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Reading Drawings II - Sections, GD&T & BOMs

Cut open parts with section views, read tolerance symbols from GD&T, and decode the bill of materials that lists every piece in an assembly.

Beginner ~12 min
1 Experience
2 Reflect
3 Theorize
4 Apply

Section Views - Looking Inside a Part

Sometimes a part has pockets, holes, or walls you can't see from the outside. A section view shows the interior of a part by cutting through it with an imaginary plane — like slicing it with a saw and then looking at the cut face.

How to Read a Section
  • Cutting Plane Line: A thick dashed line with arrows shows where the imaginary saw cuts through the part.
  • Letters (A-A, B-B): Match the letter on the cutting plane to the section view with the same label.
  • Hatching: The diagonal lines inside the section show exactly where material still exists after the cut.
Why it matters: Section views reveal hidden features like bearing pockets, internal ribs, and wall thickness - things you can't dimension from an outside view.
⚠ Predict First

When a cutting plane passes through a solid block with a drilled through-hole, how does the section view show the hole?

GD&T - Geometric Tolerancing

Regular ± tolerances tell you how big or small a dimension can be. GD&T (Geometric Dimensioning and Tolerancing) tells you how straight, flat, round, or square a feature has to be.

Flatness (▭)

How flat a surface has to be. Useful for surfaces that bolt against each other.

Perpendicularity (⊥)

How square one face must be to another. Keeps holes aimed straight.

Position (⊕)

How close a hole has to be to its target location. Crucial for bolt patterns.

Concentricity (◎)

How well two round features share the same center. Keeps shafts spinning true.

Feature Control Frame

GD&T shows up in a rectangular box with three sections:

  • Symbol (what kind of control - flatness, position, etc.)
  • Tolerance (how much variation is allowed, usually in mm)
  • Datum (the reference face or edge, labeled A, B, or C)

The Bill of Materials (BOM)

An assembly drawing usually comes with a BOM - a table listing every part in the assembly, how many of each, and often which material.

Reading a BOM
1
Find the Balloon Numbers

Circled numbers on the drawing point at parts. Each number matches a row in the BOM.

2
Read the Part Column

Every row has a part name and/or part number so you know exactly what to order or make.

3
Check the Quantity

The QTY column tells you how many of that part the assembly needs.

4
Note the Material / Source

Will it be machined, printed, or bought off-the-shelf? The BOM usually says.

Engineer's habit: When you look at a new assembly drawing, always start with the BOM. It tells you how many pieces you're actually dealing with before you read a single dimension.
Pause and Reflect
✓ Your reflections are saved automatically
Apply What You Learned

Find an assembly drawing online (search "engineering drawing with section view"). Decode it.

  • Locate a cutting plane line and match it to its section view
  • Find at least one GD&T symbol and identify what it controls
  • Count the balloon numbers on the drawing
  • Read the BOM and list every unique part and its quantity
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