Machine Tool Metrology Made Simple
By Bruce Morey
Ever tighter part tolerances and the need to make parts ‘right the first time’ are prompting more offerings in metrology devices for use directly on or near CNC machine tools. At the same time, increasing use of four and five-axis machines presents its own challenges in calibrating machine tools.
To make sense of the equipment offered and how to use it, Renishaw (Hoffman Estates, IL) developed a descriptive framework it calls the Productive Process Pyramid. "There are four key elements in this description," explained Dave Bozich of Renishaw. "These are Process Foundation, Process Setting, In-Process Control, and Post-Process Monitoring."
The basic process of calibrating and certifying a machine—Process Foundation—has become more popular as devices for it have become easier to use, according to Clive Warren of Renishaw. Ballbar calibration devices have become standard equipment for certifying CNC machines. "A ballbar system measures deviations from a programmed circular path while the machine is in motion," he explained. The ballbar software then uses mathematical algorithms to analyze those deviations, and present them as commonly understood machine errors such as squareness, straightness, linearity and backlash, in addition to dynamic errors such as servo mismatch and reversal spikes. The software also combines these errors into a single "circularity" value that is an overall measure of the machines performance.
The wireless QC20-W replaced Renishaw’s earlier QC10. That device used a wired connection to transmit data back to the PC while the test was running on the machine. "Wireless eliminated a lot of health and safety concerns," he remarked. "It also allowed you to do volumetric or ‘partial arc’ testing. This provides an estimate of how well the machine is performing throughout its working volume. This is a contrast to a 2-D assessment in a single working plane, in a single setup. He also notes that the new wireless system takes just 10–15 min to set up and take measurements. "It has become very popular since its introduction, and we have sold multiples of our wireless system over the older system," he said, attributing the increased sales in large measure to simplicity and ease-of-use.
However, the ballbar test has its limits. It does not test the rotary axes of four and five-axis machines. To make rotary axis testing more accurate and easier, Renishaw recently launched the XR20-W. "It uses an ultra-high accuracy rotary encoder designed and developed in-house," said Warren. Designed to work with their XL-80 or ML-10 laser interferometer systems, the wireless XR20-W measures to better than ±1 arc sec.
Warren also points out that precision requires knowing the location of the centers of rotation of those rotary axes relative to the three primary axes of the machine. These are sometimes called pivot points. Renishaw’s AxiSet software, released in 2009, uses a spindle-mounted probe to measure a calibrated sphere. Results from the 10-min test are sent to a PC.
What to do with all of this data? "We are seeing more CNC machines with volumetric compensation capability that uses error maps derived from data collected by laser interferometer systems (like Renishaw’s XL10), ballbars and AxiSet," he said. Special software combines the collected data to either create an error map and uploads it to the CNC control, or uploads the raw data, which the CNC control uses to create its own error map. "That is the way the industry is headed," he remarked. To keep tabs in-between measurements, users perform periodic testing with a ballbar. Alternatively, they measure a master artifact during the calibration process. The artifact is then stored and re-measured periodically.
IQL (Ashaway, RI) specializes in machine tool performance analysis and manufacturing process improvement. Robert "Buz" Callaghan started Independent Quality Labs, Inc. (IQL) in 1984 after a personal history that spanned aerospace engineering, the then emerging coordinate measuring machine (CMM) industries and machine tool industries. Over the past 28 years, IQL has developed a comprehensive hardware and software package, Locus Metrology Systems, for the measurement and analysis of machine tools. Included in the Locus Systems package are Laser Interferometers, Electronic Levels and Gages, Ballbar, Spindle Analyzer, Rotary Calibrator and Ceramic Artifacts—for calibrating the principle (X, Y, Z, A, B, C) axes. The Locus Metrology Systems measure a CNC machine’s 3-D positioning, angular, straightness, and alignment errors to ISO 230 standard, according to the company.
Recognizing the recent growth in multiaxis machining, IQL is also introducing new to North America device to aid in certifying and calibrating CNC machines: the R-Test measuring system from IBS (Eindhoven, The Netherlands). The R-Test from IBS measures the dynamic position of a rotary axis with respect to the other machine’s axes. One of the features of the R-Test is providing kinematic tests according to ISO 10791-6.
Testing to these international standards is important to Callaghan. "I am thrilled that we have an international standard that incorporates these techniques," he said, referring to the updated ISO 230 standard addendums. These specifically incorporate ballbar testing and other modern test practices. "The growing awareness of the [newer] ISO 230 standard for characterizing machines will become very important. Applying these metrology methods on machines ensures correctness," he said.
He also stresses that how one uses the data from tests is important. While there is a tendency to adjust the control system with an error map that creates offset adjustments, he cautions against this outside of tool setting or part alignment. "When you find a mechanical problem—fix the mechanical problem. Do not hide it. There are certain mechanical elements that, if they are misaligned, will seriously degrade the life of the machine. If the machine ways system is bent for some reason, that will induce extra stresses in the bearing system, and will ruin the life of the machine tool," he said. He cited an example with an aircraft engine manufacturer that aggressively implemented a program, with IQL’s expertise, to measure and physically tune all of their machines every year. At the end of the third year of the program, they had increased machine availability by 20% while decreasing time to adjust the machines from five days to one. "It was like giving them six more CNC machines just through maintenance."
"While new sensors are important, easier programming through better software is a key element of the future."
Providers are also making on-machine, in-process metrology both easier to use and more practical. "On machine probing is becoming more of a practical option because probes are better, more rugged, and are capable of reaching into difficult spaces better," explained Rob Caron, president of Caron Engineering (Wells, ME.) Caron is a company that provides add-on products for CNC machines. On-machine probes can help ensure or measure the location of a part before cutting begins, measure tool wear (tool setting), and using them to measure features of the parts. Probing for part location is especially useful for castings or forgings and other parts with odd shapes. In these cases, it would be easy for the part to be misloaded. Using probes also ensures the part is clamped and positioned correctly.
"Probes today can live in tough environments like lathes, where plenty of chips are flying," he remarked. "Some also have long shafts, up to 2' [0.6 m], made of carbon fiber that can reach into difficult parts." As a result, he notes more use than in the past years. "There is also a much higher understanding of on-machine probes and how to use them," he said. "Everyone is moving towards less-attended operation," he added. "To do that, all kinds of things need to be checked on the machine and that is forcing on-machine probing to be used more."
He also notes there are drawbacks in the minds of some users. There are two drawbacks in the minds of potential users, he notes. One is the time to program the probing routines and the other is the time that the probe takes to measure the part or tool. If the machine is measuring, it is not cutting chips. That is one reason Caron offers their Tool Monitor Adaptive Control (TMAC) system, which measures tool wear by measuring power at the spindle. The horsepower required to cut a part typically increases as the tool’s cutting edges deteriorate. For more accurate tool wear adjustment, they offer their AutoComp software that uses gage data from any device. This could be something as simple as a micrometer or as complicated as a CMM or a device like Renishaw’s recently released Equator portable CMM.
"Even though [using a probe on a machine tool] looks like it takes more time, it really can save a lot of time," he explained.
Following this trend to make probing faster, accurate, and easier to use, the Hexagon company m&h (North Kingstown, RI) recently introduced its new LTS35.65 standard laser tool setter for noncontact measurement of rotating tools. "Lasers offer a lot flexibility in tool setting, we introduced LTSS35.65 to further reduce its physical size, and increase its reliability," explained Dave Jeffers a global product manager for Hexagon.
Blum LMT (Erlanger, KY) is introducing to the North American market for use on machine tools a switchable probe that measures in point mode (digital) or scanning analog mode. The Blum-Novotest TC76 is called "Digilog" to capture this switchable digital or analog feature. The system captures data in its "single point" mode at feed rates up to 79 ipm (2007 mm/min), according to the company. In analog, it is much faster. In one application cited, one of Blum’s standard single-point probes measured 40 points on a surface in around 30 sec, whereas the TC76 Digilog measured the same feature in around 2–3 sec with 100,000 measurements.
The advantage of using a scanning analog probe is when surfaces or curves need to be checked, where greater density of points is needed to capture surfaces that are more complex. The TC76-Digilog is IP68 rated and therefore, able to hold up in the harsh environments of a CNC machine tool. Applications for this new approach to on-machine probing include gears, valve seats, turbine blades—where typical scanning analog probes are best used.
While new sensors are important, easier programming through better software is a key element of the future, according to Jeffers from Hexagon. Improvements to PC-DMIS NC software, from the Hexagon Metrology, and the recent introduction of PC-DMIS NC GAGE targets easier usability. With PC-DMIS NC, a user programs on-machine probes off-line using an imported CAD model. "PC-DMIS will create a probe path necessary to measure features selected by the operator," explained Jeffers. "It translates that path into code the machine tool will read. Data is then fed back into PC-DMIS and analyzes the part that was measured and compares it to a CAD model." The company in particular targets ease-of-use with its PC DMIS NC GAGE. "People at the machine tool with a few hours of training can do the inspection at the machine tool using PC DMIS NC GAGE," said Jeffers. "It does not do everything that PC DMIS NC will do, but it does do a lot."
Easier programming will lead to more acceptance. ME
This article was originally published in the January 2012 issue of Manufacturing Engineering magazine. Click here for PDF.