What is a CMM machine?

Apr 05, 2021
12 min read
140
summary 

For decades now, companies in need of highly accurate measurements have often turned to CMMs (coordinate measuring machines). But more recently, another solution has entered the playing field, one that offers many compelling advantages over CMMs, including go-anywhere portability, ease of use, surface-safe/no-contact needed, 10X-50X faster measurements, and more.

Cmm’s strengths
high accuracy, reliability, durability
Cmm’s drawbacks
costs, slow speed, long learning curve
Alternative: 3d scanners
affordable, fast, easy-to-use, contactless

Introduction

Accurate linear measurements have been a foundation of society for thousands of years now. The very first metrology instrument was the human body itself, whether a finger width, a foot, or a cubit (distance from elbow to middle fingertip). We’ve come a long way since then.

Eventually we invented rulers, calipers, and tape measures. These tools are still with us today, and they have a well-earned place in the metrology toolbox. But then, in the late 1950s-early 1960s, the first three-axis CMMs appeared on the market. This led to portable CMMs with measurement arms showing up in the early 1980s.

With each passing year, the technology has become that much better, making modern CMMs a time-honored quality inspection favorite among thousands of users worldwide.

Beyond the realm of quality control, coordinate measuring machines, whether controlled or programmed, have also been used in reverse engineering, rapid prototyping, and other fields.

How CMM machines work

 CMM measurement of hardened steel worm gear

Imagine taking your finger and tracing it across an object, let’s use a raised relief map as an example. You slide your fingertip around the edge of a mountain range, through a deep valley, and then ascend right up to the highest peak. As you do this, the longitude (X), latitude (Y), and altitude (Z) coordinates of your fingertip are changing as you move.

Key point

A touch-trigger probe records points across the surfaces of the part or object being measured.

A coordinate measuring machine functions in a similar fashion. To illustrate this, we’ll focus on using a CMM with a touch-trigger probe, unquestionably the most popular CMM probe in use today. This highly-sensitive probe records points across the surfaces of the part or object being measured.

The three axes (XYZ) where the probe is touching are recorded by the CMM and, when combined with dozens to hundreds of other XYZ measurements, are used to precisely measure the object or part, often for inspection purposes. These measurements are then compared with specifications to verify that the part conforms to acceptable standards. 

Categories of CMM machines

The four basic types of CMM machines include bridge, cantilever, gantry, and horizontal arm. Of these, bridge CMMs are, without question, the most popular, delivering micrometer-precise measurement capabilities. Their relative affordability is due to their low manufacturing costs.

 A batch of engineered machine parts

Cantilever CMMs, designed for working with smaller objects and parts, are normally suited for shop floor usage, where their open access on three sides makes it comfortable for operators to use. Gantry CMMs deliver the optimal precision of a bridge CMM, but for large and heavy parts. Being floor-mounted, gantry machines demand a solid foundation for installation.

Horizontal arm CMMs are the least accurate of all, yet offer multiple advantages when inspecting larger objects and components, particularly with increased production volumes, such as those often found in the automobile, shipping, and railway industries. As long as critical tolerance specifications don’t exceed the chosen CMM’s performance, horizontal arms can be cost-effective inspection solutions.

Kinds of CMM probes

Various types of CMM probes are available, including touch-trigger probes, displacement- measuring probes, proximity probes, as well as multi-sensor probes.

Let’s start with touch-trigger probes. In order for them to transmit their coordinates back to the CMM’s processor, they must be brought into contact with the surface of the object being measured. Whereas displacement-measuring probes work by scanning the part or component, with the probe relaying a stream of surface measurement data to the CMM.

 CMM technician and robot capturing an automotive part

Proximity probes operate in a similar way to displacement measuring probes, in that they don't require the probe to make physical contact with the object. Where proximity probes differ is in their use of lasers or video technologies rather than the optoelectric sensors that displacement measuring probes employ.

Strengths of CMM machines

Turn a few pages in any metrology journal and you’ll be reminded of the fact that CMMs can deliver incredible submillimeter accuracy and precision across a variety of geometries. On top of that, the leading coordinate measuring machines will keep running for years. Investing in such technology can be viewed in the light of expecting to have the equipment around for a decade or more.

Key point

CMMs can deliver incredible submillimeter accuracy and precision across a variety of geometries.

Coordinate measuring machines can be an effective solution for checking parts and components, whether during pre-production, inline, or final inspection. When used within an established quality inspection workflow, CMMs, with their steadfast ability to precisely measure well into the submillimeter range, can save both time and money, and even prevent lawsuits and catastrophes from arising.

Disadvantages

Costs, installation, programming

The price range of a quality coordinate measuring machine can stretch anywhere from around $50,000 on up to $250,000 or more, depending upon features and performance. Even when considering the purchase as once in a decade, that can still be a substantial expense. Installation is a secondary cost that must be factored in. CMMs need to be properly installed in a room with adequate ventilation, on a vibration-damping slab, with a steady supply of compressed air at hand.

 Inspecting an engine block with a CMM

All in all, equipment plus installation can kick up the price of a dependable, yet mid-range coordinate measuring machine to the $500,000 mark, if not beyond. Then there’s the training involved. Being metrology instruments, CMMs need to be operated by technically capable experts. This takes both time and training. Additionally, such experienced technicians are in high demand across the industry.

Once a CMM machine is in place and ready to go, with a qualified operator alongside, besides initial calibration, the machine will also need to be programmed for the kinds of parts and objects to be measured on a regular basis. One-off inspections of unusual components can be time-consuming to set up, as they normally involve special programming ahead of time.

Key point

Large objects, such as those commonly seen in aerospace, automotive, and even archaeology, can demand excessively long stretches of time to adequately measure.

Because CMMs require parts to be properly positioned and then fixed in place prior to measurement, operators need to take this additional time into account for each object. As well, large objects, such as those commonly seen in aerospace, automotive, and even archaeology, can demand excessively long stretches of time to adequately measure. This may easily lead to operator fatigue.

Slow measurements, operator fatigue, risk of damage

One such example of this is when maritime archaeologists attempted to use a CMM machine (Faro Arm) to accurately measure 8-meter-long ship timbers from a Viking-era ship. Each timber demanded more than two hours of measurement with the CMM machine. With 228 timbers in front of them, it would’ve taken the 4-person team more than one year to process everything via that device.

 A maritime archaeologist capturing ancient wooden ship timbers with Artec Eva

Having a fast-approaching deadline, the archaeologists turned to 3D scanning for the task. In contrast with their CMM machine, the handheld 3D scanner they chose captured and processed all 228 timbers in high-resolution color in just one month’s time. Each timber was captured in just 5-10 minutes. And the level of detail in the scans was enough for dendrochronological analyses to determine specifically where the ancient wood originated.

Aside from the lengthy measurement times, when inspecting parts with a CMM machine, using the most common types of probes, it’s a necessity to bring the probe into contact with every surface to be measured. This can prove challenging, especially when some sections of a part are blocked or in difficult-to-reach places. This can result in not being able to measure that surface, instead having to recreate it later on via software. As a result, accuracy is bound to suffer.

 CMM machine measuring the dimensions of an auto part

That same need for contact by CMM machines precludes it from being used in a number of applications. Not only can the measurement probe deform rubber or soft parts when touching them, resulting in inaccuracies, but it’s not uncommon for CMM probes to inflict damage upon a variety of materials. Scratches, scrapes, and other surface defects can easily arise.

For this reason, non-contact measurement methods such as 3D scanning are go-to measurement solutions in fields as diverse as reverse engineering and archaeology, quality control and historical preservation, as well as forensics and beyond. Experts around the world understand that when high-value objects or crucial evidence is at stake, no chance should be left for irreversible damage to occur.

3D scanning as a viable alternative

As mentioned above, professional handheld and desktop 3D scanners can be an effective solution for even the most demanding of quality assurance, inspection, or reverse engineering tasks. At any stage of a production workflow, whether on the factory floor, the loading dock, or elsewhere, in just minutes, such a scanner can capture in submillimeter 3D what a rigidly-fixed-in-place CMM machine could require an hour or more to measure.

Speed, ease of use, portability

A vivid illustration of this took place at the British Museum, when a two-person team was tasked with capturing more than 400 fragile nineteenth-century plaster casts of ancient Maya monuments in just weeks. Physical contact measurement was out of the question, due to the risks of damage, not to mention the time constraints. Accuracy down to the submillimeter was essential, with researchers and museums anxiously awaiting the true-to-life virtual and 3D printed models.

Key point

Professional 3D scanners are easy enough to operate and can be a quick and effective solution for even the most demanding of quality assurance, inspection, or reverse engineering tasks.

Each medium-to-large plaster cast was scanned in only 10 minutes. All the surfaces, including the deep, difficult-to-access engravings and curved edges were meticulously captured in high-resolution color 3D. Even if a CMM machine had been a safe option, the time required for each cast would’ve been well beyond the 1-hour mark, pushing the urgent project past the deadline by months.

 3D scanning casts of Maya monuments with Artec Eva at the British Museum

The best 3D scanners are easy enough to operate, and after just an hour or two of training, even complete beginners can be capturing their way towards 3D proficiency. This means existing personnel are more than capable of learning and using the technology for quality inspection tasks.

Affordable prices, intuitive software, surface-friendly data capture

Unlike CMMs, 3D scanners require no installation fees, training is usually included, and the final price tags are a fraction of the cost in comparison. Lightweight and fully portable for use around the office and out at client sites near or far, high-quality 3D scanners are built to last for many years.

If the device you choose comes with regular scanning software enhancements that multiply resolution and accuracy, then even years later, your scanner will deliver better results than the first day you took it out of the box.

 3D models of a metal part created with the same scanner, Artec Eva. An upgrade to Artec Studio featuring HD Mode improves the results dramatically.

When it comes to capturing clear, unambiguous data of your objects, especially for inspections, the professional 3D scanner you select should include scan processing software that’s easy to use, with straightforward visual reporting features such as surface distance mapping. In addition, being integrated with full-featured inspection packages, for example, Geomagic Control X, is a sizable advantage.

100% non-contact scanning translates to zero risk of damage to whatever object you’re working with. This could be tightly-toleranced silicone aerospace parts, millennia-old fossils, crucial forensics evidence at a crime scene, priceless museum objects, etc. Insurance companies, quality inspectors, and museum staff are among the many who breathe a sigh of relief about this, especially if they’ve had to deal with the aftermath of contact measurement damage in the past.

 Capturing an ancient, fossilized crocodile in Kenya, with Artec Eva

Quicker time-to-market, myriad applications, automation possibilities

In recent years, portable 3D scanning solutions have gained a foothold among manufacturers of all kinds. Such technology gives management the power to achieve carefully-defined standards early on in the production cycle. By doing so, you can optimize the number of design-manufacture-test cycles to a bare minimum, thus saving significant time and money, while eliminating customer rejection rates and avoiding lawsuits.

The high degree of mobility of handheld and desktop 3D scanners, coupled with their rapid capture times, opens up the possibilities of sharing such devices across multiple departments. As soon as a company realizes that its 3D scanner can be used not merely for high-precision quality inspection, but also for reverse engineering, VR/AR training, rapid prototyping, and more, the ROI of that technology can be pushed to the max.

Recent developments in AI and 3D scanning technology have pushed automated 3D scanning solutions to the forefront. With almost no human input required, batches of identical, highly complex objects and parts are captured using the ideal scanning paths, resulting in fast and complete surface captures with the utmost accuracy. In terms of inspections, whether pre- or post-production, professional desktop and robotic arm 3D scanning solutions can reduce turnaround times while precisely spotting any variations and defects. The result is a smoother, leaner workflow, with lower material costs, fewer returns, and superior quality standards.

Conclusion

 Artec Leo: the fast submillimeter-precise 3D scanner with wireless automatic onboard processing

Undeniably, CMM machines can be effective solutions for today’s metrology workflows. However, careful consideration of other measurement technologies should be made prior to any purchasing decisions. As outlined above, for many businesses and organizations, modern handheld and desktop 3D scanning solutions offer a constellation of benefits over CMMs.

Key point

For many businesses and organizations, modern handheld and desktop 3D scanning solutions offer a constellation of benefits over CMMs.

The best way to decide is to see for yourself. Start off by arranging a demonstration of a CMM machine, in close proximity to a professional 3D scanning solution, either handheld or desktop, depending upon your application needs. Draw up a list of your requirements beforehand. Then, after each demo, with the price quotes for each solution in hand, be sure to factor in the operator costs and possible device usage by other departments in your company.

In summary, professional handheld and desktop 3D scanning solutions can give anyone, regardless of professional training, the power of portable, submillimeter digital capture and metrology-level measurement, at a fraction of the costs and ongoing maintenance responsibilities of a CMM machine.

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Written by: 

Matthew McMillion

Tech reporter

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