The ins and outs of in-line automated inspection
Automation needs to adjust to meet 21st century demands.
The challenges to manufacturing as it evolves into the 21st century are now familiar, and impact how metrology must contribute. Manufacturers face uncertain production volumes with roller-coaster demand, shorter production runs and faster product development cycles. Automation, while alluring as a way to reduce cost, needs to adjust.
At the same time, the marketplace demands ever higher quality and parts made to more precise tolerances. All these factors, and more, are driving the need for more flexible yet more automated production metrology equipment.
For example, the automotive powertrain production line of the future hardly demands the traditional dedicated hard gauge purpose-built for a single part or subsystem. The traditional concept of 100 per cent hard gauging disappeared along with production runs in the millions.
“Part runs [today] are in the hundreds of thousands, even in automotive. Some companies are configuring flexible production setups to run multiple engine components on the same line: a six-cylinder crank one week, a five-cylinder the next day, and a four-cylinder the next,” agrees Andreas Blind, vice-president of Hommel-Etamic in Rochester Hills, Mich.
Flexible, reconfigurable metrology is the demand of the future. Hard gauges and fixture gauges are still very useful, he explains, but today they may be built to work in conjunction with flexible solutions or with retooling and reconfiguration in mind. There remain tradeoffs between speed, flexibility and cost.
“This does not mean hard gauging fixtures go away,” he explains. “It does mean you build them with more flexibility, so that you can retool them very quickly.”
The company uses three basic kinds of technologies for metrology: pneumatic, tactile and optical. Each is integrated into flexible or dedicated gauging solutions, depending on a customer’s budget and expectations towards flexibility.
At the same time, companies like Hommel-Etamic are expanding off-the-shelf production metrology. A good example Blind points to is its Opticline series of optical metrology systems. These are designed to measure long, round workpieces such as crankshafts, camshafts and driveshafts, directly on the production floor.
An LED light source illuminates a shaft rotating on a fixture. A high-resolution CCD line sensor captures a silhouetted shadow, digitally measuring the contour as the piece rotates. Models in this line measure parts as small as fuel injector nozzles or medical needles, and parts as large as 2,500-millimetre (mm) aircraft engine shafts. Programmers use a teach-in method to create parts programs. They call up programs when needed and workpieces are measured in seconds.
Specifically for Use in Automation
Important for this subject, the company designed Opticline Contour Automatic (CA) 300/500/800 series machines specifically for use in automation for 100 per cent in-line inspection. Accuracy in diameter is (two + D/100) microns and length is (five + L/100) microns, both to ISO 10360. Measurements include lengths, diameters, angles, radii, runout, roundness, eccentricity, cylindricity, straightness, form and profiles.
The Wavemove automated surface roughness and contour measuring system is another measuring device the company released in 2011 and recently introduced to North America to meet the growing need for automated measurements. The device includes up to seven CNC axes for complete high-accuracy measurement and evaluation of complex shaft or prismatic parts such as automotive crankshafts, cylinder heads, engine blocks, or transmission housings.
“We make the distinction between retoolable and flexible,” says Gary Sicheneder, manager, New Market Development, Marposs Corp., in Auburn Hills, Mich. “Retoolable is where you move components or change components [of a process gauge] to make it ready for the next part.” A flexible gauge, on the other hand, means it is ready to go for all the parts it is designed to check. Marposs delivers both, depending on the application need.
Marposs, too, has seen changes in how manufacturers approach lines and gauging. The growing trend, according to Sicheneder, is to plan for production lines to last for five or 10 years, that will have a short production duration (life span), while planning on running multiple parts that will change over that time.
This means delivering a system that allows for minor retooling each time a part—within a basic geometric envelope—is changed. They might need gauging easily retooled for, say, a family of camshafts, crankshafts or brake pads. While this is limited flexibility, it is cost-effective and meets the needs of the manufacturer, as long as the line is planned correctly at its inception.
While what used to be called hard gauging is becoming more flexible, delivering digital data rather than simple go/no-go checks, on the other end of the measuring spectrum, CMMs are finding their way to the shop floor. One example is the recent release by Hexagon, in North Kingstown, R.I., of its new line of Brown & Sharpe CMMs, the 4.5.4 SF, designed for portable use on the shop floor. It has a small footprint for a CMM, just 1326 × 833 × 2019 mm, allowing it to fit through a standard door. The measuring envelope is 355 × 514 × 353 mm with a best probing error of MPEE of 3.1 + (3L/1,000) microns at an ambient temperature (ISO 10360).
As CMMs become familiar on the shop floor, automating them is a logical next request.
“We are seeing three to four times more requests for automated CMMs in just the last few years,” says Scott Everling of the Special Systems Group at Hexagon Metrology. Why?
“As CNC machine tools become more automated, people are asking if they can automate the CMM that measures the parts they make,” says Everling.
He sees the requirement for such precise in-process control becoming more common in off-highway, heavy equipment, medical, and aerospace engine industries.
“The major OEMs are levying this requirement onto their suppliers,” he explains. Fully automated CMMs are useful for job shops or other organizations that have part runs of 100–200 parts.
“They can put these in a pallet or tombstone and run them efficiently on the second or third shift without people around,” says Everling. He has also seen some robot loaded automation of CMMs in high-volume plants as well, such as lines producing automotive axles.
To automate their line of CMMs, Everling points out that Hexagon has developed an add-on kit to work with automation, aimed at third-party automation integrators. This includes conveyor automation as well as robotic material handling.
“It works directly off the CMM controller, sending information to the integrated automation system on its status, such as if it is operating, stopped or safe to load,” explains Everling.
He also cautions that a CMM on the shop floor must be treated as a gauge R&R device. Quality professionals could miss bias or offset even while using the shop-floor CMM to ensure perfect repeatability. To check absolute accuracy requires a separate CMM, ideally located in a controlled environment some distance away, according to Everling.
Because of speed and throughput, CMMs on the shop floor are more “near-line” metrology stations than “in-line.” They are better used—depending on cycle-time—for process control with SPC techniques than 100 [er cent inspection.
Still, there are advantages. “CMMs are inherently more flexible and provide more data than other kinds of metrology equipment,” says Everling, “but that also makes them somewhat constrained for automation.”
To overcome some of those constraints, Hexagon offers another tool for automated metrology on the shop floor, the Cognitens WLS400A white light scanner. Using digital cameras and structured light (that is actually blue nowadays), the data capture is so fast that vibration is not a factor in data accuracy. Specifically designed for automation, some of the benefits include its 500 × 500-mm coverage, measuring full surfaces, and producing 3-D models as output.
To help automate CMMs on the factory floor, Carl Zeiss Industrial Metrology in Maple Grove, Minn., created a standard product they call the Factory Automation Control System (FACS). The FACS toolkit provides most of what manufacturers need for automating a CMM into a production cell, with some custom software required for the individual manufacturer. FACS manages the interface between a robotic system’s controller and the Automation Application Interface to the CMM’s control system through Calypso and CMMOS. Both are Zeiss software products for programming their CMMs. The intent is to reduce the amount of code the automation integrator needs to write by using a standard interface. The FACS system runs in semiautomatic, “human as loader,” and fully manual modes.
They also offer a FACS Light system for customers who require only discrete I/O communication for their fully automated systems. Jose Torres, senior software engineer reports the company installing over 300 FACS worldwide in the four years it has been available, attesting to the growing popularity of automated CMMs.
“The most popular CMM [in FACS installations] to date is the Centermax CMM,” he says. “This machine is specifically designed to tolerate harsh factory environments and still remain a high-precision CMM. The second most popular CMM is the Prismo Navigator with VAST Gold within an environmentally controlled enclosure.”
Benefits of Automated In-Cell Inspection
Why even consider automating an inspection process within a cell? Torres recommends it for reducing factory setup time, manufacturing defects, product lead time and in some cases direct labour costs.
“[Users] must also consider that these type of cells have a by-product; these processes require knowledgeable employees or service providers to integrate and maintain them over the long haul,” he says. Not more expensive labour, but re-educated labour. “I have personally witnessed a company transition from three inspectors to one inspector; however, the other two inspectors became automation process experts after the company purchased three cells,” he explains. He believes all customers considering automation should be prepared for this unavoidable “re-education” paradigm shift.
Another new concept in versatile, flexible gauging is the Equator from Renishaw in Hoffman Estates, Ill. The odd looking device pairs a parallel kinematic machine (PKM) with a Renishaw SP25 scanning analog probe. It measures an infinite variety of parts within its 300 × 300 × 150-mm working volume, collecting data at 1,000 points/second. “It is not a CMM and it is not intended to deliver the accuracy of a CMM,” says Dave Emmett CMM/Equator business manager of Renishaw. “It is a flexible gauge.” He believes there is a need for both high-speed “hard” or fixture gauging systems as well as the slower, but more accurate, CMMs. There was also an unmet need in a technology gap between the capabilities of those two technologies, according to Emmett.
He believes the Equator is ideal for job shops or manufacturers with part runs in the thousands with a large variety of individual part designs. As Emmett explains it, the Equator has all of the versatility of a CMM without the same level of absolute accuracy. “The Equator is first mastered, usually with a part that has been measured carefully on a CMM, and then subsequent parts are measured in comparison to the master,” he explains.
Repeatability vs. Accuracy
Making the Equator repeatable without making it accurate in the absolute sense makes it economical, portable (it weighs 25 kg) and ideal for workcell gauging. The comparison uncertainty, or repeatability, is ±0.002 mm, but to report accuracies in the absolute sense requires periodic remastering during a work day. “The accuracy of the Equator is held, if you will, in the master part,” says Emmett, just like many hard gauges or fixture gauges. “It is a comparative technology in a versatile, software driven [device].” Emmett says dozens of Equators have reached the field since the system’s 2011 introduction, and regular software enhancements are being introduced.
Programming is done either on the machine or off-line using Renishaw’s MODUS package. It is also tailor-made to work with automation, including robots. A 24V I/O system allows robotic system integrators to include an Equator in an automated combined inspection and machining cell. Fixture clamps are automatically engaged for measurement, co-ordinated with the automation control program. Alternatively, a number of base mounts with fixtures attached could be swapped-out by robots.
Unlike hard gauging, fixturing does not need to be precise with the Equator. The Equator uses a probe to measure and establish a datum using best-fit mathematics. “However there is a cycle-time hit when you do that,” explains Emmett. Investing in a dedicated, precise fixture that locates the part within repeatability limits means no part probing to establish datums. Precise fixturing means faster measurements. “These fixtures can cost anywhere from a few hundred up to $2,500, depending on your needs,” says Emmett.
Renishaw configured the Equator to send offsets from measurements to a CNC machine tool controller to compensate for tool wear or temperature changes.
This article was provided courtesy of Manufacturing Engineering Media, a division of the Society of Manufacturing Engineers.