Eliminating defective fasteners with high-resolution, 3D-inspection

As automotive, trucking, and heavy equipment manufacturers look to reduce weight, materials such as aluminum, plastics, zinc, and magnesium have become more prevalent. Fastener manufacturers must produce systems with zero defects since production speed and quality depends on them.

Cold heading, moving, and inscribing metal with threads is not entirely predictable, so manufacturers have long relied on sorting systems to remove defective fastnerers before they enter production. Each piece must be inspected for burrs, scratches, thread damage, or head cracks; and dimensional measurement such as thread parameters, lengths, diameters, tapers, radii, straightness, perpendicularity, recess depth, and head protrusion must be confirmed.

Modern high-volume, auto-feed systems and pass-through parts require fasteners to have even more integrity. To maximize production uptime, critical part safety, and prevent potential rework, recall, or liability, manufacturers expect fasteners to have zero defects. And the use of lightweight materials on the mating hole creates an even bigger challenge.

Those trends have led many in the fastener industry to adopt high-resolution, 3D inspection systems for billions of fasteners ranging from bolts, screws, studs, and cylindrical parts to nuts, washers, fittings, and bushings. Gaging, sorting, and cylindrical part inspection systems – incorporating laser, vision, and eddy current for dimensional measurement and determining metallurgical defects – are being used for high-speed inspection.

Challenging materials

The introduction of lightweight materials such as aluminum body panels, engines, and manifolds has made supplying zero-defect parts a challenge when the fasteners are made of steel.

“With the switch to aluminum bodies, such as in the Ford F-150, you can’t weld as you did with traditional steel,” says Mike Nygaard, president of General Inspection, a developer of high-speed measuring and sorting fastener inspection systems.

“Instead, billions of self-piercing rivets (SPRs) are used, and their dimensions, bore size, and other attributes must be exact with no obstructions,” Nygard adds. “It’s critical to ensure not only that steel fasteners don’t damage the aluminum or lightweight material they’re inserted in, but also that they’re flush, fastened properly, and not loose.”

Nygaard adds that high-speed automotive production lines rely on automatic torquing systems, so even a few fasteners that fail to meet dimensional specs out of a million used per hour can stop assembly if they jam and require troubleshooting to get production going again. The wrong sized pitch diameters on threads, burrs on the thread, or similar issues can trigger auto-torque failure, stopping thread insertion.

Quality control is critical because even small defects can cause big problems in working with lightweight materials, such as having to rework engine blocks if male-threaded fasteners strip out of threaded holes. If there’s an obstruction in the SPR bore or the dimension is wrong, the rivet may also fall out or not sit flush, which can become a serious, visible quality issue.

High resolution, 3D inspection

To address these production and quality issues, the increasing use of lightweight materials requires high-speed, 100% fastener sorting, and more inspection capability from fastener sorting machines.

“In the past it was OK to look at a fastener’s silhouette to see if it had a thread or gross thread damage. Now sorting machines need the ability to find any defect,” Nygaard says. “Thread damage can be anywhere, and if you’re not looking at it from 360°, that burr will get you.”

Technologies being used for 100% inspection include:

Multi-laser scanning to gather 3D information to detect defects that may only be on one side of the part such as damaged threads. Laser topography can provide a detailed 3D image of the part, detecting dents, flatness, chips, and dimensional characteristics.

Advanced vision systems can provide images offering greater than 360° coverage of the part, measure all dimensions, and find visual defects such as cracks, scratches, porosity, and laminations.

Axial view vision allows the camera to see the side of the fastener head or flange to detect cracks or recess defects. In addition, an internal lens can expand the view inside the part to detect very small defects by providing a 360° view of the entire length of ID threads.

Eddy current sensors can detect metallurgical defects in plating or heat treatment, which can help to prevent premature fastener rusting and cosmetic defects. This can be vital in safety-critical parts such as a wheel bolt, which may be subject to constant vibration or weather exposure.

The most precise, high-resolution, fastener inspection machines can measure with as little as 2µm of total error, but typical equipment can be several orders of magnitude less precise.

“Eliminating defective parts is all about precision and repeatability,” says Nygaard, whose company, General Inspection, has more than 40 patents or patents pending for high-speed measuring and sorting fastener inspection systems.

“A typical part today might have 10 to 30 dimensions checked, all important to how it assembles and functions,” Nygaard adds. “Any error in measurement increases the odds of falsely failing the part or falsely passing it, either of which is bad for profitability and quality control.”

Key considerations

To minimize error and gain precision in fastener inspection equipment, it is important to ask questions such as: How well is the part positioned in the mechanical handling apparatus? How flat is the surface that part is riding on? Is there any vibration distorting the image? Is the part properly aligned with the optics? But the most important question: Is the sorting machine National Institute of Standards and Technology (NIST) traceable?

ISO 17025 accredited companies must have gaging systems that comply with NIST traceability. Calibration and NIST traceability are mandatory, even though many fastener companies do not realize this requirement.

“A sorting machine is a gage when you’re using it to check dimensions on a part, but the dirty little secret in the sorting industry is that most machines are not traceable to NIST,” Nygaard says. “That can wreak havoc on your production and quality control because what you think you’re sorting for isn’t really what you’re sorting for. It can be like using a mis-marked 13" ruler to measure and sort for 12" parts.”

Cost considerations

Despite the increased precision of today’s high-resolution fastener inspection equipment, they can be as fast as or faster than traditional sorting machines.

“Fastener companies can expect to sort about 300 to 1,000 parts per minute with the best fastener inspection machines, but with better precision and coverage of the part than typical sorting machines,” Nygaard says. “The machine should be flexible enough to accommodate thousands of different part numbers, but there’s a tradeoff in flexibility and speed.”

While the cost of fastener inspection systems is higher than traditional sorting machines, throughout a typical system 5-to-7-year lifespan, the net cost, in high-volume operations, is often $0.001 or less per fastener.

“If you’re making safety-critical, lightweight parts or parts functionally checked by auto-torquing mechanisms, why play Russian roulette with your company’s good name? Would you pay 50% less for a life insurance policy that covered everything, with the exception of cancer and heart attack?” Nygaard asks. “Relying on the most precise 3D fastener inspection systems for 100% sorting will help ensure that fastener companies remain profitable, top-tier suppliers.”

General Inspection

www.generalinspection.com