Measure It On The Machine
By Bruce Morey
Why use a metrology device on or near a machine tool? It isn’t just useful for making sure a tool is present or monitoring tools for wear or breakage. On-machine measurement technologies can save time and money, by speeding up processes and eliminating extra personnel, and they are a critical step in the movement towards “lights-out” manufacturing. Makers of monitoring products are continually adapting their technology to ever more complex machining, such as with five-axis or six-axis CNC controls, as well as a widening range of machining options.
For example, low-cost machine tools are proliferating. “A number of [machine tool] companies are pioneering lower-cost machining centers,” explained Jean Zangao, product manager for Hexagon Metrology (North Kingstown, RI). “Today you can find, for example, VMCs in the $75,000 range. If you compare that to standard tool setting units that cost $3000–$5000, including installation, that is a significant fraction of the cost of the machine.” In response, the Hexagon brand m&h launched their new Tool Setter TS35.20. For roughly a third of the cost of higher end systems that feature RF or IR wireless communication, they are aimed squarely at the low-cost VMC market. “You would want a wireless system for machines with more than three-axis or more complicated than a VMC,” explained Zangao.
According to the company, they fix the TS35.20 to the table in the machine, sending its signals by cable to the control unit. The company reports deflection force down to 2.2 N•m, which the company says allows users to measure tools with a diameter as small as 0.1 mm to a system accuracy of 0.5 µm. Measuring is done at speeds of up to 3 m/min. To measure ever-smaller tools, m&h recently launched the Laser Tool Setter LTS35.65-23, specifically designed for machines with small footprints. It boasts an accuracy of 0.2 µm and measures lengths and radii on parts with a minimum diameter of 25 µm.
In another nod to the low-cost end of the market, Marposs (Auburn Hills, MI) also recently introduced its Mida brand Tool Break Detector (TBD) that uses laser beam triangulation.
The system projects a laser beam onto the tool surface that reflects onto a receiver. The company says that even in the presence of coolant, its algorithm for detecting breakage will “accurately analyze the reflected light signal created by a rotating tool.” The probe works with tools operating at 200–5000 rpm, with diameters as small as 0.2 mm. Tools may be positioned at a distance of between 0.3 m to 2.0 m from the TBD unit based on the installation. The unit requires a wire connection to the machine tool and is suitable only for center cutting or solid tools, according to Sharad Munda, Mida probes product manager for Marposs.
Machining Goes Small
Machining smaller is another trend to which metrology providers need to adapt. Marposs responded to this trend by recently launching its new higher precision Mida ML75HP noncontact laser tool-setting system for on-machine tool checking. This is a variation on its existing Mida ML75P laser tool setter. This unit features a laser beam that checks tools as small as 10 µm in diameter. “To set this in context, our ML75Pico is good for measuring tools that are greater than 30 µm and the standard ML75P is good for tools greater than 50 µm in size,” said Munda. “We have a customer in San Francisco measuring tools to a repeatability of 0.2 µm that are 25 µm in diameter.”
The ML75HP generates a laser beam between an emitter and a receiver mounted inside the machine tool working area. When a tool mounted on the spindle interrupts the laser beam, the laser system outputs a signal to the machine CNC. Checks the system performs include tool length and radius measure; tool wear compensation; tool breakage detection; insert/cutting edge integrity; insert/cutting edge profile; and thermal drift compensation.
To avoid issues with lasers in machine tool environments, GF AgieCharmilles (Lincolnshire, IL) offers its Intelligent Tool Measurement (ITM) system for use on its own machine tools. At full spindle speed, cameras take high-speed digital pictures and process them. The ITM does not measure just one single point; it optically inspects the whole tool contour. Features measured include length, diameter, radius and tip quantity, and quality. By comparing the results against a previously stored image of the tool, it can compensate for pollution, such as chips or coolant drops, by digitally ‘cleaning’ the image of the tool before making a measurement. The company said it typically offers an absolute Z-reference and tool radius below the range of ±1 µm in the process.
“There are many such optical systems that reside outside of the machine,” explained Eric Ostini, product manager for GF AgieCharmilles. “The problem with outside of the machine is that the system does not take into consideration where the tool sits in the spindle, what the heat of the tool is during normal operation.” He also pointed out that tool blends are especially important in moldmaking and other machining operations, and the ITM is especially good at measuring the blends of radii and curvature, determining tool wear in these critical areas.
The improvements of their machining centers themselves drove the need for improved measurement. “When we went to a linear motor drive system [on our machine tools], we were seeing precision in our movements that were above the capability of a normal machine,” he explained. Touch probes and laser systems were not providing accuracies needed to get the best finishes, tool blends, and accuracies.
Conserving Cycle Time with Alternative Sensing
There are other ways of determining tool wear or breakage, too. Measuring the part, for instance, during or after machining, is often used to infer the condition of the tool. The advantage is that the machine does not stop while tool wear is measured—theoretically, there is no sacrifice in cycle time, a key advantage.
While air gaging as a measuring tool is not new, Jenoptik Industrial Metrology (Hommel-Etamic; Rochester Hills, MI) has taken the concept to a new level with its TPE200 pneumo-electronic transducer. This component converts pneumatic pressure signal variations into numerical values with a response time of less than 15 ms using a #10-air nozzle. Using an application-specific pneumatic system, the sensor achieves repeatability of better than 0.01 µm (to DIN standard 2271), depending on the application.
Air gaging, by its very nature, is limited in its range of measurement. However, this nifty device measures in ranges from ±5 µm to ±500 µm, according to Andreas Blind, vice president of sales for Jenoptik, well outside the normal range of air gaging. Included in the package is a second sensor for measuring the air supply pressure to compensate for variations, and a third sensor measures temperature.
“This is ideal when paired with grinding or honing operations for inner diameters,” explained Blind. “Our goal is to get as close to the machine tool as possible, so the TPE200 is only 1" square, compared to traditional air gage transducers that are as large as 10" square.” He reported that companies that provide honing or inner diameter grinding are finding the transducer as attractive components of their machine tools. The TPE200 also contains three pre-defined, numerical filters. These filters can be set to optimize the output for either standard, high-precision, or high-speed measurements.
Another way of measuring tool wear and breakage indirectly during operation is through monitoring the torque and power on the tool spindle. For example, Caron Engineering (Wells, ME) has long offered its Tool Monitoring Adaptive Control (TMAC) that operates on the principle that horsepower required to cut a part increases as the tool’s cutting edges deteriorates. “Our newest product is a multiprocessing version of that principle we call TMAC-MP,” explained Rob Caron, president of Caron Engineering. “[TMAC-MP] is really targeting Swiss-type machines and multifunction, multiuse machines where there are multiple processes happening simultaneously.” He noted that a Swiss-type lathe often runs untended, making process control vital.
“For example, operators might put a bar feeder on the end of the machine that holds 20 bars that are each 12' [0.3-m] long while the part is only ½" [12.7-mm] long. It is going to run awhile before an operator needs to do anything,” he explained. Detecting worn or broken tools automatically is vital. “Our new system allows you to monitor all of these motions and processes simultaneously. The number is limited only by the size of the processor we select,” he said, adding that most Swiss-type lathes run no more than four simultaneous processes. Why use such methods over direct sensing? “A laser or touch probe are difficult to fit into such small spaces as a Swiss-type lathe. It is a very tight environment.” Calibration curves are determined through running a tool to failure for the operation, making these systems better for high-volume operations.
Another indirect method his company offers are strain sensors embedded in the toolholders—as the tool wears, compression on the holder increases and is measured. “We are measuring very small tools, 0.004–0.006" [0.10–0.15 mm] in diameter,” he explained. He described horsepower sensors with resolution down to 0.001 hp, which may not be sensitive enough for such small tools.
Communications and Growing Use of RF
Advances in communication technology are also refining the use of on-machine probing. While data transferred by wires is often replaced by wireless transmission using infrared/optical, radio frequency (RF) has distinct advantages and is growing in demand, according to Dave Bozich of Renishaw (Hoffman Estates, IL). “Optical requires line of sight [LOS], whereas RF does not,” he noted. “This makes it more adaptable and convenient around machine tools, especially for larger machines, like large VTLs, or five-axis CNCs. IR/Optical is also limited by distance. Renishaw can deliver IR systems with 6 m of range. In contrast, our RF systems deliver reliable signals out to 15 m.”
To satisfy this growing market, Renishaw recently introduced its new RMI-Q multiple probe radio transmission system for automated on-machine tool setting, tool breakage detection, part setup, and part verification. The RMI-Q (the Q stands for Quad) operates four separate radio transmission probes on the same CNC machine. The RMI-Q is an enhancement of Renishaw’s existing RMI. Renishaw also introduced a Radio Tool Setter (RTS), an RF tool setting probe the company offers especially for CNC machining centers, or machines with rotary tables or twin pallets where it is difficult to install wired setters. The RTS can measure both tool length and diameter of milling cutters, twist drills, and end mills, according to Renishaw. Powered by two AA batteries, the company says it is compatible with Renishaw’s RMP60 and RMP600 spindle probes.
“A better indication of tool wear is measuring the part against the nominal,” said Bozich, rather than measuring the tool itself. That is why they developed a series of part probes, the newest of which is the RLP 40. It was designed for use in turning centers that use RF for communication. “RF is much better in those hostile turning center environments,” he explained. “With an 8 or 10" [203 or 254-mm] chuck and chips flying, you need something rugged and reliable.”
If you are undecided about the choice between IR/Optical or RF, Heidenhain (Schaumburg, IL) will offer in the first quarter of 2013 hybrid probes with both IR and RF, the TS 460 for workpiece measurement and the TT 460 for tool measurement. Users can select either mode: radio when extended range and large amounts of data are required; IR when higher accuracies and fast signal transmission are needed. “Customers were asking for this,” explained Dan Vitullo product specialist, CNC products of Heidenhain.
“Even though there are LOS issues with IR, however, there are still applications where they still want the IR technology because it is a faster transmission and you can trigger the probe faster. You can get a more accurate and repeatable triggering using IR,” he said.
Who in the industry is looking for this? “We see both aerospace and automotive customers. Aerospace is especially excited about this. It is good for any application where the part on the machine is large enough to impede LOS for the probe to transmit to the receiver unit,” he answers. “Or in five-axis machining where you are down in the part trying to get a measurement.”
Integration and Operators
Integrating probes and sensors on machining centers and programming them easily may be just as important as creating new ones. For example, Marposs introduced their new process and monitoring solution called Genior Modular control system to meet this requirement. “The principle behind the Genior system is to provide a solution that can monitor processes without human intervention,” said Jorge Pena-Mena, general manager for Artis Systems of Marposs that produces the Genior system.
Automation reduces training requirements as well. The Genior system is designed for high-volume operations through providing a system that requires such little minimal human interaction. Genior consists of a main module that connects with the machine control’s PROFIBUS system for monitoring the machine’s spindle and feed drives, and a universal measuring transducer for use with various ARTIS sensors for measuring force-strain, acoustic emission and vibration, and acceleration.
As Pena-Mena describes it, the Genior system is aimed at creating a cell with five or six machining centers operated by only one person. “It is turning into a world where humans are less involved in the machining process,” he said. ME
This article first appeared in the January 2013 issue of Manufacturing Engineering. A PDF of the original article can be found here.