Section Views, Auxiliary Views, and Detail Views

Why Section, Auxiliary, and Detail Views Exist

A standard orthographic drawing—front, top, and right-side views—shows the outside of a part. When internal geometry such as bores, counterbores, keyways, or internal threads must be dimensioned, dashed hidden lines crowd the view and become hard to read. Section views, auxiliary views, and detail views are the drafting tools machinists and CNC programmers use to expose that hidden geometry cleanly. Reading them accurately is a prerequisite for correct setup, workholding, toolpath planning, and quality inspection.

Cutting-Plane Lines and Hatch Patterns

A cutting-plane line is a thick chain line (alternating long dash and two short dashes) drawn across the parent view. Arrowheads at each end point in the direction the viewer looks after the imaginary cut is made. Capital letters at each arrowhead—such as A–A—identify the section. The resulting section view is labeled SECTION A–A.

All material that the cutting plane actually slices through is shown with section lines (hatch lines): thin, evenly spaced lines drawn at 45° by default per ASME Y14.2. Adjacent parts or different materials use different hatch angles or patterns to distinguish them. Voids and open holes are left blank—do not hatch air. Features that lie behind the cutting plane are drawn as visible object lines in the section view unless they would cause confusion.

Full Section Views

A full section view passes the cutting plane completely through the part. The entire view shows the cut face. Full sections are the clearest way to expose a part's internal profile—a cylinder with a through-bore, for example, is far easier to dimension in full section than with hidden lines.

CNC application: When setting up a turning operation on a complex housing, a full section confirms bore diameters, wall thickness, and shoulder locations you cannot measure externally.

Half Section Views

A half section uses a cutting plane that turns 90° and removes only one quarter of the part, leaving the other half as an external view. The center line of symmetry forms the boundary between the external half and the internal half. Half sections are used only on symmetrical parts.

Important rule: Do not draw a visible object line at the center-line boundary in a half section—use a center line there, not an object line.

CNC application: A half section of a flanged hub shows bolt-circle diameter and bore depth without hiding the external flange profile you need for fixture design.

Removed and Broken-Out Sections

A removed section is a cross-sectional slice lifted out of the part and placed elsewhere on the sheet, labeled with the same A–A convention. It shows only the cross-sectional shape at one specific location along a rib, shaft, or web—useful for profiles that change along their length.

A broken-out section exposes a small local area by showing a jagged break line (the irregular freehand line per ASME Y14.2) to reveal a single feature—a keyway, tapped hole, or undercut—without cutting the entire view. No cutting-plane line is required for a broken-out section.

Auxiliary Views

An auxiliary view is projected perpendicular to an inclined or oblique surface, so that surface appears true size and true shape. Standard front, top, and side views cannot show a face that is not parallel to a principal projection plane without foreshortening—meaning every dimension on it would be distorted.

The auxiliary view is drawn parallel to the slanted surface. Reference lines (thin, with slash marks) connect the parent view to the auxiliary view, and only the features on the angled face are typically shown—not the full part profile.

CNC application: An angled boss with a drilled and tapped hole shows its true bolt-circle pattern and true hole diameter only in the auxiliary view. Programming the tilt angle of a 4-axis or 5-axis setup begins with reading this view correctly.

Detail Views

A detail view is an enlarged portion of an existing view, circled in the parent view with a thin line labeled D (or any letter), then redrawn at a larger scale—commonly 2:1 or 4:1—nearby on the sheet. The enlargement reveals geometry that would be illegible at the drawing's main scale: small radii, fine thread reliefs, undercuts, or chamfers.

The detail view label states the view letter and its scale: DETAIL D — SCALE 2:1. All dimensions shown in the detail view govern; they override any scaled measurement from the parent view.

CNC application: A detail view of a threaded port shows thread form, relief groove width, and minor diameter—data needed to select the correct tap or thread mill and set depth stop.

Practical Safety Note

When interpreting any drawing at the machine: confirm dimensions against the physical drawing, not a printout that may have been scaled or photocopied to a different size. Scaled or distorted prints have caused scrapped parts and tool crashes. Always retrieve dimensions from the dimension text, not by measuring the paper. Before making any setup, verify the revision block to ensure you are working from the current drawing revision. Bringing incorrect geometry into the control can cause rapid-traverse collisions—always verify tool-path simulation against the correct model.

Putting It Together

A single industrial housing drawing may contain a full section for the main bore, a half section for a symmetric pocket, a broken-out section for a keyway, an auxiliary view for an angled port face, and a detail view of the port thread relief. Reading all five view types together gives you the complete geometry of the part before you write a single line of G-code or set a single work offset.