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Metrology Tools for In-Process Inspection

Bruce Morey
By Bruce Morey Senior Technical Editor, SME Media

While suppliers are under more pressure than ever to produce precision parts faster and with less scrap, in-process metrology means manufacturers can detect as soon as possible when a part is going wrong, correcting the issue quickly and saving it from scrap. “I am seeing an increasing push for in-process inspection,” stated Jerome-Alexandre Lavoie, product manager for Creaform (Lévis, Québec), echoing a trend in the industry. He noted there’s both a need for in-process inspection where it has never existed, and a need for improved systems where it does.

Using robots to move scanners for in-process inspection provides a convenient means to extend existing technologies, such as this MetraScan 3D-R from Creaform.

There are other challenges for in-process metrology. “The market has changed where parts are more frequently undergoing model year changes, and there is more diversity of parts,” he said. “Every time you change a part, even a little, that means new CAD data, new dies or molds, and constant changes in measurements.” Other challenges lie in the complexity of tolerancing and the skill sets of shop-floor operators. “Starting around the year 2000 and later, there was a renewed push for quality control, especially in using the new GD&T format,” he said, referring to the Geometric Dimensioning & Tolerancing standard as documented in ASME Y14.5. “Starting in aerospace, practically every drawing started controlling critical features like holes with a GD&T callout rather than simple line dimensional tolerances,” he explained. “Now that change is seen everywhere—in automotive, in the recreation vehicle industry, even in jet skis.”

Using GD&T is both superior to the older tolerancing methods and is more exacting, requiring a more knowledgeable technician who can interpret the symbols, establish datum planes, and create an inspection routine. “But there is a lack of qualified people in many areas of manufacturing, including metrology,” said Lavoie. These reasons combined require simpler metrology solutions.

A good example is Creaform’s MetraScan 3D and its robot-mounted variant, the MetraScan 3D R Series. The newest version of the hand-held system, released in 2016, offers a 25% lighter product that measures up to a volumetric accuracy of 0.064 mm (0.0025″) in a 16.6-cubic-meter volume, boasting a measurement speed of 480,000 points per second. The robot-mounted system offers similar accuracies and speeds. The beauty of a volumetric scanning system like the MetraScan 3D is its ability to provide complete measurement coverage in a simple way, just pointing the collection system at the part and waving it around until all surfaces have been measured. The accompanying software creates a point cloud system in the proper reference frame and can further process it. No rigid setup or fixturing is required; the part and the system can be moved freely at any time during measurement, according to the company. It also claims that the system can scan any type of color and material, even black, multicolored and shiny surfaces.

A probe on a CNC machine provides the ultimate in-process inspection, making adaptive machining a reality.

“We are starting to see a lot of interest in replacing existing 2D laser line profile solutions that have dominated the automotive industry for decades with systems like ours that can measure large areas with volumetric accuracy,” Lavoie said. He went on to note that what will be important in that case is not simply measuring parts, or even flagging where measurements are out of tolerance, but how that data will fit into manufacturing systems. “It must be integrated into the entire ecosystem of actual production lines that are running right now, today,” he said.

In-Process, In-Machine

The ultimate in-process monitoring is a probe integral with a machine tool, such as those available from Blum-Novotest Inc. (Erlanger, KY). “In the last few years, in-process inspection on the machine tools themselves has been growing in demand,” said Lilian Barraud, president. He ascribes this growth to many reasons—but first and foremost is improved machine tool performance. Machines cut or grind parts faster, with more precision and more efficiently than ever, which increases the need for in-process measurement to ensure defective parts are not being mass produced. The improvement and variety of available probes is also a factor—it is easier than ever to get the right measurement on the machine.

Blum-Novotest offers probes for locating, measuring and inspecting workpieces, as well as probes for tool setting or detecting cutting tool breakage. It also offers laser systems for tool monitoring that operate within the machining environment. Like others in the industry, Blum-Novotest offers optional means of transmitting signals, via cable, or more conveniently by radio or infrared transmission.

Most important, perhaps, is that Barraud is seeing a change in the mentality of his customers. “For example, in the automotive industry, in the past they would only talk about cycle time and production,” he said. In-process on-machine measurement, no matter how fast the probe, still eats into a machine tool’s cycle time.

Today, companies are more aware that their objective is producing good parts. “They are now willing to accept a slight increase in cycle time to include in-process inspection because it is the most efficient and intelligent way to ensure that all parts are produced or manufactured within tolerance, avoiding scrap, scrap recycling, and postprocess manual gages,” he said. It also means not having to worry about bottlenecks that often appear at the CMM. One example he gives is Blum’s new Digilog probes, which offer an adaptive machining feature.

Expanding their portfolio of such probes is just as important. “There was a demand to measure surface finish of parts, especially complex parts, so that the surface finish could be re-worked without taking the part out of the machine, measuring it, and then resetting it back in the machine,” he explained. Blum’s TC64-RG was developed to resolve this issue, according to the company. It is a variant on Blum’s TC64 Digilog scanning probe. With a special stylus and the gage’s integrated Digilog technology, a technology that allows analog scanning of the part, the gage can accurately determine a part’s surface roughness with the part in its original orientation, thereby eliminating resetting of the part.

The gage inspects surfaces in various axes, unlike post-process conventional profilometers. The company says it is especially useful on five-axis machines, where the gage uses the machine axes to maintain stylus-to-part surface perpendicularity, allowing surface analysis on various contoured planes and profiles in seconds, according to Barraud.

Connecting various sensors into a comprehensive information network is becoming vital for in-process inspection, such as in the BLÚ system from Marposs.

Another way manufacturers are cutting costs and speeding up making parts is to combine operations in a single setup, according to Frank Powell, manager of grinder products for Marposs Corp. (Auburn Hills, MI). “Back in the day, a part would undergo one or two operations on a single machine tool before passing onto the next one. Now, there are machine tools that combine turning, milling, or grinding. Even a dedicated machine tool like a grinder will create many multiple features in a single setup. The higher the number of operations in a single platform, the more measuring systems you need to monitor the operation,” he explained. This requires not just more accurate probes, but ones that provide their data faster and more conveniently.

Examples of improved probes from Marposs are the WRP60P and WRP45P high-accuracy touch probes with multichannel radio transmission. Announced in June 2017, the probes are designed for high-accuracy five-axis machining centers and milling machines. They automatically detect the machine axes position, according to the company, for automatic part and workpiece orientation. But there is a larger issue Powell pointed to—connecting those sensors into a comprehensive information network.

Sensors Need Networks

“In the past, all we [and our customers] worried about was having a device that connected to the sensors and gages and gave specific information to the machine tool and its controller—it was all encompassed in the machine,” he said. “Today, you cannot look at it piecemeal. That is the biggest problem; you need to bring all the parties together.” That is why a comprehensive information network is so important, and why industry is moving into Smart Factory and Industry 4.0 concepts in their various incarnations.

The Marposs solution to this networking need is its BLÚ distributed control network for machine tools, first introduced in 2015. The BLÚ system architecture enables real-time communication between various internal and external machine functions, according to Marposs. The modular system is comprised of a central processing unit located in the machine’s control panel with nodes to control functions, such as part flagging, gage electromechanical actuation and in-process measurement, acoustic emission sensing, wheel balancing, and process monitoring.

“BLÚ incorporates practically every sensor that Marposs manufactures for machining and quality processes, connecting them via a distributed system,” explained Powell. He stressed that the communication between the master processing unit and the functional nodes is handled by a single cable. “It connects either through Ethernet IP or through various field busses for machine communication, and ties the machine’s functionals to the process, providing the machine designer whatever they need for machine control, as well as network to the outside world,” he said. “BLÚ is our first introduction into the concept of Industry 4.0.” With a signal sampling frequency of 45,000 Hz and an ultimate measurement resolution of 10 nm, it is clearly designed for a future of faster, more accurate sensing.

Vision Systems Prove Themselves

Toolmakers are becoming in-process tool inspectors, requiring inspection tools such as the pomBasic from Zoller.

While on-machine probes and portable scanning systems clearly provide benefit for in-process monitoring, so do systems that might seem to have less flexibility, such as staged vision systems. John Kaminsky, area sales manager for Inspec Inc. (Plymouth, MI), observed that most of his business is customer initiated. The key fact is that manufacturers concentrate on “making chips” to make money, but quality is becoming just as important to their customers as part deliveries. “It is often the case that someone will get a job to produce parts in volume, but then as part of the bid package they need to install inspection equipment to prove the parts meet spec,” said Kaminsky. “That is when they call me.”

Inspec provides a variety of services and equipment, from calibrating torque wrenches to refurbishing CMMs and optical comparators. One of its main lines of business is acting as an authorized reseller of vision systems from Micro-Vu Corp. (Windsor, CA). “For the right application, it is hard to beat the price and accuracy of a vision system,” explained Kaminsky.
Kaminsky noted that the sort of parts well suited for automating with vision systems are small, intricate parts, where access with a touch probe is difficult or where noncontact is a must because the part might be affected by the probe itself.

Vision systems are available from Micro-Vu and range from the smaller Vertex machines with 250 × 160 × 160-mm measuring capacity with MPe of (2.0+L/250) microns to the larger Excel units with a maximum 1600 × 2500 × 200-mm measuring capacity with MPe of (5.0+L/200) in XY. The Micro-Vu machines boast multisensor capabilities, common in the industry today. Available options include a Renishaw TP20 touch probe and laser probing kits with point or line scanning. Rotary indexers are available for more flexibility in presenting parts to the vision system.

“Using a vision system is often faster than alternatives—for the right part. What is important is that the part needs to be presented to the camera in the right orientation for it to measure the part. Sometimes that requires special fixturing, or needs an extra setup,” Kaminsky explained.

Hand in hand with in-process inspection is automation, with robots handling high-volume and tedious production operations. “The vision system is installed near the line with some special enclosures to ensure coolant spray or other contaminants are not affecting the measurements,” Kaminsky said. While vision systems are robust, there are limitations depending on the accuracy specification required. Large changes in temperature during an operating shift or even humidity—depending on the work material—may well require a controlled environment to keep the machine and measurements stable.

However, he also noted that the manufacturing processes themselves are now starting to require environmentally controlled conditions to make the part. As the factory floor becomes a clean, air-conditioned, and controlled environment, moving precision metrology equipment to that floor becomes more viable.

Don’t Forget Tooling

While it is true that machined parts are in general requiring higher accuracies, the cutting tools that make those parts must have equally exact tolerances. But there is more to their story than simply higher accuracies. “Cutting tools have also become more sophisticated,” stated Michael Stepke, product specialist, inspection machines for Zoller Inc. (Ann Arbor, MI).

Cutting tools today are designed to cut multiple operational steps in a single pass or setup. Unique tool features like optimal flute forms and rake angles are ground to ever tighter specifications. Many tool geometries must be accurate to microns in order to cut part features to equivalent levels of precision. “Today, in industries like aerospace, you will often see an incoming tool inspection department that measures the cutting tools to ensure they are accurate before they begin using them in expensive parts,” he explained.

For inspection applications, including in these incoming inspections, Zoller offers its genius line of universal measuring machines. Based on image processing, tools like grinding wheels, milling cutters, or drills can be rapidly tested. The measuring machines can also be used for complete, fully automatic, operator-independent checks in a lights-out operation. Zoller’s full CNC robotic tool inspection cell, called roboSet, addresses high inspection throughput for high-volume production of cutting tools, according to Stepke.

As important as that process is, Stepke also stressed how complex the process of creating sophisticated tools is becoming. “It is getting more complicated and it is important that the toolmaker have measuring equipment to help them,” he explained. For example, Zoller’s caz virtual measuring machine allows the tool designer to create the measurement sequence while composing the tool using 3D models before the tool is manufactured. “That saves programming time at the inspection machine,” he said.

According to Zoller, common methods of using hand calipers, optical projectors and toolmaker’s microscopes need upgrading as toolmakers evolve into in-process tool inspectors. “In the past, it was easy to separate those functions, and the grinder operator sent the tool over to final inspection, but that is no longer possible with high-quality, precision tools,” he said. “The toolmaker is now also more and more becoming a tool inspector.” Therefore, more sophisticated inspection machines move closer to the CNC grinder, right to where the toolmaking process is happening.

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