relative to a datum
⊥	Perpendicularity	90° relationship to a datum
∠	Angularity	Angular relationship to a datum
⊙	Position	Location of a feature relative to datums
◎	Concentricity	Axis alignment of coaxial features
↗	Runout	Surface variation during rotation

Datums

GD&T tolerances reference datums — theoretically perfect planes, axes, or points established from actual part surfaces. Datums are labeled A, B, C and called out in the feature control frame. They define the coordinate system against which all geometric tolerances are measured.

The datum reference frame: typically three mutually perpendicular planes. Primary datum (A) — most contact points; Secondary (B) — two contact points; Tertiary (C) — one contact point.