CourseWiki/Engineering Graphics and CAD/Assemblies

Exploded Views and Bills of Materials

Show how components fit together and connect drawing balloons to clear part identities and quantities.

An exploded view separates parts to explain their relationship

An assembled view shows the working arrangement. An exploded view spaces components apart so the reader can identify them and follow how they fit. The separation is for communication; it is not an operating clearance or a change to the part geometry.

Separate components along understandable directions, keep related parts aligned, and use explode lines where the connection would otherwise be unclear. Preserve the working assembly and create a saved exploded view rather than deleting mates to pull the model apart.

Side schematic of a pivot stack, assembled and exploded along one axis. Balloons identify support 1, lever 2, pin 3, washers 4 at two positions, and retainer 5. — Original teaching schematic: the pin passes through the aligned holes; a retainer holds its end. Both washers use item 4. No manufacturing dimensions or fit are implied.

Use the saved view in a drawing

Onshape: Exploded Views describes saved explode steps and selecting the result in a drawing. An exploded presentation does not demonstrate that the parts can physically be installed in that order; check access separately.

A balloon finds a BOM row; a part number identifies the component

A bill of materials (BOM) lists the components required for the stated assembly. A drawing balloon points to a component and usually shows its BOM item number. The item number is a row identifier in that drawing's list. The part number identifies the part for reuse, manufacture, or purchase.

One teaching pivot assembly — quantities per assembly
Item	Part number	Description	Quantity
1	PV-110	Support	1
2	PV-210	Lever	1
3	PV-310	Pivot pin	1
4	HW-410	Thrust washer	2
5	HW-510	Pin retainer	1

Balloon 4 identifies HW-410, not four washers. Its quantity is two per assembly. This list has five distinct rows and six component instances. Repeated instances of the same specified part share an item number; different sizes or variants need distinguishable identities.

Linked tables reduce manual mismatches

Onshape: BOM Table describes inserting the assembly's BOM and using balloons to pull its identifiers. Keep the drawing, table, and view tied to the same intended design state.

Decide whether the BOM lists units or their internal parts

A top-level list can show a purchased subassembly as one unit. A structured list can show the assembly hierarchy, while a flattened list collects the underlying components without that hierarchy. State the scope before interpreting a quantity. Do not order a complete unit and all its contents accidentally.

Verify part numbers, descriptions, quantities, and relevant configuration or revision information. A library model's filename is not automatically a useful purchasing description. The teaching identifiers above are examples, not an orderable hardware specification.

Subassembly display changes what the table means

Onshape: Bill of Materials documents structured and flattened views and subassembly BOM behavior. A hidden or excluded row can change what the reader sees, so review the table's scope instead of trusting a row count alone.

Review the assembly drawing as an identification tool; Checklist; Practice

Review the assembly drawing as an identification tool

Show an assembled view that makes the working arrangement understandable.
Add an exploded or section view where hidden relationships need explanation.
Follow each balloon leader to the correct component and match its number to the BOM.
Count repeated components and compare the counts with the stated BOM scope.
Check the drawing's design state, units, scale, and necessary assembly notes.

An assembly drawing explains how parts belong together. Individual part drawings supply the manufacturing detail for those parts; do not crowd every part dimension onto an assembly view. See Choosing Drawing Views and the Engineering Graphics and CAD collection for related drawing topics.

Read a balloon Count repeated parts

Appears in

CAD Assemblies DET 102 DET