Imagine an electric motor moving through an assembly line. Its internal components are installed, the wiring is in place, and the housing is ready to be closed.
But there's a problem. A gasket that should sit inside the housing is missing.
At this stage, the mistake is easy to correct. Once the housing is closed, however, finding the same problem could mean reopening the assembly and repeating several operations.
This is why checking a product only after assembly is complete isn't always enough.
In-process inspection helps manufacturers identify assembly errors while components are still accessible. By inspecting at the right stages, teams can catch missing parts, incorrect positioning, and other issues before production continues.
1. What Is In-Process Inspection?
In-process inspection involves checking a product at selected stages during manufacturing rather than relying entirely on final inspection.
During assembly, individual components are installed, positioned, connected, and secured. Each operation introduces opportunities for mistakes. For an overview of how assembly operations are organized, see Loopr's guide to the assembly process.
Typical checks include:
- Confirming the correct component is installed.
- Verifying that parts are positioned and oriented correctly.
- Checking for missing fasteners, seals, or connectors.
- Identifying visible damage or assembly defects.
Inspections may be manual, camera-based, or supported by AI. Choose the method that can verify the requirement under the actual production conditions; AI is not necessary for every check.
The objective isn't to inspect every operation. It's to identify problems at points where they can be detected and corrected effectively.
2. Where Should Products Be Inspected During Assembly?
Not every assembly step needs a separate inspection. The right checkpoints depend on the product, its requirements, and the consequences of an error.
A useful starting point is to consider three questions:
Will the component become hidden later? Internal fasteners, wiring, and seals may no longer be visible once another component is installed.
How difficult would the mistake be to correct afterward? A missing gasket is easy to install before housing closure but may require disassembly if discovered later.
Could the error affect functionality or safety? Critical components and connections may need additional verification.
For an electric motor assembly, inspections might be placed at the following stages:
| Assembly stage | Inspection requirement |
|---|---|
| Component preparation | Correct parts and product variant |
| Internal assembly | Component placement and wire routing |
| Before housing closure | Gasket presence and positioning |
| After fastening | Fastener presence and required torque verification |
| Final inspection | Appearance and functional tests |
Each requirement should be matched to an appropriate inspection method. For instance, a camera can verify that a bolt is present, but its appearance alone cannot confirm that the bolt has been tightened to the correct torque.
3. How Are Assembly Inspections Performed?
A reliable inspection starts with clearly defined acceptance criteria.
For a gasket inspection, this might mean checking that the correct gasket is present, seated in its designated channel, and not visibly folded or displaced.
Once the requirement is established, the next step is choosing how to inspect it.
Manual visual inspection
An operator examines the assembly using work instructions or a checklist.
This approach works well when products vary frequently or when a check requires human judgment. However, consistency can become challenging with repetitive inspections.
Conventional machine vision
A camera captures an image, and software checks for predefined visual features.
For example, a vision system can verify whether a fastener is present at a specific location or whether a component is positioned within an expected area. KEYENCE describes this type of check in its overview of presence/absence vision inspection.
These systems work particularly well when components and imaging conditions are consistent.
AI-based visual inspection
AI-based systems can help evaluate more complex visual patterns, including certain variations in component appearance or positioning.
For example, a wiring harness may have several acceptable routing configurations. Depending on the inspection requirements, an AI model may help identify arrangements that differ from the approved configurations.
AI isn't necessary for every inspection. Simple presence checks may be easier to implement using conventional machine vision.
Regardless of the method, image quality matters. Poor lighting, reflections, or an obstructed camera view can affect results.
As explained in NI's practical guide to machine vision lighting, lighting is an important part of reliable image-based inspection.
For a closer look at the technology, read Loopr's article on the key components of machine vision systems.
4. What Happens When an Inspection Fails?
Finding a potential defect is only useful if there's a clear process for handling it.
An inspection generally needs to distinguish between three outcomes:
- Pass: The requirement is satisfied, and the product can proceed once other necessary checks are complete.
- Fail: A potential defect has been identified. The assembly is held or referred for review according to the approved quality procedure.
- Unable to inspect: The result is inconclusive, perhaps because the image is blurred or the component is obscured. The inspection must be repeated or reviewed.
Example: Detecting a missing gasket
Consider an electric motor assembly where a gasket must be installed before the housing is closed.
After the gasket installation step, a camera captures an image of the housing. The inspection checks whether the gasket is visible and correctly seated in the required locations.
Suppose the inspection reports that the gasket is missing.
The operator reviews the assembly and confirms the issue. Following the approved work instruction, the correct gasket is installed, and the inspection is repeated.
Once the gasket passes inspection and any other required checks are complete, the assembly can proceed.
The original failure and subsequent correction should both be recorded. Keeping only the final passing result would remove information that may be useful during a quality investigation.
This example also highlights why inspection timing matters. Detecting the missing gasket before housing closure avoids having to reopen the assembly simply to verify and correct that requirement.
5. What Information Should Be Recorded?
Inspection records help manufacturers understand what happened at each stage of assembly.
A useful record typically includes the product identifier, inspection station, requirement checked, result, and any action taken.
For the gasket example, a record might look like this:
| Field | Example |
|---|---|
| Product ID | MTR-001 |
| Inspection station | Housing assembly |
| Inspection | Gasket presence and seating |
| Initial result | Fail |
| Issue identified | Missing gasket |
| Corrective action | Gasket installed |
| Reinspection | Pass |
| Final status | Released for next operation |
Illustrative example, not actual customer data.
Depending on the application, manufacturers may also retain timestamps, images, operator details, and inspection criteria.
These records support traceability and make it easier to investigate recurring assembly issues.
For example, repeated failures at one inspection station may indicate a problem with component handling, work instructions, or the assembly process itself.
Loopr explores this in more detail in its article on AI and real-time data for manufacturing traceability.
6. Limitations of Visual Inspection
Visual inspection can verify many assembly characteristics, but it cannot confirm everything.
A camera may show that a connector is installed without proving that the electrical connection works. Similarly, visual inspection cannot replace required torque measurements or functional testing.
Automated inspection systems can also produce incorrect results. An acceptable assembly may be rejected, or a genuine defect may go undetected.
Before deployment, inspection methods should be tested using representative acceptable and defective assemblies under realistic production conditions.
Changes in lighting, component positioning, or product variants may also require the inspection to be reviewed.
Human review remains important for uncertain results and situations where the inspection cannot reliably establish whether a requirement has been met.
Conclusion
Effective assembly inspection isn't about checking everything at every stage. It's about identifying the right points in the process where errors can be detected before they become harder to correct.
Whether inspections are performed manually, using conventional machine vision, or with AI, the fundamentals remain the same: clear requirements, appropriate checkpoints, reliable results, and useful records.
Choose a system only after confirming that it supports the required check, evidence record, review path, and handoff to the next production step. Product capabilities vary, so verify them against current documentation before selecting a specific platform.
Sources and Further Reading
- KEYENCE, Presence/Absence vision inspection.
- National Instruments, A Practical Guide to Machine Vision Lighting.
- Loopr, The Assembly Process.
- Loopr, Machine-vision system components.
- Loopr, AI and real-time data for manufacturing traceability.
