Precision Measuring Instruments: 3D Scanner and CMM compared

Quality assurance in the realm of manufacturing plays a pivotal role, particularly when we turn our focus towards the production of hardware components. In the landscape of hardware production, the lynchpin for determining the quality and competitiveness of the produced items resides within the oversight of the Product Quality Control Department. The Quality Department diligently manages the technical parameters inherent to production processes, as well as the inherent characteristics and properties of the manufactured components using various measuring tools. Measurement processes constitute a noteworthy share, accounting for up to 15% of the total production expenditure. Overlooked or poorly executed measurements at one juncture can result in grievous errors during the assembly process downstream. Historically, the quality of measurements remained vulnerable to a single factor that remained unpredictable – the human element. Even when investing in instruments of supreme precision, such as rulers, calipers, and micrometers, the outcome hinged on human proficiency and vigilance. The consequences of even a minute human error or miscalculation in dimensions could potentially translate into staggering financial losses for the company. One of the well-known cases occurred with the fifth-generation Lockheed Martin F-22 Raptor fighter.

A thickness measurement error transpired during the application of radio-absorbing material (RAM), a material pivotal for ensuring the stealth attributes of the aircraft. A mere oversight in the measurement process resulted in a few millimeters' deficit in the RAM application. Because of the measurement mistake, the material was too thin. This made the jet less invisible to radar. The correction of the error necessitated a labor-intensive and costly process of retrofitting already-produced aircraft, causing budget overruns and project delays. The program has been subjected to public and political scrutiny due to high costs and performance problems caused by measurement error. The prominent defense contractor, Lockheed Martin, experienced a tarnished reputation as a result of this incident, which cast a pall on its future contractual engagements. This highlights the importance of selecting the right measuring tools either prior to or during the pre-production phase.

Nowadays, industry leaders are striving to eliminate the element of human unpredictability from the realm of measurements. To this end, Coordinate Measuring Machines (CMMs) and 3D scanners have been universally integrated into production lines. These advanced apparatuses are equipped with the capability to autonomously execute and archive measurements, thus obviating the need for human intervention. Additionally, they are equipped with real-time monitoring functions to promptly flag discrepancies in component dimensions.

Beyond their utility in quality control, CMMs and 3D scanners serve as pivotal tools in the field of reverse engineering. Reverse engineering involves the generation of a three-dimensional model based on an existing physical prototype. Reverse engineering is instrumental for preserving and replicating objects, especially when they are aged or have suffered wear and tear. Doing this kind of reverse engineering work, especially on complicated shapes, would be really hard without CMMs and 3D scanners.

Modern Methods of Measuring

In today's world, we can broadly classify measuring techniques into two categories: manual and automatic. Manual ones involve humans using various tools to measure the size and characteristics of objects. Think of tools like rulers, calipers, tape measures, micrometers, and calipers – these are good examples of manual measuring tools. Even though we've been using these tools for many years, there's a growing trend in modern manufacturing to shift toward automatic measuring systems. One reason for this change is that manual tools often struggle to measure complex curved surfaces accurately.

Automatic Measurement Systems

Automatic measuring systems offer a significant advantage in terms of accuracy and consistency when compared to manual control methods. Human operators, while capable, are prone to errors owing to factors such as fatigue, skill disparities, or personal judgments. Automatic measuring systems, on the other hand, allow for rapid and accurate measurement and control of components, often achieving results in a fraction of the time required by manual techniques. The increased measurement speed not only enhances productivity but also reduces production timelines, which consequently yields cost-saving benefits.

Presently, the market boasts a diverse array of automatic measuring systems, spanning from conventional coordinate measuring machines to advanced visual inspection systems integrated with artificial intelligence.

Coordinate Measuring Machines (CMMs)

CMMs represent versatile, high-precision machines employed for the precise measurement of an object's geometric attributes. They utilize a measuring probe that makes contact with various points on the object's surface, gathering data to construct a digital 3D model. The main component of the CMM is a measuring probe, typically affixed to a mobile mechanism. These probes manifest in diverse forms, encompassing contact-trigger and continuous-contact variants. Contact-trigger probes establish point contact with the object's surface to procure data, whereas continuous-contact probes sustain contact throughout the entirety of the measurement process. CMMs operate within a three-dimensional Cartesian coordinate system, comprising the X, Y, and Z axes, which are mutually perpendicular and collectively delineate the spatial dimensions wherein measurements transpire.

During a measurement operation, the CMM shifts the sensor in the X, Y, and Z directions following a predefined measurement trajectory.

In the case of contact-trigger sensors, the sensor descends to make contact with the object at designated measurement points. Upon contact, the sensor relays a signal to the CMM, registering the coordinates of the contact point.

In continuous-contact sensors, contact with the workpiece's surface is sustained as the sensor traverses a pre-programmed path, continuously accumulating a stream of data points. These data points encompass three-dimensional coordinates in the X, Y, and Z dimensions for each location.

Subsequently, the amassed data undergoes scrutiny by specialized CMM software to calculate diverse measurements, such as distances, angles, and deviations from specified (nominal) dimensions. This software can further generate a three-dimensional digital representation of the object's surface, often referred to as a 'point cloud'. Armed with a point cloud and dimensional information, mechanical engineers can engage in reverse engineering activities, which involve creating a solid model within a Computer-Aided Design (CAD) software program.

Advantages

High accuracy and repeatability: CMMs excel in providing remarkable accuracy and consistency, rendering them an optimal choice for applications necessitating stringent tolerances, as found in industries like aerospace and automotive. The accuracy of the CMM reaches 1-2 microns.

Control of concealed elements: CMMs possess the ability to scrutinize internal and concealed components of objects, making them invaluable for industries requiring precise assessments of parts that aren't visible from the outside.

Standardization of measurements: CMMs have a well-established history and are widely trusted within various industrial sectors and quality control facilities. They're often employed as the gold standard for measuring accuracy, especially in places like calibration centers.

Disadvantages

Direct physical contact: CMMs rely on sensors that must physically touch the object during measurement. This contact can sometimes be detrimental to fragile or delicate components.

Slow data acquisition: CMMs typically take more time to measure an object, especially if it's complex, compared to the faster data acquisition capabilities of 3D scanners.

Size constraints: CMMs have limitations in terms of the size of objects they can effectively measure. To assess very large objects, you may need specialized equipment, which can involve additional expenses and logistical considerations.

Stationary setup: CMMs are typically stationary machines, meaning that objects need to be brought to the machine for measurement.

3D scanning

3D scanners are categorized into two main types: laser and optical scanners. They operate on distinct principles, but both have the capacity to convert physical objects into digital representations. Here's how they function:

Laser 3D Scanning: Laser scanners emit a focused beam of light onto the object's surface. When this laser beam hits the object, it bounces back after interacting with the surface material. The scanner measures the time it takes for the laser beam to travel from the scanner to the object and back, using methods like time-of-flight or phase shift. A detector captures the reflected laser radiation and records the time delay or phase shift resulting from the laser's interaction with the object's surface.

The scanner repeats this process for millions of points across the object's surface, creating a 3D point cloud. Each point in this cloud represents a specific location on the object's surface and has X, Y, and Z coordinates. To generate a complete 3D model of the object, the scanner is moved around the entire object. Some laser scanners are manually operated, while others are fixed to robotic arms or stationary bases.

Optical 3D Scanning: Optical 3D scanners project a pattern of structured light or a laser grid onto the object's surface. This pattern contains known geometric features recognized by the scanner.

High-resolution cameras installed in the scanner capture images of the object's surface illuminated by the projected light pattern. As the light pattern interacts with the object's surface, it becomes distorted and altered. The scanner's software analyzes these distortions and changes in the projected light pattern visible in the images. By comparing the known geometry of the projected pattern with the altered pattern observed in the images, the scanner calculates the 3D coordinates of points on the object's surface. These points collectively form a 3D point cloud, similar to what laser scanners produce. Just like with laser scanning, the data from the point cloud is processed by software. The software aligns the images, filters out any noise, and generates a complete digital 3D model.

Advantages

Non-contact measurement: 3D scanners operate without direct physical contact with objects, making them particularly suitable for fragile or delicate objects. This non-contact approach mitigates the risk of damage to the subject.

Rapid data acquisition: 3D scanners enable the swift capture of a large volume of data points from an object, providing a comprehensive representation of its geometry in a relatively short time frame. In fact, a sizable object spanning several meters can be scanned in just 10 minutes.

Handling complex geometry: 3D scanners excel at measuring and digitizing intricate and irregular shapes, which can pose challenges for traditional CMMs.

Color and texture information: Some 3D scanners not only capture geometry but also gather information about color and texture, which is especially valuable for reverse engineering and digitizing old parts.

Portability: 3D scanners are portable, facilitating their use in diverse locations. This mobility makes them well-suited for fieldwork and on-site measurements.

Disadvantages

Limited accuracy: While 3D scanners offer high accuracy, typically on the order of 40-50 microns, their precision falls short of CMMs, which can achieve accuracy as fine as 1-2 microns. This disparity is particularly pronounced when dealing with tight tolerances.

Surface-limited scanning: 3D scanners can only capture data from the visible surface of an object, rendering them unsuitable for measuring concealed or internal components.

Challenges with reflective or transparent surfaces: Highly reflective or transparent surfaces can pose challenges for 3D scanners, resulting in data inaccuracies. Special sprays are sometimes required to mitigate these challenges, and the cost of such sprays can be as high as $50 per canister.

Measurement Methods at EnCata

In our line of work, precision manufacturing of components often assumes a paramount role, particularly in projects with the potential to impact human safety. In such projects, strict adherence to design specifications and dimensional tolerances is imperative. To fulfill these requirements and oversee the geometric attributes, we rely on the Faro Coordinate Measuring Machine (CMM). The equipment also proves invaluable for reverse engineering. We are oftentimes approached by large production facilities with requests for reverse engineering of parts and assemblies of large industrial installations. In numerous cases, manufacturers of technological equipment cease to offer support or discontinue the sale of individual spare parts for their equipment post-release. Consequently, when clients seek repairs or replacement parts, they are invariably presented with the option to acquire entirely new equipment. This is precisely where reverse engineering comes into play. Often, the original parts exhibit signs of wear, damage, or operational usage. Using the capabilities of the CMM, we can not only measure the geometry of these components but also employ specialized software to reconstruct any missing elements.

Recently, we were approached by an industrial enterprise necessitating the reverse engineering of a device integrated into their production line. Given the device's exposure to corrosive environments, it was imperative to choose the right materials and coatings.

There were damages on the customer's sample that had to be repaired during modeling with the least deviations, since the device is part of the production process and will be installed in the production line. Furthermore, since the device operates in aggressive environments, it was necessary to update the nodes, taking into account their chemical resistance.

The work began with the chemical analysis. Parts of the device were dispatched to a specialized chemical laboratory to ascertain the composition of the material. These analyses established the use of AISI 304 stainless steel, a material well-regarded for its resistance to acid and its capability to withstand short-term temperature excursions of up to 900 degrees Celsius. AISI 304 is widely employed in the food industry, but it did not exhibit sufficient durability for the particular application in question. During the reverse engineering process, we promptly introduced a more chemically resistant variant — AISI 316L. AISI 316 is developed on the basis of AISI 304, improved by the addition of ~2.2% molybdenum, which makes the steel particularly resistant to corrosion, high temperatures and aggressive environments.

The moment we sorted out the materials, we began measuring the device. The choice of the CMM was deliberate, as the components were predominantly metallic and lent themselves to measurement via contact instruments, thereby assuring a higher precision level in the resultant model. The device was disassembled into its constituent parts, and we generated their three-dimensional models with the help of the CMM. During the modeling process, we updated all the components and optimized the design for manufacturing on modern equipment. After that, we produced a prototype and subjected it to rigorous testing. The device has successfully passed all the tests, ultimately finding its place within the customer's technological workflow.

In closing,

Measurements play a vital role in today's development and production processes. In fact, it's hard to think of any modern business that doesn't rely on automatic measurement systems for tasks like product development, manufacturing, and quality control.  

These systems are invaluable in our daily work, especially when we reverse engineer parts and assemblies. With the help of CMMs and 3D scanners, we're able to not only recreate faulty equipment but also bring it up to date with modern standards.