How to Read Assembly Drawings: BOMs, Exploded Views, and Design Intent
(Level 5)
Mechanical engineering drawings ultimately describe systems of parts, not isolated components. Level 5 introduces assembly drawings and the concept of design intent, which explains how and why parts are arranged to function together.
At this level, drawings are read as representations of mechanical systems rather than individual geometries.
What You'll Learn
Why Assembly Drawings Exist: Parts Don't Explain the System
While part drawings are excellent for manufacturing each part individually, they can not explain how all those components come together to form a system. That's where the assembly drawings step up.
Assembly drawings, as opposed to part drawings, also show interfaces (what mates to what), constraints (fixed or free parts), and assembly part sequence to facilitate assembly, maintenance, and troubleshooting.
What good are part drawings without assembly drawings to go along with them? Where does the shaft go in relation to the bearing? Are bolts fully seated or just tightened? What is the first sub-assembly? Traditional part and detail drawings are useful tools for designers and purchasing departments, but engineering, manufacturing, and quality control personnel need more information in order to assemble a product correctly. The assembly drawings fill that information gap.
How to Read an Assembly: Function → Major Parts → Interfaces
Start by determining the functionality of the system, and then proceed to identify individual components before examining details of each part.
Break down and identify the major parts or components of the system and explain their roles. Some parts are structural (frames, main supports), some are moving parts, other are fasteners or connectors. Seeing the system and not just individual pieces or shapes is important.
Now examine how individual parts relate to each other. How do parts fit or limit together? Are two parts fixed together rigidly or does one part slide inside the other? Are any parts designed to rotate? The relationships between individual parts determine how the assembly will function. For example, a bearing that has been pressed into a housing is very different from a bearing that is a slip fit inside a housing.
When working with an assembly it is important to understand how it operates, and what the relationships are between the different components. Viewing the assembly drawing is only the first step. Close components on the drawing do not necessarily mean the same in reality. For example, one part may be fixed while another part rotates. This type of functionality is important to verify in the assembly drawing.
Exploded Views: Assembly Order, Stack Direction, and Interfaces
Exploded views show assembled parts exploded and arranged in a view sequence in which they remain aligned along the assembly axis. The effect of the exploded view is that the parts are pulled apart and can be easily viewed while still maintaining proper stacking arrangement.
It shows the order in which the parts go (part sequence), the order in which to assemble the product (assembly sequence), and how individual components interface with each other (fit sequence).
Note: exploded views are NOT realistic assembly positions. These views are use to assist in the understanding of component relationships that would otherwise be obscured in an exploded view. Realistic assembly relationships are indicated by part alignment.
Looking at an exploded view? Follow the assembly axis - the parts will travel in that direction from one end of the view to the other, and should generally be assembled in the same order that they appear.
Balloons and BOMs: Identifying Parts, Quantities, and Specifications
This section introduces the critical step of part identification and reviewing required counts and details. Detailed review of all balloon components and specifications is essential, ensuring accuracy throughout the design and production phases.
In addition to describing the overall view of the assembled product, assembly drawings are used to reference specific parts with identifying numbers called balloons. These balloons are typically circular in shape, and are numbered in sequence corresponding to a BOM or Bill of Materials; a parts list that typically gets included with the assembly drawings.
The BOM (Bill of Materials) breaks down your board into the individual parts that you need to order. The BOM tells you the name of each part, how many of each are required and the material or specifications for that part.
It's a recurring theme with the documents for this assembly: a lot of cross-referencing is required. Looking at a balloon with "3" pointing to a component on the drawing, I would then refer to Item 3 in the BOM which happens to be: Housing, Aluminum 6061-T6, Qty: 1. Very helpful to know the part name, the material, and the required quantity.
Are the shapes in the guide just shapes? I see a BOM listed, is that included in the downloadable PDF? Or are the shapes usable to create my own BOM, so I know what each shape represents and where to purchase or find materials (or patterns) to complete the craft.
From the BOM it appears that item 3 is: Housing, Aluminum 6061-T6, 1 of.
On the detail drawings there are balloons that reference the housing part number as shown here, and on the assembly drawing the balloons with the "3" points to the housing component as well, which cross references to the BOM entry.
Interfaces That Define Behavior: Fixed, Sliding, Rotating, Locating
Assembly drawings illustrate how individual parts interact to form the complete system and, when examined closely, provide insight to the overall system behavior.
Fixed connections are elements in which parts are bolted, welded, or press-fit together. All parts move as a unit; if one part moves, then all of the parts move also.
Static or movable interfaces: Bearings for rotary or linear motion, pistons moving in cylinders. Seals separate sliding or rotating interfaces where contact with fluid occurs.
Pins, keys, shoulders and other locating features are used in assembly design to ensure components are properly aligned. Some are used for overall positioning, while others are specific to orientation. Typically these features do not carry significant loads and are treated as static components.
By understanding how all these relationships behave, you can predict how an object will move, how loads will be transferred and how the object will function. A clearance fit allows parts to move whereas an interference fit does not allow any movement. A keyed shaft is used to transfer torque, and a pinned connection is used to control the alignment of parts. This is how you read the function from the geometry.
So the two parts are connected via an interface. Looking at that interface, a few very simple questions arise. Are the two parts fixed, or are they mobile? Does one part try to locate the other? How will the parts be aligned, and which part will have control over that? These questions have a lot to do with the design intent—why a particular configuration has been assembled and is being presented the way it is.
Sectioned Assemblies: Seeing Fits, Engagement, and Internal Interfaces
Section views are a standard feature of most assembly drawings. The purpose of the section view is to show internal relationships that are not apparent from the flat plane of the assembly drawing. This would include views of the bearing within the bearing housing and the detail of the bolt's threads in the corresponding nut.
Sections in assemblies are very useful for highlighting internal interfaces (how components interrelate) as well as fasteners and fits (eg. hidden bolts and press fits) which would otherwise be obscure. Sections can also be used to simplify the view by eliminating superflous geometry that does not contribute to the assembly's functionality.
Threads are often simplified in drawings for purposes of simplification. Small fillets are often removed for simplification purposes. Minimize minor chamfers for simplification. Not all features on details need to be fully detailed. Assembly drawings are for relational purposes, simplification of views, and simplification of feature description. If it doesn't affect the assembly or function of the assembly, then it can be simplified.
I've also seen sectioned assemblies where each part has a different hatch pattern or angle. In one example, part A was 45° left hatched and part B was 45° right hatched. It helped to distinguish between the two parts and enabled you to see where one ended and the other began.
Design Intent: Which Features Locate, Which Retain, Which Carry Load
Assembly drawings are often thought of simply to show the fully assembled product, but they can also serve to explain the reasons behind the design configuration.
Whether you were designing from scratch or updating an existing product, a clear design intent was embodied in the creation and communication of the final drawing. This intent was evident through strategies such as highlighting critical interfaces, clearly indicating aligning features and their associated positions, and correctly delineating features which control position and those which are themselves controlled by datums or reference features.
For example, in drawings for assembling a rotating shaft with bearings, the detail feature of a machined shoulder is shown positioned to locate a bearing. A section view or detail callout may be used to illustrate this non-functional feature. If a production machinist were to mistakenly machine the feature in an incorrect location or at an incorrect angle, a bearing would not seat properly.
The assembly drawings expose these decisions and show how all the parts come together to create the end result. Many of the features were designed for a reason – the geometry serves a purpose. These decisions expose the design intent.
One of our assemblies shows how a simple shaft with a shoulder can be used to position a bearing along its axis.
Design intent: The shoulder controls bearing location. This means:
• The shoulder width is a critical dimension
• The shoulder of the adapter must be at a right angle to the shaft axis.
• manufacturing tolerance of this feature affects assembly function.
This text helps to avoid errors of misunderstanding the shoulder as a simple fillet or transition.
Why This Matters: Most System Failures Are Interface Failures
8.5 Where is the Risk?
Understanding the specific points at which system failure is most likely to occur can help IT teams manage limited resources and deploy capacities wisely. Typically, most failures occur at interfaces.
This level of problem solving shifts your perspective from individual parts to whole systems. Real world engineering problems are rarely just about a single component, instead they are systems where many components interact in complex ways.
With Level 5 capability, one is able to view drawings as a fully integrated system, and understand how individual components operate in relation to the overall function of the assembly. The ability to identify fully fixed components, and distinguish them from parts that move, as well as follow the transfer of loads through the assembly, indicates advanced skills. Furthermore, at this level, one can read assembly intent from detailed assembly documentation, perceiving the purpose of individual features within the design.
If you have only part drawings to reference, you are only looking at half of the total picture. Assembly drawings give critical insight into how the machine functions, how it is assembled, and underlying design decisions that need to be considered when making design modifications, troubleshooting, or evaluating manufacturability.
How Level 5 Builds on Levels 1—4
The first 4 levels of PartPro trained you from the ground up in reading part drawings to understand part geometry. Beginning with Level 1 where we covered basic features such as title blocks, line types, scale and basic drawing standards, each level expanded on the information needed to read part drawings to understand part geometry. For example, in Level 2 you learned how to read orthographic projections in space. In Level 3 you applied your new reading skills to dimensions, tolerances and fits controls that tell manufacturers how to assemble parts. In Level 4 you learned to read sections and details in part drawings to determine internal features that are not visible in the exterior view of the part.
Now we have moved to Level 5 where we see that all of the above comes together in a system. There are still the same parts (projections, dimensions, sections, fits, . . . ) that you studied at the earlier levels but here you will see how they all interact in a complex system where many individual parts work together as a system. Once again, questions change at this level as you consider the individual parts but more importantly ask how all the individual parts are assembled, what purpose they serve and the purpose that the designer had in assembling the system that you are studying.
This course will prepare you for working with actual design assemblies and even altering them based on new design requirements. It teaches you how to first understand the assembly makeup, constraints and relationships before delving into the details of individual components and explain their function to machinists, other engineers or manufacturing engineers as required.
These five levels of understanding progress from simple (reading a single line off a print) to very complex (understanding the functionality of an entire machine) and are what I consider to be the core skills necessary to become a proficient machinist.
Task: Identify Motion, Fits, and Design Intent in a Shaft—Bearing—Housing Assembly
Scenario: You are reviewing an assembly drawing for a simple to moderate complexity shaft-bearing-housing assembly.
- Part 1: Housing (fixed to machine frame)
- Part 2: Bearing (press-fit into housing, but a clearance fit on the shaft).
- Part 3: Shaft (rotates within bearing)
- Part 4: Shoulder on shaft (washes out bearing at axial position)
Questions:
- Moving component: Motor bearing. Stationary components: Bearing housing, rubber foot pads.
- What is the purpose of the press fit between Bearing and Housing?
- What is the reason for the clearance fit of bearing to shaft?
- What is the intention of the shoulder on the shaft?
Answer (1): Motion Analysis
Here, only one part moves: the shaft (Part 3), which rotates during the movement.
Everything else in the setup remains unchanged. The housing is securely screwed to the machine frame and cannot move. The bearing is a press fit in the housing and so cannot rotate relative to the housing. The shoulder is a feature of the shaft and thus rotates with the shaft but prevents the bearing from rotating (i.e. sliding axially) beyond this point.
So, there is a relationship between the shaft, bearing and housing. Namely, the shaft rotates within the bearing and the bearing is fixed within the rotating housing.
Answer (2): Press Fit Function
The press fit between the bearing and the housing achieves three important functions. Firstly it fixes the rotational position of the bearing within the housing, secondly it maintains correct alignment, and finally it provides a solid load path to transfer radial loads from the shaft through the bearing into the housing.
Why does this matter? Because the bearing outer race cannot rotate. Choosing the fit of the bearing requires understanding how an interference fit achieves the bearing's desired function of preventing the outer race from rotating. The interference fit is what presses the bearing into place to prevent any fretting or wear on the bearing itself and prevents premature bearing failure. For this bearing, an press fit achieved by having the bearing diameter slightly larger than the bore of the part was chosen.
Answer (3): Clearance Fit Requirement
Bearing design dictates that, when the bearing is press fitted into a housing and fixed in place (stationary), the shaft is then free to rotate provided there is adequate radial clearance; this is the difference between the shaft diameter and the bore of the bearing, i.e., a small gap between the shaft and the raceways in the bearing's inner ring.
The clearance between the shaft and bearing is provided for several reasons. It is required to allow the rotational shaft to be smoothly inserted into and rotated without interference. The internal rolling elements of the bearing require radial space to rotate properly. Finally, the lubricant in the form of oil or grease requires space in this interface – the clearance provides for a thin film.
The typical bearing mount consists of one side being a pressed interference fit ( stationary) and the other being a pressed clearance fit (rotating). You cannot press fit both sides, that would defeat the purpose of having a rotating assembly.
Answer (4): Design Intent of Shoulder
One feature on this bearing that might have been easily overlooked is the shoulder on the shaft. The purpose of this feature is to allow the bearing to be axially positioned on the shaft, to locate during assembly, and to transfer axial loading from the bearing to the shaft.
The design of the shoulder on a part is very important and typically the dimension from the face of the shoulder to other features on the part are designed to placement of bearings or other components in an assembly. If these dimensions are not accurately manufactured the bearings can end up in the wrong place which can have unintended consequences on the overall assembly and its operation including possibly changing the overall stack up and misalignment of components.
Another area where tolerances need to be very close is the dimensional relationship between the shoulder diameter and its angular relationship to the shaft axis. This is not simply a surface finish issue and is not something a machinist can simply fillet and call it good since it is a critical locating feature in an entire assembly of parts.
What you should understand from this task:
assembly drawings are used because the part drawings don't adequately show the functional relationships between the parts, which are shown in the assembly drawing to allow understanding of how parts fit together.
Second, fit types: A press fit type bearing is fixed in the housing so that bearing and housing rotate as a single unit. A clearance fit type bearing allows the shaft to rotate freely inside the bearing.
Third, the design intent is inherent in the features provided by the geometry of the part, such as how the shoulder allows the bearing to be placed axially, the press fit prevents rotation, and the clearance fit allows for motion. None of that was random.
Fourthly, knowing these relationships between parts is crucial to assembling and having the carburetor work as intended. Misunderstanding the fits or the purpose of the shoulder on the float tube can cause serious problems.
🎉 Engineering Drawings Course Complete!
You've mastered the fundamentals of reading and interpreting mechanical engineering drawings. You can now understand line types, orthographic views, dimensioning, tolerances, and assembly relationships—the complete foundation for professional engineering work.
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