In the example, what is used to draw the visible outline of the machine parts?

Visible contour lines are drawn with thick solid lines.

In machinery, the visible line is a thick solid line; Invisible lines are dotted lines; The imaginary line has a two-dot chain line; Auxiliary lines (reference lines) are indicated by dotted lines, etc.

There are thick solid lines and thin solid lines in the view, and only the visible contour lines are represented by thick solid lines; The thin solid line is mainly used to represent dimension line, extension line, section line, thread bottom line, root line, etc.

Related definitions:

Every object has a different outline, even the same object has a different outline shape from different angles. In film creation, according to the requirements of directors and photographers, we should choose the most characteristic and expressive outline shape of this object and shoot it at a suitable photography angle.

Generally speaking, the factors that affect the outline of an object are: the choice of lighting conditions and the configuration of light sources, the configuration of light and shadow tones, the camera angle and shooting direction, the setting of exposure value and the printing method.