Orthographic Projection and Mechanical Drawing Conventions

Orthographic Projection and Mechanical Drawing Conventions

Orthographic projection is the foundation of every engineering and manufacturing drawing. It is the method of representing a three-dimensional object on a flat sheet by projecting its features onto two or more planes that are at right angles to one another. Mastering projection and understanding the line conventions defined by ASME Y14.5 are non-negotiable skills for any drafter working in mechanical, architectural, or marine environments.

First-Angle vs. Third-Angle Projection

The world uses two standard systems. Third-angle projection is the standard in the United States, Canada, and other countries following ASME practice. First-angle projection is standard in most of Europe and countries following ISO convention.

The easiest way to remember the difference is to think about where the view lands relative to the object:

• Third-angle projection: The object sits inside the glass box. You look through the glass and the view is projected onto the near face — the face closest to you. When the box is unfolded, the front view is center-left, the right-side view unfolds to the right of the front, and the top view unfolds above the front. The symbol is a truncated cone drawn with the small end facing the viewer.
• First-angle projection: The object sits in front of the projection plane. The view falls on the far face — behind the object. When unfolded, the right-side view ends up to the left of the front view — the opposite of third-angle. The symbol shows the cone with the large end facing the viewer.

Every drawing sheet must display the correct projection symbol in the title block or near it. Mixing systems on a single project is a serious error that can cause parts to be machined or fabricated incorrectly. When working in SolidWorks Drawing, select the projection type in Sheet Properties before placing views.

The Six Principal Views

Orthographic projection can produce up to six principal views: front, top, right side, left side, bottom, and rear. In practice, most mechanical drawings show three views — front, top, and right side — because they are sufficient to fully describe the object without redundancy. Choose views that reveal the most features with the fewest hidden lines.

ASME Y14.5 Line Types

ASME Y14.5-2018 (Dimensioning and Tolerancing) and the companion standard ASME Y14.2 (Line Conventions and Lettering) define the line types every drafter must use consistently. The major line types you will annotate and apply are:

| Line Type | Appearance | Purpose |

|---|---|---|

| Visible line | Thick, continuous | Outlines and edges you can see |

| Hidden line | Medium, dashed | Edges hidden behind surfaces |

| Center line | Thin, long-short-long dash | Axes of symmetry, hole centers |

| Phantom line | Thin, long-short-short-long dash | Alternate positions, adjacent parts |

| Section line (hatching) | Thin, angled | Cut surfaces in section views |

| Cutting-plane line | Thick, long-long-short | Shows where a section is taken |

| Dimension/extension line | Thin, continuous | Dimensioning |

| Break line | Thin, freehand (short) or thin zigzag (long) | Shortens a long feature |

Line weight matters. Visible lines are drawn heavy (0.5–0.7 mm typical). Hidden, center, phantom, dimension, and extension lines are thin (0.25–0.35 mm typical). AutoCAD and SolidWorks control line weight through layer settings and lineweight properties — always set these by layer, not by object, so drawings are easy to edit later.

Title Block Standards

A title block is the official record of a drawing. Industry practice, guided by ASME Y14.35 and company standards, requires a minimum set of fields:

• Drawing title and part or assembly name
• Drawing number and revision letter
• Scale (e.g., 1:1, 1:2, 2:1)
• Sheet size (ANSI A through E in the US)
• Projection symbol (first- or third-angle)
• Material (if applicable)
• Drawn by, checked by, and approved by with dates
• Tolerances — general tolerances applying to all dimensions not otherwise specified
• Company or school name and logo

In SolidWorks, title block fields linked to drawing custom properties update automatically when the model changes. This reduces manual errors. Always verify every field before releasing a drawing.

Drafting Safety and Good Practice

The drafting lab contains computer workstations and plotters. Follow these safety points:

• Electrical safety: Do not place drinks near workstations. Report damaged cables immediately.
• Ergonomics: Adjust your chair and monitor to avoid repetitive strain. Take short breaks — prolonged CAD work without breaks is a recognized source of musculoskeletal injury.
• Large-format plotters: Keep hands and loose clothing away from the paper feed. The cutting blade on some plotters is exposed — treat it as you would any sharp edge.
• File management: Save work frequently and back up to the network or cloud. Loss of a drawing file can have real cost consequences on a job site.

Setting Up in SolidWorks Drawing

1. Open the part file you want to document.

2. Choose New → Drawing and select the correct sheet template (ANSI B, C, etc.).

3. In Sheet Properties, confirm the projection type is Third Angle and set the sheet scale.

4. Use View Palette or Model View to place the front view, then project the top and right-side views.

5. Assign layers: Visible, Hidden, Center, etc., with correct lineweights.

6. Fill in all title block fields.

7. Verify the projection symbol appears in the title block area.

Precision in these setup steps prevents costly revision cycles later in production or fabrication.