Hidden Detail and Centre Lines
≈ 36 minHidden Detail and Centre Lines
An orthographic view is a projection, not a photograph — it shows edges you could not actually see, because the reader needs to know they exist.
Hidden detail is drawn as short dashes. Every internal feature that does not break the surface in this view — a drilled hole seen from the side, a counterbore, an internal step — appears dashed. Omitting it is not simplification; it deletes a feature from the part.
Centre lines mark axes and symmetry with a long-short-long chain. Every circular feature gets crossed centre lines through its centre in the view where it appears as a circle, and a single centre line running through it in the views where it appears as a rectangle.
Worked example. A blind hole (one that does not go through) versus a through hole: in the side view, a through hole's dashed lines run the full depth of the material, while a blind hole's stop short and are closed off by a dashed line at the bottom. Those few millimetres of dashes are the only thing distinguishing two very different parts.
Core checkpoint: Draw every hidden edge and every centre line. A view is complete when nothing about the object is left to inference.
In a side view, how do the hidden lines of a blind hole differ from those of a through hole?
A learner omits hidden detail because the view looks cluttered. What is the correct response to genuine clutter?
How should a drilled hole be marked with centre lines in the view where it appears as a circle?
Why is an orthographic view described as a projection rather than a photograph?