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Engineering Graphics and CAD · Lesson 27 of 35

Exploded views and bills of materials

Communicate an assembly's parts and structure through an exploded view and a bill of materials.

Readiness check

Learning objectives

By the end of this lesson you can:

Create an exploded view that shows assembly order.
Add item balloons keyed to a bill of materials.
Build a bill of materials with item, quantity, and description.
Ensure the bill of materials, balloons, and model stay consistent.
Explain the bill of materials' role in procurement and assembly.

Check your starting point

Five to ten minutes.

When you buy furniture that needs assembly, what shows how the parts fit together?
What is a parts list, and who uses it?
If a drawing balloons part number 3 but the parts list has no row 3, what is wrong?

Interpretation.

Q1: An exploded diagram, separating the parts along their assembly paths. That is exactly an exploded view.
Q2: A list of every part and its quantity; used by purchasing and assembly. This is the bill of materials.
Q3: The balloons and the list disagree; the documentation is inconsistent. Skill 27.4 covers consistency.

You need L25-L26 (a working assembly).

0 or 1 weak itemsContinue with this lesson.

2 weak itemsReview Lesson 25, then return.

3 or more weak itemsWork through the prerequisite examples before continuing.

The core idea

What it is. An exploded view separates an assembly's parts along their assembly paths so their arrangement and order are visible. Item balloons label each part with a number that keys to a bill of materials (BOM), a table listing every part, its quantity, and a description.

Why an engineer needs it. An assembly drawing must tell someone how to build the product and what parts to buy. The exploded view shows how it goes together; the BOM says what and how many. Together they are the assembly's build-and-buy documentation.

What problem it solves. It communicates assembly order and the complete parts list in a form that purchasing and assembly can act on directly.

What goes wrong when it is ignored. Without an exploded view, assembly order is unclear; without a consistent BOM, the wrong parts or quantities are ordered. Mismatches between balloons, BOM, and model cause build errors.

A simple mechanical example. The clamp assembly exploded shows the base, the jaw, the pivot pin, and the screw separated along their assembly directions, each ballooned to a BOM row: item 1 base (qty 1), item 2 jaw (qty 1), item 3 pin (qty 1), item 4 screw (qty 1). Anyone can see how it assembles and what to order.

Key ideas:

Exploded view: parts separated along assembly paths, in explosion steps, to reveal order.
Balloons: numbered bubbles with leaders, each keyed to a BOM item.
BOM: a table of item number, quantity, description, and part number, derived from the assembly.
Consistency: balloons, BOM, and model must agree; the BOM is generated from the assembly, so it stays accurate if not overridden.

Part 5: Parts, assemblies, and drawings.

Check: explain the decision in your own words before using a CAD command.

The lesson map. Exploded views and bills of materials becomes manageable when you move through the four checks in order and verify each result before continuing.

The skills, taught in order

Skill 27.1 - Create an exploded view

Concept. Separate parts along their assembly directions in steps. Terminology. Exploded view, explosion step, assembly path. Procedure. Move each part outward along the direction it is assembled, in a sensible order, creating explosion steps. Reasoning. Separation along assembly paths reveals how the product goes together. Failure mode. Scattering parts randomly, hiding the assembly order. Check. Explode a two-part joint along its assembly direction.

Skill 27.2 - Add balloons keyed to items

Concept. Each part gets a numbered balloon that keys to a BOM item. Terminology. Balloon (item bubble), item/find number, leader. Procedure. Attach a balloon with a leader to each part, numbered to match its BOM row. Reasoning. Balloons link the picture to the parts list. Failure mode. Balloons that do not match BOM numbers. Check. Balloon a part to its BOM item number.

Skill 27.3 - Build the bill of materials

Concept. The BOM lists every part, quantity, and description. Terminology. Bill of materials (BOM), quantity, description, part number. Procedure. Generate the BOM from the assembly; confirm each row's quantity and description; add part numbers. Reasoning. The BOM is the authoritative parts list for purchasing and assembly. Failure mode. A hand-typed BOM that misses a part or a quantity. Check. Read a BOM row and state the part and quantity.

Skill 27.4 - Keep BOM, balloons, and model consistent

Concept. The three must agree; the BOM derives from the assembly. Terminology. Consistency, traceability. Procedure. Generate the BOM from the model, key balloons to it, and check every part appears once with the right quantity. Reasoning. Consistency prevents ordering or building the wrong parts. Failure mode. A balloon or quantity that disagrees with the model. Check. Confirm every balloon number has a matching BOM row.

Worked example 1: explode and balloon the clamp

Problem. Create an exploded view of the clamp (base, jaw, pivot pin, screw) and balloon each part to a three-part-plus-fastener BOM.

Planning. Separate parts along assembly paths, balloon them, and generate the BOM.

Solution.

Explode. Move the jaw off the base along the pivot axis, the pivot pin out along its axis, and the screw out along its axis, in explosion steps that show the assembly order.
Balloons. Attach balloons: 1 to the base, 2 to the jaw, 3 to the pivot pin, 4 to the screw, each with a leader to its part.
BOM. Generate the BOM from the assembly: item 1 base (qty 1), item 2 jaw (qty 1), item 3 pivot pin (qty 1), item 4 screw (qty 1), each with a description.
Consistency. Every balloon number (1 to 4) matches a BOM row; every part appears once.
Result. An exploded clamp with four balloons keyed to a four-row BOM.

Result. The clamp exploded along its assembly paths, with balloons 1 to 4 keyed to a four-row BOM (base, jaw, pin, screw), all consistent.

Why the method works. Exploding along assembly paths shows the order; balloons keyed to a generated BOM keep the picture and the list in agreement.

How to verify independently. Count parts in the exploded view and rows in the BOM: both should be four, with matching numbers. Agreement confirms consistency.

Worked example 2: adding fasteners and catching a mismatch

Problem. The clamp gains two cap screws and two washers to secure the base. Update the BOM quantities, and show how forgetting a washer creates a balloon-BOM mismatch to catch. The complication is keeping quantities correct as fasteners multiply.

Planning. Add the fasteners to the assembly, let the BOM update, and check quantities against the balloons.

Solution.

Add fasteners. Insert two cap screws and two washers into the assembly. The BOM, generated from the assembly, now shows item 5 cap screw (qty 2) and item 6 washer (qty 2).
Balloons. Balloon one representative cap screw as item 5 and one washer as item 6 (a single balloon per item, with the BOM giving the quantity).
The mismatch scenario. Suppose one washer was not actually placed in the assembly (only one inserted). The BOM would then read washer qty 1, while the design intends 2. The balloon still says item 6, but the quantity is wrong.
Catch it. Comparing the intended design (two washers, one under each screw) with the BOM quantity (1) reveals the missing washer. Because the BOM is generated from the assembly, the fix is to add the missing washer to the assembly, and the BOM updates to qty 2.
Lesson. Because the BOM derives from the model, quantity errors trace back to missing or extra components in the assembly, not to the table.

Comparison. A generated BOM keeps quantities tied to the actual assembly, so a missing part shows as a wrong quantity; a hand-typed BOM could hide the error. Generating from the model makes mismatches visible and fixable at the source.

Result. The BOM updates to cap screw qty 2 and washer qty 2; a missing washer shows as washer qty 1, caught by comparing with the intended design and fixed by adding the component.

Independent check. Count the screws and washers in the assembly and compare with the BOM quantities. Equal counts confirm consistency; a difference points to a missing or extra part.

Misconceptions and diagnostics

Misconception	Why it seems reasonable	Why it is wrong	Evidence that reveals it	Correction	Diagnostic question
"The BOM is typed by hand."	It is just a table.	The BOM is generated from the assembly, so it stays accurate and traceable.	A hand-typed BOM misses a part the model has.	Generate the BOM from the model.	"Does the BOM come from the assembly?"
"The exploded view is decorative."	It looks like a picture.	It communicates assembly order and part relationships.	Without it, assembly order is unclear.	Explode along assembly paths to show order.	"Does the explosion show how it goes together?"
"Balloon numbers can be arbitrary."	Any number labels a part.	Balloons must key to BOM items for consistency.	A balloon number has no matching BOM row.	Key every balloon to its BOM item.	"Does every balloon match a BOM row?"

Practice ladder

Level A - Recognition

Task. On an exploded view with a BOM, map each balloon to its part and BOM row. Deliverable. A mapping table. Success criteria. All balloons correctly mapped. Answer guidance. Follow each leader to its part and its BOM row. Common errors. A balloon without a matching row. Difficulty. Low.

Level B - Guided application

Task. Build a guided exploded view of a small assembly and add balloons. Deliverable. The exploded, ballooned view. Success criteria. Parts separated along assembly paths; balloons keyed to items. Answer guidance. Explode in assembly order. Common errors. Random separation. Difficulty. Medium.

Level C - Independent application

Task. Explode and BOM the clamp (Project P5) independently. Deliverable. The exploded view with balloons and a generated BOM. Success criteria. Consistent balloons, BOM, and model; correct quantities. Answer guidance. Generate the BOM from the assembly. Common errors. Quantity mismatches. Difficulty. Medium.

Level D - Transfer and design

Task. Audit a supplied exploded view and BOM for mismatches (missing part, wrong quantity, unkeyed balloon) and correct them. Deliverable. A corrected exploded view and BOM with a note on each fix. Success criteria. All mismatches found and fixed; consistency restored. Answer guidance. Compare balloons, BOM rows, and the model part by part. Common errors. Missing a quantity error. Difficulty. Medium to high.

Working with AI, and proving it yourself

Use AI as a tutor

Useful AI support:

Ask it to explain BOM fields and their use.
Ask it to describe good explosion order for an assembly.
Ask it to generate a BOM-consistency checklist.

Limits:

A text assistant cannot see your assembly or BOM.
It may treat the BOM as hand-authored.

Verify AI output against: the generate-from-model principle, balloon-to-BOM keying, and part-count consistency.

Prove it yourself

A plausible but incorrect AI answer, and how to catch it. You ask, "My BOM shows one washer but I need two. Should I just edit the BOM to say two?" and the assistant replies: "Yes, change the BOM quantity to two."

This hides the real problem. Detect it with the generate-from-model principle: the BOM reflects the assembly, so a quantity of one means only one washer is actually in the model. Editing the table would make the BOM lie about the assembly. The evidence is the part count in the assembly. Correct conclusion: add the missing washer to the assembly; the generated BOM then correctly reads two.

Retrieval and spaced review

What does an exploded view communicate?
What links a balloon to the parts list?
What fields does a BOM carry?
Where does BOM data come from?
Why must balloons, BOM, and model agree?
How do you fix a wrong BOM quantity?
Cumulative (L25): Why does a well-structured assembly (L25) make a correct BOM easier?
Reconstruction task: From memory, list the clamp's four BOM items and quantities.

Answers. 1: how the assembly goes together (order and part relationships). 2: the item/find number. 3: item number, quantity, description, and part number. 4: it is generated from the assembly. 5: to prevent ordering or building the wrong parts. 6: add or remove the component in the assembly so the generated BOM updates. 7: a clean assembly with all parts present yields a complete, accurate generated BOM. 8: base (1), jaw (1), pivot pin (1), screw (1).

Suggested review intervals. 1 day, 3 days, 7 days.

Reference mapping and next step

Read further

Onshape docs (BOM, exploded views)
Giesecke ch.13.

Standards details must be checked against the current official edition used by your institution or employer.

Finish the lesson

You can now: create an exploded view; add balloons keyed to a BOM; build a BOM from the assembly; keep the three consistent; and explain the BOM's role.

Self-assessment checklist.

I explode along assembly paths.
I key every balloon to a BOM item.
I generate the BOM from the model.
I check part-count consistency.
I fix quantity errors in the assembly, not the table.

Next lesson: L28 - Drawings from CAD: views, sections, dimensions, annotations. Why it follows: with parts and assemblies modelled and documented, you now produce the working drawings from those models, applying everything from Part III (views, sections, dimensions, tolerances) to model-derived drawings.

Required files or submissions: submit your Level C exploded view and BOM (Project P5). Optional extension: in Onshape, generate a BOM for your clamp and confirm the quantities match the assembly.