With the ever-increasing influx of automation in the manufacturing industry, the process of prioritizing accurate measurements is needed now more than ever to ensure that products are not only being manufactured on time, but that production is repeatedly accurate. In this continuous, rapid-paced environment, time is becoming an even more valuable commodity, making the time-saving information provided by dimensional inspection an increasing necessity.

What is Dimensional Inspection?

Dimensional inspection is sometimes referred to as dimensional measurement or dimensional metrology and is a specialized field encompassing a broad range of applications from large, complicated parts such as jet engine turbines to extremely small parts with tight tolerances that are part of life-saving medical devices. In manufacturing, this object might be a prototype for R&D, or a custom-made component for a new production line, or even a finished product. Simplified, dimensional inspection can be seen as the process for comparing what an object actually is to what it is supposed to be, and uses quantifiable values to measure virtually any physical characteristics such as:

  • Length, width, and height.

  • Angles and perpendicularity.

  • Roundness, flatness, and other geometric characteristics.

  • Position.

  • Shape profiles.

  • Surface characteristics.

  • Edge sharpness.

  • Thickness and uniformity.

Why is Dimensional Inspection Important?

In general, when dimensional measurement is required, the results must be both accurate and precise. Although multiple techniques and countless applications exist, there are four main categories of dimensional inspection.

First Article Inspection. Implementing a manufacturing process requires extreme attention to detail, especially if the process is new or if the equipment has been specially designed. First article inspection is required to ensure that the equipment was properly installed and calibrated, and to verify the manufacturing process. Dimensional measurement is used to compare the first objects from the manufacturing line to 3D CAD models, engineering drawings with tolerances, and/or other specifications.

Quality Control. When objects coming off a manufacturing line must meet certain quality standards, measurement equipment is used to confirm that the dimensions fall within the required tolerance levels. In some cases, when the tolerance levels are more rigorous, each object is measured automatically. In other cases, batches might be spot-checked for quality.

Regulatory Compliance. In industries for which regulatory compliance is necessary, dimensional measurement ensures that the required specifications are met. Regulatory bodies such as the FDA or FAA often have requirements regarding the dimensions of certain components.

High-Precision Engineering. When creating a larger assembly from smaller parts, high precision is often required. If a minor flaw or inconsistency can impact an entire process, ensuring that such imperfections do not exist is critical.

The entire purpose of manufacturing is to create a process that is repeatable, scalable, and reliable. When successful, it results in lower production costs and lower pricing for the end user. However, precise, accurate measurements are required for any manufacturing line to be successful.

Dimensional inspection during the production process can help prevent costly errors such as:

  • Flawed batches. A production line that creates duplicate products requires precision setup and alignment. Errors in the production line can lead to entire batches of flawed goods, which can be quite expensive in both time and cost to correct the production error. Dimensional measurement is worth the investment to prevent these expensive mistakes.

  • Defective parts. Using dimensional inspection as part of the quality control process for high-precision manufacturing ensures that each part meets the required specifications.

Quality control is essential for producing consistent products, but implementing dimensional measurement even earlier in the process can help prevent costly manufacturing errors that can lead to problems.

What Factors Are Involved in Using Dimensional Measurement Equipment?

Coordinate measuring machines (CMMs) range from basic XYZ readouts utilizing hard-probes to fully automated systems with articulating continuous contact probing that can perform CAD model-based inspections. (Credit: Q-PLUS Labs)

After establishing which metrics you need to measure, the next step is determining what dimensional measurement equipment can meet your needs, and whether you have the capabilities to conduct these measurements in- house or need to outsource the process to an accredited dimensional inspection lab. You must also consider a broad range of factors and prioritize the ones that are most important for the specific application.

Sensor Type. The most important principle in the field of dimensional inspection is that the item being measured must not be altered during the course of inspection by the measurement process itself. This is metrology's prime directive.

Therefore, one of the most important decisions when integrating dimensional inspection equipment is selecting the right type of sensor. This will depend largely on the characteristics of the object being measured and may include touch sensors, noncontact sensors, or a combination of both types. To narrow down the options, answer a few key questions about the nature of the object:

  • Is the object rigid or pliable? Rigid objects can often be measured with either a touch probe, laser, or camera sensor.

  • Is the object's surface reflective? If the surface is reflective, many laser sensors may not be appropriate because the measurement results may be incorrect or even completely invalid as a result of light scattering.

  • What color is the object? If the object is matte black, difficulties may be encountered with structured light scanners. If the object is clear glass, there may be complications with focusing using vision systems.

  • Can the object be touched? Softer objects such as elastomers typically require noncontact sensors that will not compress or distort the surface during the measurement process.

  • Is the object very small? Not all sensors can produce the same resolution. Small objects may require a more sensitive sensor or a different type of equipment altogether.

  • Does the object have internal geometry that cannot be seen? If the object has cavities or passageways, specialized equipment that incrementally slices the object — or can see through it using computed tomography — may be needed to measure these surfaces.

Tolerance Requirements. Equally important as selecting the right sensor type is ensuring that you understand and meet your tolerance requirements. In dimensional measurement, tolerance refers to the acceptable deviation from the desired outcome. For example, a medical device may need to be within 5 or 10 μm of the required diameter in order to pass an inspection. On the other hand, a toy part may have a larger tolerance in the range of millimeters.

Understanding the required tolerance levels will help you select the right type of dimensional inspection equipment. Think about the difference between measuring with a ruler versus calipers. You can achieve a much higher degree of accuracy and precision with calipers. Now think about measuring with calipers versus an indicating micrometer with even higher accuracy and precision. While an indicating micrometer might be able to achieve the highest level of accuracy and precision, if your tolerance level is not so tight, then calipers may suffice (and will be more affordable).

Portability. This is typically an easier question to answer, but it does narrow the type of equipment you need to purchase. Will you be taking measurements in the field or at a single location where the equipment can remain stationary? Most of the time, the choice will be obvious and is driven by variables such as part weight, size, and delicacy. However, if you are uncertain, think carefully about how you will use the equipment in the future. While it might be tempting to purchase portable equipment to have more flexibility, if you truly do not require portability, you may be sacrificing other features that you could get with stationary equipment. For example, the accuracy of a stationary coordinate measuring machine (CMM) will be higher than the accuracy of a portable CMM articulating arm.

Medical Design Briefs Magazine

This article first appeared in the December, 2017 issue of Medical Design Briefs Magazine.

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