Why Assembly Drawings Still Matter
With full 3D model data now standard in most design environments, it might seem that assembly drawings are becoming obsolete. In practice, they remain the primary communication medium between design and manufacturing for complex products, and they are the document that gets carried onto the shop floor, pinned to the assembly station, and referenced during troubleshooting. A poorly structured assembly drawing causes confusion, incorrect builds, and expensive rework. A clear one eliminates ambiguity before it causes problems.
This guide covers the fundamental elements of assembly drawing practice: projection angle conventions, section views, balloon callouts, parts list structure, and the most common mistakes that create shop-floor confusion.
First Angle vs Third Angle Projection
Engineering drawings use either first angle (ISO standard, common in Europe, Japan, and most of the world) or third angle (ANSI standard, common in the US, Canada, and some Asian markets) projection to arrange the standard orthographic views. Confusing the two is a fundamental error that causes mismatch between dimensions and actual geometry.
In third angle projection (ANSI), the views are arranged as if you are looking at the object from the outside through a glass box. The front view is in the center; the top view is above it; the right side view is to the right; the left side view is to the left. The projection symbol shows the truncated cone with the narrow end toward the viewer.
In first angle projection (ISO), the object is between the viewer and the projection plane. The top view appears below the front view; the right side view appears on the left. The projection symbol shows the truncated cone with the wide end toward the viewer.
Always include the correct projection symbol in the drawing title block. For international supplier communication, including a note (Third angle projection per ASME Y14.3 or First angle projection per ISO 128) eliminates ambiguity completely.
Section Views: When and How to Use Them
Sections are cut through the assembly to reveal internal geometry that would otherwise require many hidden lines. Overusing hidden lines in assembly drawings makes them difficult to read; section views are almost always clearer. Common types:
- Full section: The cutting plane passes completely through the assembly. Most useful for symmetric assemblies to show the full internal cross-section.
- Half section: Used for symmetric assemblies to show both external and internal geometry in the same view, cut along a centerline.
- Offset section: The cutting plane steps (offsets) to pass through features that are not in a single plane. Useful for assemblies where the key features are not aligned.
- Revolved and removed sections: Used for shafts and structural sections to show cross-sectional shape at specific locations.
For assembly sections, hatching (cross-hatching) is applied to the cut faces of each component. Different components should use different hatch angles (45°, 30°, 60°, etc.) or hatch spacings to make it visually clear where one part ends and another begins. Fasteners, shafts, pins, and balls are typically not sectioned even when the cutting plane passes through them — this is a standard drafting convention, not a mistake.
Balloon Callouts and Parts List
Assembly drawings identify components using balloon callouts (circles with item numbers connected to components by leader lines) that correspond to rows in the Parts List (also called Bill of Materials or BOM).
Balloon Placement Rules
- Each component that appears in the drawing should have at least one balloon callout
- Leader lines should point to the actual component, not empty space or a hidden line
- Balloons should not cross each other and should be arranged in a logical order (typically left to right, or following the assembly sequence)
- Standard components (screws, washers, pins) that appear many times may have a single balloon with a note (Qty: 8 — see Item 12) or may be called out individually depending on drawing practice
Parts List (BOM) Structure
The parts list is typically placed in the upper right of the drawing or above the title block. Minimum content for each row:
| Column | Content | Notes |
|---|---|---|
| Item No. | Number matching the balloon | Sequential numbering preferred |
| Part Number / Drawing Number | Unique identifier for the component | Link to detail drawing or catalog |
| Description | Brief functional description | Material or spec as applicable |
| Quantity | Number used per assembly | Per assembly, not total program qty |
| Material / Specification | For standard purchased items | e.g., ISO 4762 M6×20, class 10.9 |
Key Dimensions vs Reference Dimensions
Assembly drawings should show only dimensions necessary to verify the assembly: interface dimensions (mating lengths, spacing, envelope dimensions), critical assembly clearances, and any dimensions that cannot be verified from detail drawings alone. Do not duplicate all dimensions from detail drawings — this creates conflicts when detail drawings are revised and the assembly drawing is not updated.
Reference dimensions (shown in parentheses) are informational — they are driven by the design and not independently toleranced on the drawing. They help the reader understand the assembly context without creating additional inspection requirements. Use reference dimensions for overall envelope dimensions and distances that are the result of other toleranced dimensions.
Common Mistakes That Cause Shop Floor Confusion
- Missing projection symbol — shop doesn’t know if they’re reading first or third angle
- Undefined balloon items — a balloon number that doesn’t appear in the parts list, or vice versa
- Hatching all the same angle in a section — makes it impossible to distinguish adjacent parts
- Dimensioning individual part features on the assembly drawing — creates conflicts with detail drawings
- Omitting assembly-critical clearances — the only dimensions that actually need to be on the assembly drawing
- Using the wrong revision status in the BOM — sourcing parts from an old revision because the assembly BOM was not updated
FAQ
Q: Should I show all fasteners in the assembly drawing or just representative ones?
A: Show representative fasteners with a note or balloon indicating quantity. For a flange with 8 identical bolts, show one fully assembled bolt with a balloon and note 8x Item 7, and indicate the bolt circle with a centerline. Showing all 8 individually adds clutter without adding information. The exception is when fastener access, installation sequence, or orientation varies — in that case, show the specific configuration that needs to be communicated.
Q: What level of detail should I show in an assembly drawing versus in detail drawings?
A: The assembly drawing shows how components relate to each other and to the assembly envelope. Individual part geometry and dimensions belong on detail drawings. The assembly drawing should show: component locations, critical interface dimensions, assembly sequence notes if required, and any dimensions that only exist at the assembly level (e.g., the gap between two components after assembly). A good test: if a dimension appears on both the assembly and a detail drawing, remove it from one of them.
Q: How do I handle proprietary supplier assemblies in my BOM?
A: For commercial purchased assemblies (motors, gearboxes, actuators, valves), use the manufacturer’s part number or the purchasing specification number in the BOM and note Commercial assembly or Purchased item in the description. Do not explode a purchased assembly into its internal components in your BOM unless you are legally permitted to do so and have a specific reason. Reference the supplier’s installation drawing in the notes section of your assembly drawing.
コメント