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Modern CNCs Make Easy Work Out of Tough Stuff

Ed Sinkora
By Ed Sinkora Contributing Editor, SME Media

CNCs are getting faster, smarter and easier to use

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A Heidenhain TNC 640 CNC.

It’s the machine tool acronym you never bother to put into words: CNC. And much of the time it’s probably OK to view your “computer numerical control” as a black box doing magic. But if you’re struggling with high-speed machining, need better surface finishes or higher accuracy, have training and retention problems, or want a better handle on your production efficiency, the answer just might be the latest iterations of those three little letters.

Who Needs the Speed?

Like everything computer related, CNCs are constantly getting faster. To take a few examples from the dizzying array of specs you’ll see, Heidenhain controls process data blocks within half a millisecond, the Siemens SINUMERIK 840D sl can control up to 31 axes and 10 machining channels with just a single processor, and the pulse count is up to 32 million per revolution in FANUC’s latest drives and motors. Who needs this kind of power?

First, anyone trying to move multiple axes simultaneously, especially if they need to do so rapidly. Tom Maxwell, senior application engineer, Fagor Automation-USA (Elk Grove Village, IL) pointed specifically to the increased use of composites in aerospace where “faster high-speed five-axis routers are commonly requiring more processing power. We also see a need for high-speed machining of aluminum in five axes on high-performance machining centers.”

Or as Gisbert Ledvon, Heidenhain’s TNC business development manager, put it: “High-speed machining requires fast feed rates and constant chip loads. You want to transfer the heat into the chip and not into the tool. But if the control is too slow to manage the data coming from your CAD/CAM system or post-processor, you won’t be able to guarantee a constant chip load, which burns out the tool very quickly. You also don’t get the same surface finish or accuracy. Faster controls are also needed to run spindles as high as 80-100,000 rpm, again to maintain constant chip load. The CNC is a big component of success in these areas.”

Machines that combine machining methods, like mill-turn machines or machines that both mill and grind are also driving the need for ultrafast processors. “If, for example, the part is not centered on a milling machine’s table and you want to do a turning operation, you would have movement in all axes, not just the rotational axes,” Ledvon said.

Steve Holmes, business developer, Siemens Industry Inc., Digital Factory Division, Machine Tool Business (Elk Grove Village, IL) added another example: Robots controlled by a machining center’s CNC for both part handling and also light machining, such as deburring. The SINUMERIK 840D control has a function called “Run My Robot” that eliminates the need for a separate control or PLC for another robot. The control simply dedicates another channel to that task.

On the other hand, take care that the speed you think you’ll get from a control isn’t hindered by another component. As Jody Michaels, national sales manager, FANUC America Corp. (Hoffman Estates, IL) explained: “We have competitors that claim features like a 25,000 block look-ahead. That’s fine, but if you don’t have processing speed in the CPU and the drives and motors to respond quickly, it doesn’t matter how far you’re looking ahead. You could put a 1000 hp motor on a John Deere tractor, but that doesn’t mean it’s going to go 150 miles per hour. You need the whole system.”

Before leaving the subject of speed, it’s worth noting that most modern CNCs include a function generally called “adaptive feed control,” which uses measured spindle load to adjust the cutting speed. For example, if you’re cutting “air” the control automatically speeds the feed to the maximum you’ve set. When the tool is buried in the cut the control slows the feed to maintain constant, safe torque. All on the fly. As Ledvon sees it, the feature is particularly good for “unattended machining and trochoidal milling. It even further optimizes CAD/CAM packages that adjust machining speeds based on predicted material removal volume.”

Higher Accuracy, Better Finishes

As we’ve already suggested, advanced controls offer much more than the ability to handle kinematic complexity and multiple processes, or the ability to machine faster. Perhaps most importantly, they offer higher machining accuracy and improved surface finishes for mold and die and other applications where these factors are critical. One common approach, exemplified by Fagor, is dual feedback that uses both the digital motor’s encoder and a separate absolute linear encoder to inform the control. Maxwell said this “allows for much higher accuracies, yet maintaining smooth motion.”

Paul J. Gray, manager, path planning, front-end design, R&D for machine builder Hurco (Indianapolis) said the company’s WinMax control needed more processing power to support its patented UltiMotion tolerance-based control algorithms, which in turn delivered a 25% reduction in cycle time for four- and five-axis machining. But more than that, Hurco’s new five-axis machines feature direct-drive rotary axis torque motors that take full advantage of UltiMotion’s high dynamic response to deliver smoother surface finishes.

The mention of “algorithms” brings us back to the common perception that modern CNCs are a black box. Indeed, there are often sophisticated programs running in the background that go well beyond simply executing the moves defined by your machining program.

A key example is the reinterpretation of the spline curves that come out of your CAD/CAM post-processor. As Ledvon explained, such contours typically have points that fall just outside the radius of a given curve.

“Forcing a machine to go from point to point along such a contour can create vibration and gaging marks on your workpiece. On the Heidenhain control, you can establish a bandwidth of a few microns and allow the machine to move smoothly within this tolerance band around the programmed contour. The control also optimizes the speed and feed depending on the shape of the contour, while maintaining the accuracy.” Heidenhain calls this feature Advance Dynamic Prediction (ADP), and it’s common on advanced controls. FANUC, for example, calls its version Smooth Tolerance Control.

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Before and after: Heidenhain’s Adaptive Chatter Control automatically compensates for machine vibrations to produce a much smoother finish.

There are also a plethora of features that correct for inherent machine errors, like Heidenhain’s Adaptive Chatter Control (ACC). “We work with the machine tool builder to determine how vibrations can occur in their machines, as this differs from machine to machine,” said Ledvon. “And we determine what action the control should take to eliminate such harmonics when they occur, which again differs from machine to machine. The machine tool builder puts this data in the PLC, which feeds it back to the control, and it runs automatically.”

Heidenhain, FANUC, and others also have functions that compensate for acceleration-dependent position errors at the tool center point. For example, Ledvon said, mass and inertia can cause pitching movements during braking and acceleration, resulting in position errors that are not recognized by the position encoders.

The errors depend in part on the stiffness of the guideways, the distance between the feed force application point and the center of mass, as well as the distance between the center of mass and the tool center point. So, Heidenhain partners with the machine builder to understand the mechanics of the machine and makes calculations based on that understanding to correct for these acceleration errors. Ledvon added that such adjustments amount to only a few microns and therefore may not always be necessary, depending on the application.

FANUC’s terminology provides a good sense of some of the additional seemingly magical functions now available to the machine tool builder, and therefore the user: Smart Spindle Acc/Dec, Smart Feed Rate Acc/Dec, Smart Backlash Compensation, and Smart Thermal Control.

Michaels pointed out that Smart Spindle Acc/Dec also serves to save on energy costs because CAD/CAM programs often output big changes in spindle speeds that would otherwise consume more electricity than necessary. But producing better parts is the main benefit of most of these features. Or, as he summarized it, “With today’s tolerance control and the high response of our motors and drives, there’s so much going on in the background that your machined part looks like a polished finish.”

Advances in HMIs

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The Fagor HBH series pendant is available in a wireless configuration, giving the operator many of the same functions of the CNC, but with mobility.

As you might expect, recent advancements in the human/machine interface (HMI) include icon-driven designs and the kind of fingertip control popularized by smartphones (pitch to shrink an image, spread to zoom, etc.). Even FANUC, whose ubiquitous HMI hadn’t changed much in 25 years, now offers what Michaels described as “a very different interface. A home screen that doesn’t look anything like a FANUC screen, with rows of different icons for tool data, editing your program, maintenance, etc. You can also customize it, for example adding a programming app from another vendor. Most importantly, this makes running our control much more accepted by millennials.”

But the changes aren’t just a sop to millennials. CNC vendors have managed to not only give the machine operator more programming capability independent of off-line CAD/CAM, they’ve made it relatively easy.

Gray said Hurco’s new Graphical Conversational Programming system gives operators the ability to “import solid model CAD files and simply click on the part features they want to cut to generate the part program. It will even automatically orient five-axis machines to cut 3+2-axis features. The simplicity in programming not only improves operator productivity, it further eliminates sources of human error in the production sequence.”

Siemens, Heidenhain, and others offer the same capability and Holmes added that the ability to use any standard three-axis canned cycle on any inclined plane eliminates the need for an expensive CAM system for five-sided parts.

Holmes also observed that “today’s advanced HMIs go beyond basic visual verification. They offer 3D graphical verification of programming, tool and spindle setups, part probing, toolpath efficiency, and they support a range of new capabilities, such as collision avoidance that are made feasible by visual verification. In addition, the same visual verification experience can be had across the shop, from three-axis to 3+2 and on into full five-axis machining.”

Gray countered that on-control graphics and machine simulation systems are no longer special, and “it’s the programming side where most controls falter, particularly for five-axis machining.” To address this, Hurco “developed a universal program type for both conversational and NC programming that will cut the same part on any Hurco five-axis machine regardless of its configuration without having to make any changes to the part program,” he said. “This simplifies five-axis operator training and offers considerable production flexibility to our customers.”

The degree to which programs written for one machine can run on another (kinematic independent programming) is another focus for Siemens.

“This approach allows the same program to run across multiple five-axis machines that have different kinematics without the need for a separate post-processor for each machine tool,” Holmes said. “With kinematic-independent programming, the CNC programs generated are also machine-independent, resulting in greater flexibility for five-axis machining. So, take a part written for a machine with an articulating head and move it to a VMC without an articulating head but with a trunnion table. The control automatically reorients the program to run the part on this machine.” And the parts don’t have to be programmed with Siemens own CAD/CAM package, NX. Any standard package will do.

At the same time, Siemens is perhaps unique in offering both a full CNC system and CAD/CAM software package; the two are so integrated that changes to one are immediately reflected in the other without post-processing. With their broader reach, Siemens and FANUC also both offer “universal HMIs” that appear similar from milling to turning to grinding machines, easing training.

Maxwell said Fagor offers a new feature that “allows the user to select the material and tooling and retrieve basic speeds and feeds for the particular part they are programming. Fagor has also implemented an on-the-fly operator adjustment between speed and accuracy for increasing speed during roughing operations, but accuracy during finishing.”

Along the same lines, Holmes said Siemens controls include “high-speed machining cycles that allow the operator to easily adjust the machining process for roughing, semi-finishing or finishing, all with changing only one or two values in the cycle.”

Other Setup Aids

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Siemens MindSphere combines data from machines worldwide and provides analysis of that data, enabling predictive maintenance and other benefits.

Besides intuitive interfaces, today’s CNCs also help the operator with guidance for each function. Siemens includes more than 100 short video clips, called Animated Elements, that illustrate virtually every field in the canned cycles or set-up operations.

Heidenhain has a new system called VSC (Visual Setup Control) that takes a picture of your setup after you’ve optimized the orientation of the part in the workholding, the tightening of the bolts, and so forth. The control then compares that image to subsequent setups in the same production run and alerts the operator to any anomalies, such as a wrench being left on the part or a missing hole, giving him the option of proceeding, switching to the next pallet, or stopping for corrective action. VSC is so sensitive that the camera even recognizes a bolt head that’s not perfectly flat due to being over torqued.

KinematicsOpt is another intriguing Heidenhain feature. “Put a calibration sphere on the table,” Ledvon said. “The probe touches the circumference of the sphere at different angles and automatically calibrates the machine exactly to the center pivot point, which is critical for five-axis applications. It’s a very powerful feature, especially if there is any temperature deviation in the shop during the day. You can load a pallet with the calibration ball during the shift and the machine can re-calibrate itself automatically before cutting the next batch of parts.”

Hurco and Siemens also provide this functionality for machines with rotary axes in any kinematic configuration with the additional ability to include centerline probing cycles in the part program for automatic measurement during production.

Finally, although all controls are tuned at the factory based on the expected workholding, part size, and so forth, changes in the field (intentional or otherwise) sometimes require adjustments. So FANUC and others enable what Michaels called a “certain amount of auto-tuning you can do yourself.”

Maxwell said, “automatic tuning of the axes using Fagor Finetune software is now a standard feature. This software tunes the parameters of the CNC and drives using Bode diagrams to optimize the high-speed cutting features. Fagor and others also provide a built-in oscilloscope to the CNC for further advanced tuning.”

Whose Control Is It Anyway?

Before leaving the subject of HMIs it’s worth noting that the interface actually presented to the operator is often partly, if not entirely, the creation of the machine tool builder and not the CNC vendor. By the same token, you often have several control options from the same builder, so it pays to educate yourself on what’s out there.

For example, behind DMG Mori’s CELOS front end you’ll find either a FANUC, Siemens, Mitsubishi, or Heidenhain control, though in the last example it doesn’t run in the background but side by side on the same screen.

Michaels said, “Makino uses mainly FANUC but you wouldn’t know it. They use what we call Panel i, with our control in the background hooked into a computer that presents a video screen to the user. Standard vertical and horizontal machining centers and lathes often use the standard FANUC HMI. More specialized applications, like grinding, generally have the OEM’s own HMI.” Gleason gear cutting machines bury the FANUC control with its specialized HMI. Another example includes Weiler lathes, which run Siemens controls with an HMI that is “so customized by the builder that you wouldn’t know it,” said Holmes.

Harnessing the Power of Data

“Industry 4.0 and the Industrial Internet of Things are arguably the most important driving forces in today’s industry,” Gray said. “Information is the key to making smarter decisions and today’s CNC machines are expected to publish productivity and production-relevant information and to network with other intelligent devices to reduce setup time and costs.”

As part of this effort, Hurco freely provides an open-source interface to its control on GitHub and partners with a number of robotics and productivity monitoring companies to broaden the interconnectivity capabilities of its control.

While the Heidenhain control can tell you virtually everything that happens in great detail, Ledvon said its StateMonitor system makes it easy to get a relatively constrained set of data on things like spindle on, spindle off, and error codes that provide “very clear and decisive analytics without all the fuss and bother.” It also allows the operator to provide input on why the machine was not running, which can be a critical data point a fully automated system would not catch.

A forward-thinking take on Industry 4.0 is Siemens’ MindSphere, an open architecture cloud-based platform where machines from different manufacturers upload data securely to be stored, analyzed, and monitored from anywhere in the world. Machines with intelligent sensors that monitor vibration, current consumption, temperature, and other factors upload status reports to MindSphere.

If, for example, a sensor detects an abnormal vibration coming from the Y axis of a certain type of machine tool, it can determine (based on algorithms derived from large amounts of smart data gathered from other users) that the vibration is due to a bearing wearing out on the Y-axis ballscrew. That part can then be ordered and shipped to the customer before they are even aware of the problem. This goes beyond preventative maintenance to predictive maintenance. A major step forward.

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