Quality Control in the Manufacturing of Metal Parts

Written by Dr. Bhargav Raval | Updated: September 24, 2025

The Meaning of “Quality”

Most people would agree that there is a great deal of leeway in how one defines quality. The term has a wide range of possible interpretations.

As a manufacturer, however, you care only about perfect quality—a condition that can be precisely specified, validated, and repeated.

The quality of an object is defined as “the extent to which its innate characteristics satisfy requirements” in ISO 9000:2015, section 3.6.2.

Quality control (QC) is performed throughout the production of metal components to make sure they are defect-free and will perform as expected before being used in the final product.

A well-designed QC plan also aids in meeting production deadlines and staying under budget. It also aids in preventing difficulties with product safety and reliability that can drive up costs, lead to product recalls, or put users or consumers in harm’s way.

Since having high-quality components is crucial to the success of your end product and application, your parts supplier should fully grasp your expectations.

We hope that the quality assurance advice presented here will be useful to you and your supplier as you work together to complete your manufacturing project.

Specifications for the Individual Components

The means by which one accomplishes one’s goals

Tools for precise measurements

Extensive checks for quality

What Information About the Parts Is Necessary for Your Supplier?

The first step is to give comprehensive design specifications that outline the critical features and requirements of the parts you desire.

Specify the Features of the Constituent

Send an engineering sketch to your manufacturer detailing the specifications needed for the finished product.

Everything about the part, from its straightness to its flatness to its circularity to its concentricity to its cylinderness to its perpendicularity to its parallelism to its profile to its runout, should be depicted on the drawing. Customers care most about dimensions like length, OD, and ID (in the case of tubing) when purchasing from Metal Cutting.

Here is where you can give a thorough description of the part, along with measurements and tolerances. The tolerances you offer will indicate your supplier the range of allowed error for making items that still fit your specifications.

A hole or pin, for instance, that must be perpendicular to a theoretical axis, may have a perpendicularity tolerance noted on a part drawing. The perpendicularity of small items, like metal rods or tubes, is often stated in terms of the squareness of the end cut for Metal Cutting customers.

You must think ahead to the needs of those who will use your product or service next. A product may include multiple tubes within tubes, all of which must be compatible with one another during assembly. For this to work, the tubing parts must have excellent OD and ID concentricity.

To how many decimal places should measurements be rounded, and whether or not you utilize the standard or metric system, must also be made clear. When translating part tolerances from inches to millimeters, you may also need to account for the maximum and minimum allowed deviations from the nominal value.

Additionally, conversion and rounding can alter the accuracy of the tools employed. For this reason, at Metal Cutting, we always make sure that our measurement device is accurate to the highest and lowest possible tolerance levels that our customers have selected.

Put Necessities First

In addition, your parts supplier has to know which qualities of the parts you want are most important to you in terms of manufacturing quality.

Although it is always preferable to have all of a part’s features conform to specifications, this is not always doable. As a result, it is crucial that you rank your needs and communicate these to your service provider.

The purpose of the component, or what it must accomplish.
Its Compatibility with Other Parts
How it communicates with other components
Tolerance Variability

Don’t over-engineer the part by requesting the tightest tolerance on all characteristics; doing so would increase the difficulty and cost of making the item.

Again, the trick is to request tolerances that are just tight enough to get the job done, but not so tight that making the item would be too expensive.

It is important to prioritize which dimension is more important when a single part has numerous properties that need tolerances, such as a diameter and a radius.

In most cases, the performance of a part in its final application is determined by the most critical dimension, so it makes sense to place the tightest tolerance on that dimension. The cost of a part is often determined by the type of machine and tools required to achieve the required tolerance level.

Incompatible Needs

It’s not uncommon for a component’s design to include requirements that are at odds with one another. How do you proceed when two or more needs seem incompatible?

A customer of Metal Cutting Corporation, for instance, may request a component that requires both deburring and a certain radius. However, if a smaller radius is essential for functionality, the customer may have to forego deburring because the process of tumbling the part to remove burrs would increase the size of the radius.

Or, the buyer may specify a large radius for the part, although such a radius may reduce the available diameter. It’s back to the consumer to decide which dimension matters more: the diameter or the radius.

In other circumstances, a compromise may involve increasing or decreasing a tolerance to allow for the achievement of the most crucial dimension. To maintain a tight diameter that is critical to the part’s function, for instance, a customer may need to relax the radius tolerance.

Locate Primary Resources

Which raw materials offer the qualities required to fulfill your parts’ specifications? In other words, what resources are necessary to accomplish the desired goal?

For a piece of material to feed well through a machine, for instance, it may require a specific degree of straightness. Or the part may need a certain chemical composition to ensure it doesn’t react with other components during assembly.

Raw material suppliers should be carefully investigated, and an approved supplier list should be compiled for future use or for submission to the parts maker. Your purchase order should detail not just where you may find the raw materials you need, but also the specific size, quality, and quantity you require.

After receiving a shipment of raw materials, you or your supplier must inspect the goods to ensure they are up to par with the order and your specifications.

How Do We Plan to Meet Your Needs?

The next step is for your parts provider to detail how they intend to obtain the greatest results at the best cost once you have set the requirements of your project, including the design standards, your priorities, and the materials to be used.

Recording the Quality Assurance Procedures

Your service provider should offer written documentation of all aspects of their quality control system, including processes, general procedures, and anything else that may have an effect on quality. Some examples of information that could be included in the docs are:

Capacity, tolerance, tools, and features of equipment
Competence, education, and experience of employees
Procedures for using the tools provided
Keeping tabs on processes and records
Standards and accreditations for quality
Monitoring of resources all through production
Inspecting, Checking, and Verifying

In a nutshell, the paper trail must prove that your manufacturing partner can carry out your instructions and reach your quality benchmarks. A partner may provide information on how to reduce metal expansion due to production-related factors such as temperature, humidity, and air pressure.

You and your business partner should outline in detail the procedures and equipment that will be used to manufacture the component in question.

For instance, the degree of inspection has a significant effect on whether or not surface faults are found in a material. Defect-free surface inspection necessitates the use of a specific tool and a certain magnification level.

In addition to detailing the frequency and scope of inspections, the paperwork should also cover:

Document the findings of the inspections.

Address any problems that may develop.

Separate out any items that don’t conform.

Fix all the problems that you find.

Check to see if the problems have been fixed

Keep up with Quality Control Procedures

ISO 9000 criteria are used to establish, document, and support quality control procedures in the metal parts manufacturing industry, among many others. Auditing, risk management, environmental management, social responsibility, and food safety are just some of the other areas where ISO has developed standards.

Companies in the medical device industry, the biotechnology industry, the pharmaceutical industry, and many others must adhere to stringent standards such as the Food and Drug Administration’s (FDA) and Current Good Manufacturing Practice (CGMP) requirements. Six Sigma, Kaizen, and lean manufacturing are just a few examples of the process improvement strategies used by numerous businesses today.

Metal Cutting Corporation’s quality management system (QMS) follows the most recent version of the ISO 9000 standard, and we continue to maintain our current ISO accreditation. This ensures that the documented procedure we design for each client meets not only ISO standards but also their individual criteria and quality benchmarks.

How Will Reliability Be Kept?

An additional vital feature of manufacturing quality control is the capability to measure parts and provide assurance that they have been created according to their specifications. For this reason, it is crucial that you and your manufacturing partner employ identical instruments and that these devices are properly cross-calibrated.

Measurement Accuracy Through Careful Tolerance Establishment

The equipment that will be used to measure the dimensions of the finished items must be checked and validated to ensure that it will deliver the accuracy required.

Consider the device’s capability for tolerance, as well as the tolerance to which you need to measure, when determining the calibration tolerance.

While the full range of a laser micrometer might be from 0.005″ to 1.000″, the parts being measured might only span from 0.025″ to 0.050″. Using pin gages with diameters ranging from 0.020 inches to 0.055 inches is one example of a calibrated library that may be put to use in the calibration process.

When using a pass-fail inspection method, such as when measuring the dimensions of extremely minute components is problematic or impossible, calibration is also required. For pass-fail inspection of very small diameter tubing and other items we create, we frequently use NIST-traceable pin gages (or plug gages).

Each category of small pin gages (XXX, XX, X, Z, etc.) has its own tolerances for the manufacturing variance that is acceptable. Class XXX pin gages, for instance, have a tolerance of 0.000010″ (0.00025 mm), making them suitable for evaluating small ID items with tight tolerances due to their straightness and uniform length.

Metal Cutting’s quality control procedures include sending out pin gages on a regular basis to be calibrated by an independent, accredited laboratory. To ensure uniformity, accuracy, and dependability, we periodically send out the instruments we use to calibrate other devices internally for recalibration to NIST standards.

Examinations of the R&R Gages

The amount of variance that can be attributed to (1) the device and (2) the personnel (such as machine operators or part inspectors) doing the measuring can be determined through a repeatability and reproducibility (R&R) analysis of the gage. The producer can then take measures to lessen the variation if it determines the source.

Ten parts would be measured three times by three separate operators/inspectors using the same gage in a random order for a typical gage R&R study. Variations, such as those between parts or between operators/inspectors, can be traced back to the original cause by analyzing the measurement data.

To clarify, R&R studies can be conducted on any sort of measurement tool, test procedure, or inspection system, not just gages. The research could be used before to implementing a new tool, during the induction of a new technician, or during routine maintenance.

How Will We Know If It Works?

Naturally, inspections at various points throughout production are essential to any QC program.

Metal inspection, for instance, is commonplace in the business of producing small metal parts at multiple points, including during receiving, manufacture, and packaging and shipping. Mechanical techniques like eddy current testing (ECT) are used to discover surface defects, but other methods, such as visual inspections by eye or optical instruments, pass-fail (go/no-go), and eddy current testing are also possible.

The frequency of inspections, as well as the strategy and equipment to be employed, should be mapped out in advance and documented.

Perhaps the surface roughness of your final metal products must meet extremely high standards. To assess if the parts are up to grade, you and your business associate may decide to use a surface finish specification chart.

Or maybe you need a very thin wire that is absolutely straight for use in a medical device, but the part’s diameter is so small that measuring it with a pin gage, micrometer, or any other equipment is a laborious, time-consuming, and expensive endeavor. Metal Cutting could suggest a test for straightness based on the medical device standard ASTM F2819.

Additional tips for a reliable inspection procedure in the components manufacturing industry are provided below.

Checks at the Beginning and During the Process

Your components supplier should set inspection locations during the manufacturing process to keep the quality of the parts at an acceptable level and catch any differences before they have an effect on the product.

There will be checks at the beginning of the process, whenever tools or wheels are switched out, and at predetermined intervals during the manufacturing cycle.

Manufacturing metal components inevitably causes some tool and wheel wear. A firm can lessen the effects by replacing worn machinery before the products they produce are out of tolerance, thanks to in-process inspection.

If you are machining an item to get a specific feature, for instance, that feature will shift as the tool wears. Constant quality control checks allow a producer to:

Recognize whether the tolerance range’s upper or lower boundary is being reached

To keep the parts inside their tolerance range and get the measurement back to nominal, you should switch tools.

The deviation from nominal is constant and rather predictable, allowing for interim milestones to be established. The manufacturer of the component can also set a tool change tolerance to ensure that the tool is constantly monitored and replaced before it exceeds the acceptable range of variation.

If a piece checked at the checkpoint is acceptable, it is assumed that all pieces produced since the last inspection are as well. If the part is flawed in some way, every single one that has been made since the last inspection must undergo a full inspection.

The cycle is repeated until either the machine is fixed or the 100% examination reveals that no more parts are within tolerance.

Nonconforming materials and components must also be isolated from the production flow. Metal Cutting has a strategy in place to ensure compliance with ISO 9001:2015 by:

Stop the production line immediately and remove any parts that don’t meet quality standards.

Keep track of everything from start to finish in the manufacturing process

Keep in mind that nonconforming parts may get through a sample inspection checkpoint due to random manufacturing issues.

Something as small as a metal shaving or a particle of dirt could become lodged between the tool and the material for just long enough to ruin a single component. The in-process sample inspection would still pass the batch unless that one nonconforming component was checked at the checkpoint.

Conclusions and Sampling Strategies

Manufacturers can only tell if their entire production run is up to snuff in terms of quality if they randomly pick samples from the batch.

In high-volume production, especially for small metal parts, a sampling plan is much faster and cheaper than examining each individual item. An indicative of whether or not a lot is defect-free can still be obtained by statistical sampling.

Metal Cutting suggests an Acceptable Quality Level (AQL) sample plan for final inspections to ensure the final product is up to your standards.

At the outset of each project, we create the sampling strategy, documented technique, and other requirements with the client. Typical components of the strategy are:

What characteristics of the completed components will be checked, and how.

The schedule for all inspections, both interim and final, and when they will occur.

The percentage of each lot’s components that will be analyzed based on a random selection is determined by the AQL and index values.

If a random sample of a lot’s components fails inspection, we do a full examination of the entire lot. This is known as a zero acceptance sampling plan, or AQL 1.0 c=0.

This means that, during final inspection, a representative sample of all manufactured components is selected to ensure that they meet quality standards. Example: if there are 5,000 little metal components in a lot, we might randomly analyze 50 of them.

If a single item is found to be defective during inspection of the randomly selected sample, the entire batch must undergo a full inspection of the relevant attribute. If every component in the sample passes quality control, then the whole batch must be good.

The sample plans employed in our business are based on well-established probability theory, which suggests that there is a high possibility that all the parts in an accepted lot are good. A zero acceptance sampling plan is a statistically reliable, efficient, and cost-effective technique of insuring quality outcomes, while there is always a small chance that some of the parts won’t be up to grade.

In sum, what elements make up the whole picture of excellence?

When the parts pass inspection at the end of the assembly line, quality control doesn’t stop there. The final phase of the quality control process involves carefully packing the assembled parts to ensure they survive the journey to their final destination in one piece.

You and your manufacturing partner may guarantee high-quality components for your use case by adapting a QC program to the specific needs, variables, and problems of your project.

After that, each component can do what it was designed to do, resulting in a finished product that lives up to your standards and gives your consumers what they want. Therefore, the success of your application depends on manufacturing quality control.

Therefore, it is crucial to your company’s long-term success to collaborate with a partner who is dedicated to manufacturing quality control.