The Human Factor in Metal Inspection

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The Human Factor in Metal Inspection
Are Visual or Mechanical Inspections by Humans Ever Truly 100%?

Metal inspection is a critical step in the manufacturing process, helping to ensure the quality of small metal parts that are the components of (literally) countless products. Yet, the concept of 100% inspection of parts is a somewhat tricky proposition. That’s because, no matter what method of inspection you use, it is impossible for people to examine every tiny segment of a part’s surface — which means 100% inspection is not exactly 100%.

Whats, Hows, and Whens of Metal Inspection

Here at Metal Cutting, we typically do a sampling plan at the beginning of every project, in which we spell out what dimensions of the parts we produce — their diameter, length, and so on — will be inspected, as well as how and when those inspections will occur. As a rule, we use a c=0 sampling plan with an associated Acceptable Quality Level (AQL), which determines how many randomly selected parts in each lot will be inspected. Based on this plan, if just one randomly selected part fails inspection, then 100% inspection must be performed. (We’ll talk more about sampling plans in our next blog.)

Metal inspection may occur in receiving, at a designated step (or steps) in the production process, at the end of the manufacturing (prior to packaging and delivery), or all of the above. In addition, an inspection might be requested as a result of a chance discovery. For instance, when doing an inventory of incoming customer-supplied materials, such as long rods or tubes that will be cut off into small parts, we might notice and alert the customer to gouges, scratches, or other surface defects that could impact the end product. At that point, the customer might ask might ask to have the material returned to the supplier, give authorization for us to use it as is, or ask Metal Cutting to do a 100% visual inspection to eliminate defective materials.

Since inspecting metal parts for quality is part of our DNA, we are always ready, willing, and able to meet such requests. However, the price of the job may need to be requoted if the customer-supplied material causes increased inspection time. In addition, we always remind customers that even 100% visual inspection cannot be 100% guaranteed.

To be sure, we are well versed in and have an array of automated visual inspection machines with various capabilities that have very high accuracy, repeatability, and reliability. However, these machines are designed in advance for specific tasks. Our automated inspection machines are not designed for the kinds of unintended visual defects that occur on metal parts and are caused by our vendors or our customer’s raw material vendors. In transactional terms, no one wants to pay us to have and use a machine to find something that should not be there in the first place. Therefore, these added-on 100% visual inspections are done by human eyes.

Is Visual Metal Inspection 100%?

Visual inspection is commonly used to detect a variety of surface finish issues in metal parts, from corrosion and contamination to cracks and surface irregularities that can have an impact on product performance. This type of metal inspection ranges from examination with the naked eye to the use of sophisticated optical tools, such as high magnification microscopes.

Performing 100% visual inspection generally involves looking at small parts at the closest possible level – for example, rolling small round rods or tubes under a microscope to examine every tiny section of the part’s surface. It requires a human being to physically move the part while ensuring that the surface is being examined at all 360º of the tube or rod’s circumference along 100% of the part’s length. Blink, and a minuscule but important flaw can easily be missed. What’s more, visual inspectors, utilizing their experience, decide if a part passes or fails by judging a defect’s relative size, coloration, depth, and other traits, making the results subjective.

In addition, human beings are flawed and even the most diligent inspector can make mistakes. In fact, human error is a normal part of any process where people are involved, and especially where tasks are high volume and highly repetitive. Among a vast array of productivity studies and data, the consensus is that for high volume activities, human beings can often show accuracy rates of 80% to 90%, with a steep fall off in accuracy after a certain volume of repetitions. A human handler might misread a micrometer, put parts in the wrong bin, or simply “blink” at a crucial millisecond. Even where you have a binary, pass-fail method of inspection, judgment and human error will have an impact on results of a 100% visual inspection.

It’s not that any particular person is incapable of 100% accuracy in a particular task. But unlike a mechanized or computationally executed task, a human being finds it very difficult to be perfect. After all, to err is human!

Is Mechanical Metal Inspection 100%?

Of course, metal inspection is not limited to visual inspection “by eye” or using optical tools; many manufacturers (including Metal Cutting, as noted above) also utilize various machines that can automate much of the parts inspection process. It is possible to purchase a piece of equipment that would inspect 100% of parts perfectly for dimensions and some color characteristics. But whenever there is a need for judgment or no standard for pass-fail, mechanical inspection (like visual inspection) cannot be 100% guaranteed.

For instance, eddy current testing (ECT) utilizes electromagnetic induction to inspect metal parts for surface flaws such as cracks, pitting, and corrosion; it is also used to measure the thickness of parts such as thin-walled tubing. However, because ECT is based on indirect measurement, the correlation between instrument readings and the structural characteristics of the parts being inspected will vary and must be carefully established according to the specific application. (To learn more about ECT, you can read this blog or give Metal Cutting Corporation a call.)

There is also a human factor involved in the mechanical inspection of metal parts, creating room for human error in vital steps such as programming the machines and setting the lighting for tools like vision measuring systems. Human beings literally have a hand in tasks such as feeding parts into machines and applying pressure when using micrometers and other measuring equipment.

In addition, people often forget that computers, too, have limitations. For instance, even the best machine cannot check all points of inspection simultaneously; rather, it must work sequentially, and its speed of operation has an impact on yield and throughput. A more deliberate, slow process is more costly, but it delivers better resolution; the faster the automated process, with more pieces inspected per second and greater throughput, the lower the computer resolution.

Metal Cutting saw this firsthand when a customer’s order required us to check tens of thousands of parts every day using a laser micrometer. We found that the slower we ran the micrometer, the more accurate the measurements were; conversely, when we ran the automated system more quickly, the decimal moved to the right and the tolerances were looser.

Spell Out Your Metal Inspection Requirements Up Front

Along with the advanced equipment and optical tools that are available today, visual metal inspection continues to play an important role in quality control. Even where automated inspection methods are the norm, additional visual inspection may be done to spot-check results. But whatever inspection method you choose, it is extremely helpful to decide what needs to be inspected, how, and when, at the point when you are requesting an estimate from your small parts supplier.

Since metal inspection, and especially 100% inspection, adds to your costs, your supplier can give you a more accurate price quote if your inspection requirements are stated clearly before your project begins. It is also helpful for your parts supplier to know what you DON’T need, since excess quality — such as tolerances that are overly tight or unnecessary inspection steps — can add to production time and material costs without deliverable measurable product benefits.

For tips on specifying tolerances, materials, cutting methods, and other requirements for your small parts needs, download a free copy of our comprehensive guide, How to Fine-Tune Your Quote Request to Your Maximum Advantage: Frequently Asked Questions in Small Parts Sourcing

For tips on how to choose the right vendor for your metal parts needs, please download our free guide, 7 Secrets to Choosing a New Contract Partner: Technical Guide to Outsourcing Your Precision Metal Fabrication.

Metal Cutting Corporation manufactures burr-free tight tolerance parts from all metals. We provide the precision required by medical device, automotive, electronic, biotechnology, semiconductor, aerospace, fiber-optic, electrical and many other diverse industries.

We are specialists with over 45 years cutting, grinding, lapping, polishing and machining metal parts. Our experience, inventory and capabilities provide the skills and capacity to meet the needs of technology device manufacturers. Specialty metals, micron tolerances, low or high volumes, complex metrology--all these and more are the requirements we achieve every day for products shipped worldwide.