Regardless of how skilled the machinist is or how expensive the machine is, parts on the production line do not always match the design. Defects occur, inconsistencies arise, and factory workers have to catch these issues before it is too late.
So what is inspection in manufacturing? This critical area of production ensures that final delivered parts meet the high standards demanded by the customer. It is carried out throughout the production cycle, from the time raw materials arrive at the facility to the moment before shipment. It may involve observing parts with the naked eye or deploying complex measurement equipment, and often combines multiple strategies.
This article goes over the basics of inspection in manufacturing, looking at the main objectives of inspection, the principal techniques used on the factory floor, and the type of inspection equipment used by workers to validate parts.
The Purpose of Manufacturing Inspection
Manufacturing inspection is a form of quality control conducted by manufacturers in a production environment. It can be carried out at different stages of production using various techniques and equipment.
The basic purpose of manufacturing inspection is to ensure that products are being manufactured according to specifications. This means ensuring they match the geometry and dimensions of the intended design, that they are free of manufacturing defects, and that they are consistent within and across batches.
Inspection benefits both the producer and the customer.
From the manufacturer’s perspective, inspection helps to:
- Reduce scrap and rework costs
- Identify machinery defects or calibration issues
- Prevent product recalls or rejections
- Provide a level of liability defense in case of dispute
- Build stronger customer relations
Quality inspection also leads to a higher level of quality in the final product delivered to the buyer, reducing the need for follow-ups, retooling, and other issues that can lengthen the production cycle.
Verify Dimensional Accuracy
Manufacturers verify dimensional accuracy by using tools like calipers and CMMs to measure parts against CAD models and technical drawings. Measurements must fall within specified tolerances to ensure they function properly and fit into assemblies.
Detect Manufacturing Defects
Visual, manual, and automated checks can all help manufacturers identify manufacturing defects (surface defects, material issues, unwanted marks caused by machine tools, etc.) before it is too late to correct them.
Advanced non-destructive testing (NDT) techniques like radiography can identify internal defects that cannot be seen with the naked eye.
Ensure Production Consistency
Quality inspection in manufacturing helps to ensure that parts are consistent in dimensions and quality across an order.
From first article inspection (FAI) to final inspection, several strategies can be deployed to ensure minimal deviation across a batch. This reduces the likelihood of disputes with the customer or outright product rejection.
Support Manufacturing Standards
As a form of quality assurance, manufacturing inspection is important for supporting manufacturing standards such as ISO 2768, which provides guidelines for general tolerances on linear and angular dimensions.
Customers from certain industries may have other manufacturing standards that must be met, and inspection allows companies like 3ERP to meet those standards.
Where Does Inspection Fit in the Manufacturing Process?
Contrary to popular belief, inspection in manufacturing is not only carried out at the end of the production cycle before shipment to the customer. Different types of inspection are performed throughout manufacture to catch issues that can occur at different stages.
A reliable manufacturer will typically employ inspection methods before, during, and after production.
Incoming Inspection
Incoming inspection, or incoming quality control (IQC) is a form of manufacturing inspection that occurs before production begins. It involves inspection not of finished parts, but of materials, components, tooling, fixtures, and other items that the manufacturer will use to make the final parts.
Common checks performed during incoming inspection include:
- Material certification check verifying Certificates of Analysis (CoA)
- Material condition inspection via visual check and sampling
- Dimensional inspection of tooling
- Overall quality check
Incoming inspection is the first line of defense against manufacturing problems. It is important for manufacturers because it prevents issues down the line that could be the result of a supplier issue, not an internal issue. In this way, it keeps suppliers accountable.
Stopping bad batches at the earliest possible point also helps manufacturers maintain production efficiency. If a metal injection mold is received with small defects, then the plastic injection moldings produced with it are also likely to be defective.
Incoming inspection may be followed by preproduction inspection (PPI), though this focuses more on machinery setup and environmental conditions than the parts themselves.
In-Process Inspection
Once production has begun, in-process inspection or in-process quality control (IPQC) serves to verify dimensions, catch defects, and support process stability on the production line, long before the batch is complete.
In-process inspection of a single part is not always as thorough as final inspection, but it is more economically important to the manufacturer, as it helps to prevent scrap and rework earlier.
It is also the first form of inspection that assesses the work of the manufacturer rather than their outside suppliers, so it is essential for ensuring compliance with standards and regulations.
In-process inspection typically provides immediate feedback to the manufacturer, allowing for quick fixes or parameter adjustments.
In-process inspection can be carried out in different ways, such as:
- First article inspection (FAI) to check the first item off the machine using visual and manual inspection techniques
- On-machine inspection, e.g. with a CNC machine probe system
- Intermediate checks to verify parts at specified intervals across a batch
- Automated optical inspection (AOI) in which machine vision systems identify defects on fast-moving production lines
Final Inspection
Final inspection of manufactured parts takes place before shipment to the customer. It is the final line of defense against inconsistencies and defects and is therefore vitally important for ensuring a good relationship between manufacturer and customer.
Some visual and manual inspection methods deployed during in-process inspection (like caliper or CMM measurement) can be repeated during final inspection to improve reliability.
Other methods are more likely to be performed during the final inspection than at other stages. These include:
- Non-destructive testing (NDT) with X-rays or ultrasonic waves to detect internal defects
- Functional testing of complete and powered-up assemblies to ensure they perform as intended
Final inspection may also involve acceptance quality limit (AQL) sampling for the finished goods, determining whether a batch exceeds the maximum acceptable number of defective units, as well as any necessary regulatory audits.
A related stage of inspection is pre-shipment inspection (PSI) or final random inspection (FRI), which involves sampling of fully packaged goods to ensure product and packaging quality and catch any issues that may have arisen following the main final inspection stage.
Common Types of Manufacturing Inspection
We have so far touched upon different techniques that manufacturers can employ during inspection at its various stages. This section examines those techniques more closely, looking at how they work and what equipment is used.
Visual Inspection
Visual inspection is when workers look at products to assess their quality and identify defects. It is carried out at every stage of inspection and relies upon the expertise of the worker to recognize defects and inconsistencies.
Visual inspection typically precedes more advanced forms of inspection. If a defect can be spotted with the naked eye, it is a waste of time and money to put it through an X-ray machine, for example.
Machine vision inspection using cameras and computers may also be classified as a form of visual inspection. This involves a system of lenses, lighting, and software that can accurately assess parts.
Advantages of visual inspection include speed and a lack of physical impact on the inspected parts.
Dimensional Inspection
Dimensional inspection in the area of quality control that focuses exclusively on guaranteeing accurate sizing of parts so that they perform their intended function and fit into final assemblies.
It is a key part of in-process and final inspection and may also be deployed during incoming inspection to assess tooling and supplied components.
Features of a part to be assessed include:
- Length
- Diameter
- Angles
- Surface profile
Tools for dimensional inspection include manual tools like calipers, micrometers, and gauges, while CMM inspection involves the use of machinery.
Dimensions must fall within customer-specified tolerances. Critical features typically have tighter specified tolerances, and more precise measuring instruments may be used to inspect these features.
Technician Using A CMM Coordinate Measuring Machine For Precision Part Inspection
First Article Inspection (FAI)
First article inspection is technically a cluster of inspection processes, including visual and dimensional inspection, applied to a single unit in order to validate processes and catch any problems at the earliest point of the production run.
FAI is not strictly an internal process, as it often involves the customer. Manufacturers may sign an agreement with the customer to provide a first article inspection report (FAIR) before the entire order is fulfilled.
Non-Destructive Testing (NDT)
Non-destructive testing involves the thorough inspection of a part, including its interior, without breaking it apart. This is usually achieved with advanced equipment like X-ray machines or ultrasonic testers, or with a liquid penetrant.
NDT is important for identifying internal defects and inconsistencies and can pick up on problems that cannot be spotted by visual inspection. It is used more sparingly than other methods but is important for preventing scrap or catastrophic failure.
These techniques are particularly important when inspecting high-value components that the manufacturer would prefer not to damage.
Note that destructive testing is not used for the same purposes as NDT; it is deployed mainly to determine the failure points of parts.
Functional Testing
Functional testing is a later-stage inspection process that verifies that a part performs as intended once fully assembled, powered up, and put through real-world conditions.
It may be considered the last line of defense for manufacturers, and is typically performed during final inspection.
The exact method of testing depends on the type of product being built.
Key Inspection Equipment Used in Manufacturing
Below is a non-exhaustive list of the most important pieces of equipment used in manufacturing inspection:
Calipers
Versatile handheld measuring tool that gives precise linear dimensions of an inspected part; comes in different varieties (Vernier, dial, digital)
Micrometer
Measuring tool that uses a calibrated screw for very accurate measurement of smaller dimensions; sometimes called a micrometer screw gauge (MSG)
Go/no-go gauge
Two-ended inspection tool used to perform a go/no-go test for acceptable sizing; the part must fit with the “go” end of the tool and must not fit with the “no-go” end
Profile projector
Shines light on a small object to project a magnified shadow onto a glass screen, allowing for simple measurement of micro components
Coordinate measuring machine (CMM)
Device that uses a probe to sense discrete points on a part’s surface and determine its exact geometry; typically features a bridge and a contact probe, though some devices are optical
Optical inspection system
Coordinated system of cameras, lighting, and software that automatically inspects parts for visible defects and inconsistencies; often deployed on production line for rapid inspection of batches
Surface roughness tester
Precision measuring tool that uses a very fine contact probe or optical system to assess the texture of a part; also called a profilometer
Hardness tester
Typically handheld tool that measures resistance to deformation to verify part strength
X-ray machine
Non-destructive testing device that uses radiation to inspect inside parts or packaging
Ultrasonic tester
Non-destructive testing device that uses high-frequency sound waves to detect internal defects
Manufacturing Inspection at 3ERP
3ERP is committed to the highest levels of quality control, which has helped us establish excellent relationships with returning customers.
This quality control comes from a combination of staff expertise and strategic deployment of high-quality tools like CMMs, X-ray machines, and on-machine inspection systems. Our inspection equipment includes but is not limited to:
- Industry-leading Hexagon CMM
- YW profile projector
- Mitutoyo height gauge
Our ISO 9001:2015 certification confirms our capabilities. From procurement to final inspection, our workflow follows rigorous standards to ensure quality, traceability, and customer satisfaction.
Beyond that, customers can be reassured by our FAI policy. For bulk orders, a first article can be produced and inspected to verify that the part fully meets design and tolerance requirements before mass production, and detailed inspection reports can be shared with the customer.
For parts and prototypes subject to the highest levels of quality control, request a quote from 3ERP.
FAQs
What is first article inspection (FAI)?
First article inspection is a set of inspection processes, including visual and dimensional inspections, applied to the first part out of the machine before full production proceeds.
What tools do manufacturers use for inspection?
In addition to their own eyes, manufacturers use a range of tools such as calipers, micrometers, and gauges, as well as advanced equipment like CMMs and X-ray machines. Some inspection systems are automated; for example, computerized machine vision systems can “see” parts on the factory floor and quickly detect issues.
When does inspection take place in a factory?
Inspection happens before, during, and after production. Before production, the focus is on material and tooling quality; during production, workers try to identify defects or discrepancies; after production, parts are put through even more thorough inspection and assembled to ensure they function properly.
What is a CMM?
A coordinate measuring machine is a device that uses a probe or other component to sense points on a part’s surface and determine its exact shape and size.
What is non-destructive testing vs destructive testing?
Non-destructive testing uses techniques like X-ray or ultrasound to see inside a part (or simply within a package) without breaking it. Destructive testing intentionally breaks a part to see at what point it will fail; it is more economically feasible for mass-produced, low-cost parts, where the destruction of a few units is acceptable.
How do manufacturers guarantee tolerances?
Advanced metrology and inspection tools are used during dimensional inspection to ensure that part dimensions fall within allowable tolerances.
