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.
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?
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.
Size of the Objects Being Measured. You can put a turbofan blade into a machine to analyze its contours, but you cannot do the same with an entire aircraft. Dimensional inspection equipment can also be used to determine metrics such as the space between bolt holes, but only if you use a probe small enough to enter the holes. Knowing the sizes of all the objects you intend to measure will help you select the right dimensional inspection equipment and accessories.
Shape of the Objects Being Measured. Objects that have internal geometry require different measuring methods than those that do not. You may need to purchase accessories that enable internal measurements or use specialized equipment dedicated to that purpose.
The shape of the object can also impact the type of equipment you purchase. For example, if you need to measure the form of a small screw thread you will either need a contact sensor that has a small enough tip (typically knife edge sharp as with a contour tracer) to reach all the geometry, or a noncontact optical sensor with sufficient magnification.
Speed. Do you need to get immediate pass/fail (attribute) quality control results as objects come off a manufacturing line? Or do you require variable data results with more comprehensive data that allows you to understand by how much an object did not pass inspection and possibly why?
Of course there are countless other scenarios, but as with other factors, understanding your requirements will help you make the right choice. If you don't need real-time results, you can often get all the information you need with less-expensive equipment.
Automatic or Manual. Some applications — typically those related to quality control — require dimensional inspection equipment to operate automatically. For example, wall thickness measurements from parts coming off a line can provide real-time pass/fail information for manufacturers. Other applications, such as thread gaging, require manual operation to achieve the desired results.
Ease of Use. Many types of dimensional inspection equipment require programming or other specialized skills to properly operate, such as vector-driven measurements. While training can provide many of the necessary skills, some equipment requires a specialist. If you do not already have, or do not intend to hire specialized staff to operate the equipment, you may find it necessary to outsource some or all of your dimensional inspection needs to a qualified provider.
How Do You Know What Equipment Will Be the Best Fit for a Specific Application?
Here is where knowledge and experience are paramount. Just making the measurement of certain dimensions possible is difficult enough, let alone making the measurements feasible. For every object imaginable, from a nanoscale surface to a rocket engine, there are numerous measurement opportunities. There are also multiple ways to execute each measurement technique, and multiple devices and manufacturers from which to choose.
When investing in dimensional inspection equipment, the primary objectives are to find a device or system that:
Allows for the inspection to be performed correctly, accurately and precisely.
Allows for the inspection to be performed as quickly as possible.
Serves the functions you require.
Is reliable and has a long lifetime.
Can be easily operated and maintained.
Fits the budget.
Types of Dimensional Inspection Equipment
There are three primary types of dimensional inspection equipment: precision hand tools, contact sensor systems, and noncontact sensor systems.
Precision Hand Tools. Portable and generally easy to use, precision hand tools are often capable of providing all the information you need. Some of the advantages of using hand tools include factors such as relative low cost and high portability.
However, some of the disadvantages of hand tools include the slow speeds with which readings can be acquired, their relative inaccuracy compared to dedicated systems, and difficulties in obtaining good repeatability and reproducibility among different users compared to automated systems. Here are some of the most common dimensional inspection hand tools:
Bore and ID gages measure the internal diameter of an object, either by indicating the deviation from a predetermined standard or by providing an actual measurement.
Calipers provide inside, outside, depth, length, or step measurements using various technologies. Certain types can also be used to compare or transfer dimensions from one object to another, or to precisely mark a measurement.
Fixed gages are designed to quickly compare specific attributes to a specified standard. They might measure thickness, length, angles, gear teeth, radius, bead size, and a range of other parameters.
Micrometers are used for precision dimensional gaging and may use mechanical, digital, dial, scale, and laser technology. Micrometers can be used to measure the thickness, length, depth, internal diameter, outer diameter, height, roundness, or bore of an object.
Protractors and angle gages measure the angle between two surfaces. They can be fixed or variable depending on the intended use, and may be designed to provide other functions such as simultaneous depth measurements.
Indicators and comparators amplify the movement of a precision spindle or probe and display the results on a dial, digital display, or column. Different levels of precision are available for a range of applications.
Air gage instruments use changes in pressure and flow rates to measure parameters such as thickness, depth, internal diameter, outer diameter, bore, taper, and roundness.
Plug and ring gages provide a pass/fail assessment for holes and bores, and shafts and pins respectively, based on specified dimensional tolerances. A simple plug gage pin can do one remarkable thing that advanced continuous contact CMMs cannot do: ensure that not a single opposing point reading along the hole or bore is undersize. Since a hole or bore theoretically has an infinite number of diametrically opposed points, a CMM could sample points for a long time, but still only approximate the resultant size.
Threaded plug and gages qualitatively measure and/or verify thread size, spacing, shape, geometry, or other parameters.
Rules and length gages are used for length measurement, and much like a measuring tape or a ruler, have a flat, graduated surface.
Contact Sensor Systems. For some dimensional inspection applications, the best way to obtain measurements is by using a sensor that comes into contact with the object. Though the probing forces can be extremely light, contact sensors work best when the object is rigid and not pliable or fragile. They are also often used when the surface of the object does not lend itself to optical sensors, such as structured light scanners, because it is reflective or too dark. The main categories of dimensional inspection contact sensor systems include the following:
Coordinate measuring machines (CMMs) are mechanical systems that use a contact measuring probe and transducer technology to convert physical measurements of a surface into electrical signals that can be processed and analyzed by metrology software. 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.
Articulating arms are another type of CMM that use rotary encoders on multiple axes of rotation instead of linear scales to determine the position in space of the hard probe or touch probe (laser line probes are also a common accessory). Such systems are manual in nature but are portable and able to reach around or into geometry in a way that cannot be accomplished with a conventional CMM.
Form and contour tracers are purpose-specific devices that use high-accuracy continuous contact sensors with varied styli to obtain small part geometry such as fillet radii and chamfers or that measure roundness, cylinidricity, and other geometric tolerances.
Optical CMMs are a lesser known but increasingly common hybrid of non-contact and contact technology used for gathering measurement data for areas that are difficult to reach. The handheld device transmits data wirelessly and allows the operator to move both the part and the scanner during the measuring process. Using stereoptics to scan an object, the optical CMM uses 2-3 cameras to track either passive retroreflective or active targets through space. This process allows objects to be rebuilt in 3D via the device's reflectors.
Noncontact Optical Sensor Systems. In many cases, the object being measured cannot or should not be touched by a contact probe or any other part of the measurement device. In cases where the object is either soft, elastic, very small, or fragile, optical inspection equipment may be most appropriate.
Optical Comparators. Optical comparators use light projected upon a screen to obtain a magnified silhouette or shadow of the object of interest. Measurements can be made by articulating the table axes to obtain width and height. Physical overlays are commonly used for complex shapes and quick-checks. Modern systems can employ fiber optic edge detection, automated movement and measurement, and even digital overlays.
Vision Systems. Vision systems are similar to optical comparators, but instead effectively project images directly onto a screen while a camera with interchangeable objective lenses and/or zoom optics relays images to the display. Most systems have edge detection and other sophisticated capabilities. Automation via a computer and controller is common, and some systems even come with multiple sensors including laser and touch probes.
3D Scanners. 3D scanners use lasers or structured light to capture 3D information about the given object's geometry. In addition to measuring objects and converting them to digital images, 3D scanners can also be used for reverse engineering as well as for other applications.
3D laser scanners — A laser, either as a single point, line, or an entire field of view, is projected onto the surface of an object and a camera captures the reflection. Each surface point is triangulated, measured, and recorded to produce a 3D rendering of the shape and surface measurements of the object.
Structured light scanners — Sometimes also called white light or blue light scanners, these devices use light from halogen or LED lights to project a pattern of pixels onto the object. The pixels are distorted by the surface of the object when viewed by one or more cameras that are placed at an angle relative to the light projector, and measurements of the light pattern can be used to reconstruct a 3D image.
Range scanners — Range scanners use a time-of-flight laser rangefinder based on LiDAR technology to measure the distance between the laser and the object's surface. The laser rangefinder sends a pulse of light to the object and measures the amount of time it takes for the reflection to return in order to calculate the distance of each point on the surface. Point measurements are taken by aiming the device at the object and using a series of mirrors to redirect the light from the laser to different areas on the object. Although the process may seem cumbersome, typical time-of-flight 3D laser scanners can collect between 10,000 and 100,000 points per second, which is much faster (though less accurate) than contact sensors.
3D noncontact surface profilometers (a.k.a. nanoscanners) — With the advent of nanotechnologies and growing demand for micromanufacturing, the need for the ability to accurately measure very small objects and geometry has increased the need for microanalysis. This technology utilizes ranges from confocal laser microscopy to white light interferometric optical profilometry. Typically, surface finish has required the highest precision in dimensional metrology. In addition to surface finish in 2D and 3D, now very small geometry can be dimensionally characterized and inspected. Relevant characteristics include flatness, wear, texture, sharpness, and other conditions that can affect functionality, but might otherwise not show up in a dataset of conventional metrology measurements.
Dimensional inspection is useful for much more than just production line setup and quality control. Manufacturing (and much more) can greatly benefit from dimensional measurement in all phases of product development ranging from research and prototypes, to first article inspections and capability studies, production inspection, to final inspection of the finished product. The manufacturing process is full of potential pitfalls, especially when you are trying to develop a new product on a tight timeline.
Understanding what it takes to integrate dimensional inspection into your quality process, whether in-house or outsourcing to an accredited measurement lab, will provide value and help your manufacturing process operate more efficiently. An objective that can be measured and that gets properly measured is an objective with the best chance of being brought to fruition.
This article was written by Mike Knicker, President of Q-PLUS Labs, Irvine, CA. For more information, Click Here .