Feeding the Loop
Advances in software protocols and standardization are making automated, "lights-out" manufacturing a less complex endeavor
By Bruce Morey
Adaptive machining or manufacturing is especially attractive when considering large parts with tight tolerances or parts made of expensive materials such as titanium or composites. Such adaptive manufacturing—sometimes called "closed loop"—could comprise a workcell of machining centers, gages, and a CMM, with parts moved by a robot. Conversely, a single machining center outfitted with a single spindle-mounted probe offers a smaller closed loop. Both approaches are important.
"Spindle-mounted machine probes have been around for awhile. What we discovered is that they were underutilized," says Ken Woodbine, president of Wilcox and Associates Inc. (North Kingstown, RI). "First, there was a general lack of interest because the value was not clear. Second, it was difficult and cumbersome to program at the [controller] macro level to take full advantage of the probe." These probes, when used, are typically employed in one of three ways, according to Wilcox: pre-machining to set tools and establish the alignment of parts, tools, and machines; in-process to send data to the machine's controller; and postprocess, where the part is measured precisely while still fixtured in the machine, augmenting if not replacing a separate CMM.
The most popular use is in setup and part alignment. Using on-machine probes results in faster machining with more accurate setup and less scrap, according to Woodbine. Extending use of these probes to in-process operation means they monitor critical conditions such as errors, tool breakage, and part shift. "Aerospace is a popular customer set, and we have partnered with waterjet manufacturers to cut composites. We align the parts and perform basic troubleshooting [inprocess], alerting operators to problems before they are measured on a CMM, catching problems early and saving money.
Making such probes easy to use is vital if their use is to grow beyond setup. In response to this demand for simplicity, Wilcox introduced its PCDMIS NC product to the comany's PC-DMIS portfolio. The software simplifies writing inspection routines for on-machine verification because it eliminates any need to program the CNC directly.
"Currently, on-machine probing is primarily used for setup applications, checking the tools and the part, and establishing a work coordinate system," agrees Dave Bozich of Renishaw Inc. (Hoffman Estates, IL). "But it's not only for setup." He describes the Renishaw Productivity+ software suite as shielding the user from dealing with the CNC controller directly by using parametric programming. The Active Editor Pro offline programming tool, which is part of the Productivity suite, imports a variety of CAD models. The tool then produces probing routines that integrate into CNC cutting codes (think G-code and M-code.) "Programming the probe off-line is something many end-users are starting to demand." To achieve adaptive machining, he believes there is a need to deliver a more-sophisticated interface. For instance, probing cycles that contain conditional branching, where a measurement program detects a problem and reacts. He describes this as more difficult to program than a simple point-and-shoot probing cycle that updates a work-coordinate system. Using a PC-based programming tool such as Productivity+ simplifies this task. He also points to Gibbs-CAM from Gibbs and Associates (Moorpark, CA), which allows Productivity+ to run as a plug-in within its native environment. Other CAM systems, like NX CAM from Siemens PLM Software (Plano, TX), provide tools for programming Renishaw probes as well.
A key point is how the user needs to plan for measurement. Old ways will not do. "Instead of the user thinking about how to use the probe at the end of the manufacturing process, they need to think about it at the beginning, during planning and programming." Bozich also believes that competition is heating up. "Right now, it's a foot race to get advanced programming tools to customers. Ease of use will get the user over the hurdle of programming the probe for closed-loop machining."
The next level in closing the loop is tying together cells of machines and metrology stations. "Manufacturers are making the switch from fixed manufacturing to flexible manufacturing. That's driving the need for flexible automation," says Frank Powell, gage product manager for Marposs Corp. (Auburn Hills, MI). "Cost reduction is driving closed-loop manufacturing. The challenge is meeting the need for flexibility. Machine tools have the ability to be very flexible today. They need quality-control systems that are just as flexible."
Powell cites a recent example where a manufacturer of high-precision fluid-power devices reduced costs to keep production in North America. A manual process produced 128 separate parts of a related part family. This process used 35 individually operated grinders, typically running large lots to maximize efficiency, that needed to deliver small production lots of 10 items or less on short, random notice. The system Marposs helped deliver features automated machining—operated by only two workers—with all parts feeding robotically through a central gaging station. "The gage checked six diameters, six taper characteristics, and two runout conditions, based on our Marposs M57A automatic gage." The system's Quick SPC stored data from the gage. Connected directly to a Fanuc controller, it automatically updates offsets for compensation.
Marposs contends that digging deep into an individual machine is just as important. In January 2009, the company acquired a controlling interest in Artis (Bispengen, Germany). Artis provides technology that monitors spindle deflection, vibrations, frequencies, and horsepower draw on the spindle. Comparing it to a heart monitor for the machine, Powell explains that while measuring parts is good, delivering the next level of flexible, closed-loop systems requires monitoring machine-level data.
Marposs is also improving the capabilities of on-machine probes. Its 3D Shape Inspector (3DSI) software delivers what the company describes as CMM-like part measurements. Recent upgrades to this software include work-offset feedback, tool offset, and anticollision check, as well as support for four and five axes. It's not intended to replace end-of-line CMMs for part validation, according to Sharad Mundra, product manager for 3DSI. Instead, it reduces the load on such CMMs while giving the process developer an added tool for controlling the process. "We have two types of probes that go with 3DSI, each with 0.5 and 1 µm of repeatability." Referring to the general rule-ofthumb that a measurement capability needs to be about 10x more accurate than the tool used to cut the part, he reports meeting tolerances greater than 5 µm. "We calibrate accuracy to datums placed on the fixtures. Each time you wish to inspect on the machine, the system measures itself against these datums, and adjusts accordingly."
Following this trend towards building automated systems of machines driven by metrology data, Renishaw points to its own RAMTIC production system (for Renishaw Automated Milling, Turning, and Inspection Center). Using RAMTIC, Renishaw builds its own measurement probes in its Wotton-under-Edge plant in Gloucestershire, England. A combination of automated loading and machining coupled with automated inspection replaces human first-off inspection and manual updating of tool offsets. Automated inspection techniques using workpiece artifacts holds production tolerances down to 10 µm on its VMCs. Inspection grams are automatically created using their gage point analysis system. Machined parts are inspected with spindle-mounted probes, and the results compared to target values. Tool-compensation values are automatically calculated and entered, according to Renishaw.
Making an automated closed-loop system accessible to a wider group might turn it into an off-the-shelf commodity. That seems to be the goal of a recent partnership between Hexagon Metrology (North Kingstown, RI) and GF Agie Charmilles (Linconshire, IL). "Many companies on their own have created closed-loop manufacturing systems, piecing together the necessary elements of automation, machining, metrology, and software. They create unique, one-off solutions," explains Scott Everling, manager of special services for Hexagon Metrology. The two companies created a base design for systems that use EDM to create molds. The basic system equipment is composed of a five-axis HSM 400U milling machine, a Robofil 240cc EDM, a Brown and Sharpe Global Advantage 7.10.7 CMM, and a Fanuc robot-on-rail system for material handling. Equipped with a scanning contact probe, the CMM measures to an accuracy of 1.5 µm.
Why a CMM rather than on-machine tool probes? "The nature of the EDM process precludes using a probe. Also, having a CMM integrated directly in the cell means that it is used not only for presetting the EDM tools, but product validation as well—close to the point of manufacture," says Everling. "For instance, in medical applications where 100% inspection is required, integrating a CMM into the cell makes a lot of sense." The CAD models used for the off-line programming must contain both workpiece geometry and X, Y, Z reference elements. In addition, the user must align reference points in the pallet system. "This type of system, with a CMM in the workcell, is useful anywhere there is a need for multiple processes [whose measurements and tolerances] need to stack up correctly."
The key enabler is the controlling software and the communication protocols. GF Agie Charmilles provides its Sigma Cell workshop organizer software to manage the result of measurements from the CMM. Measurements feed the system through an interface with the PC-DMIS software that controls the CMM. What remains for the user are the true value-added tasks of designing the part and manufacturing it. "This is an example of what I am seeing—more standardization overall. It contributes to easier communication, leading to more automated machining and manufacturing systems."
Taking standardization to the next level is the emerging STEP-NC protocol, termed the STEP application protocol AP-238 "integrated CNC machining." The STEP-NC AP-238 standard is an international effort to replace the RS-274D (ISO 6983) M and G code standard with a modern, associative language connecting CAD design data directly to the CAM process, according to Martin Hardwick, president of STEP Tools Inc. (Troy, NY). Currently, most CNC machine tools are programmed using G and M codes to a standard over 40 years old. STEPNC replaces this with an integrated 3-D data format resident in the controller. Machine instructions are automatically derived from this model. STEP Tools is organizing a series of closed-loop machining demonstrations that include a cross-section of aerospace companies, university researchers, and providers of hardware and software.
One of the participants in these demonstrations is Mitutoyo (Aurora, IL). There are two elements that need to be distinguished when discussing closed-loop manufacturing, according to Shawn Lawrence of Mitutoyo. "First is process validation. The second is process control, where you are using measurement results to ensure design verification. Think of closed-loop process control as adjusting offsets for tool wear, thermal growth, or slight variations in material properties," explains Lawrence. Closed-loop process validation takes a more fundamental view; it verifies if the process would ever produce the right part. This change in view is enabled by the STEP-NC protocol, because it creates a machining CAD model from the engineering CAD model.
Based on his experience with the original STEP specification, Hardwick expects widespread use of STEP-NC within the next 3.5 years. Stay tuned for further developments. "It will require a new user mindset. The system will compute many of the parameters that they are now inputting manually themselves."
This article was first published in the May 2009 edition of Manufacturing Engineering magazine.