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Precision Machining


With recent improvements that elevate CNC accuracy, manufacturers can increase metalcutting productivity


By Patrick Waurzyniak
Senior Editor 


Productivity gains often start with machine controls, where advanced programming can quickly and easily add functions capable of greatly improving metalcutting processes. New CNC machining techniques and software for five-axis machining can help manufacturers achieve higher precision with smoother surfaces while cutting metal faster and more efficiently.

Reducing vibration can significantly improve surface finishes and reduce cycle times in five-axis machining. A new feature called Smooth Tool Center Point (TCP) control from GE Fanuc Intelligent Platforms (Charlottesville, VA) was introduced at IMTS. Smooth TCP control is said to enable extremely smooth motion and reduced cycle time during five-axis machining processes using tool center point control.New Smooth TCP control from GE Fanuc reduces vibration and improves surface finishes.

"There's a couple things at play when you're talking about five axis and smoothness, one of which is following the program path," notes Paul Webster, CNC product manager, GE Intelligent Platforms. "It's the CNC's job to figure out the best way to get around the program; the other side is controlling the machine itself, which is the servo side, so it has to make its decisions once it gets the commands from the CNC. Vibration is usually one of the biggest problems you start having with five axis, because you start going fast and trying to cut in all these different areas—the complexity of the math gets pretty high, so you get in situations where you start hitting resonance frequencies or tool chatter."

The Smooth TCP removes redundant program command points generated by tool center point control automatically, and compensates the direction of tool posture to enable smooth motion of the rotary axis, Webster notes. The TCP control reduces vibration in the velocity command to the servo system, resulting in smooth surface finish. "The result is vibration suppression that improves surface finish and reduces cycle times up to 44%," Webster says.

         With its new KinematicsOpt feature, the Heidenhain iTNC 530 control can improve precision on five-axis machining of complex parts.

Another factor in reducing vibration is the vibration suppression inherent in GE Fanuc's servosystem, which offers four levels of High Response Vector (HRV) control, Webster adds. Introduced several years ago, the current top-end HRV4 is available on GE Fanuc's 30 and 31 series controls. "That really speeds up the control loops; each iteration doubles the speed of the control loops," Webster says of HRV. "By using higher versions of HRV, you can increase the bandwidth of your servosystems, and lower the vibration.   

"As it gets more advanced, there are more filters, and also learning filters, that actually move back and forth to correct for vibration the system sees in the servosystem," Webster explains. "This is the very basic, fundamental side on the servo. Then in the CNC we're getting into the smooth compensations and features that actually smooth out the command points. By smoothing out the command points, you don't get these jerky acc/decs that add to chatter in the tool. So if you combine the two things, what's going on in the servosystem with HRV and filters, and then on the CNC side, you're getting smooth motion commands. The two of those work together to eliminate vibration."

Volumetric compensation now available on the Siemens Sinumerik 840D solution line machine controller from Siemens Energy & Automation Inc., Machine Tool Business (Elk Grove Village, IL), helps manufacturers machine parts on three-axis and five-axis machines with much greater accuracy.

"One of two technologies for aerospace that has emerged in the last 12 months is volumetric compensation," says Timothy D. Shafer, director, Aerospace Center of Competence, Siemens Energy & Automation Inc. (Mason, OH). "That's really being driven by the requirements of the Joint Strike Fighter [JSF] program. The manufacturing tolerances that their designs require are about five times tighter than what they are in the rest of aerospace, so the supply chain obviously has a very big manufacturing challenge to meet their tolerances.

"We've been working with Lockheed Martin and Northrop Grumman to come up with an integrated way to compensate a machine to help the supply chain for JSF meet their tolerances," Shafer says, "and the result is VCS, Volumetric Compensation System, for the 840D sl, the solution line control from Siemens. This feature is a derivative that came came from the CMM world. There's a mathematical model for the errors on the machine; it's well-defined and published."

With the VCS software, a Cartesian machine tool can more accurately find its tool center point and eliminate the 21 potential sources of error in not just the X, Y, and Z linear axes, but also in pitch, yaw, and roll, and three squareness errors, Shafer notes. "As you're moving down a single axis, it could be off if you're moving up and down, if you're moving side to side, basically positioned too far or too short," he adds, "so there's three linear displacement errors, and there's pitch, yaw, roll. As you're moving, you could be pitching, yawing, or rolling."

To measure the errors, Siemens teamed with laser metrology companies including Renishaw GmbH, Automated Precision Inc., and AfM (Accuracy for Machines) Technology GmbH, Shafer notes. "You put a measuring device on it and measure the 21 sources of error, with laser trackers or laser interferometers. We're not measurement experts—that's not our job. Our job is when they measure the error to enable them a way to compensate. That's where we come in; we've created that capability with VCS, and the measurement companies are finding the best way to measure the errors as accurately as possible. That's why we partnered with them."

Accurate simulations also are key to aerospace manufacturing, and Siemens has refined its Virtual NC Kernel (VNCK) to offer more realistic simulations. Shafer notes that the VNCK software plug-in module replicates the Siemens Sinumerik 840D kernel and runs within major simulation packages, including Vericut from CGTech Corp. (Irvine, CA), NX CAD/CAM software from Siemens PLM Software (Plano, TX), and in CATIA/Delmia software from Dassault Systèmes (Paris).

The VNCK capabilities enable machine tool users to much more accurately estimate the time required for metalcutting tasks. In addition, the actual cutter path, calculated by the CNC, is presented to the simulation package. Vericut and other simulation software packages have offered this simulation for years, but the simulations offered by such packages were always estimates, since the software developers lacked the actual algorithms of the 840D control and the machine's commissioning data.

"What they historically have done is to guess at how the CNC calculations are going to turn out, and they do a pretty good job of it," Shafer says. "The problem is, with today's advanced CNCs, a lot of the calculations that were in the postprocessor before now have moved over and are being done real-time in the CNC. For example, the transformation calculations now are done real-time in the CNC. You don't have to do them in the postprocessor. As the industry moved over and started using the power of the CNC, the calculations' best guess got farther and farther away from what the machine was really doing, because they just don't know how we perform the calculation. That's why we came out with the 840D relocatable software module called VNCK that actually plugs into Vericut 6.0, NX 5, and CATIA V5/Delmia/ICAM."

Short PLC and CNC cycle times make the Bosch Rexroth IndraMotion Advanced CNC suitable for multiaxis machining and rotary indexing machines.Software enhancements from Heidenhain Corp. (Schaumburg, IL) for its iTNC 530 control improve precision for multiaxis machining applications. "Accuracy requirements are becoming increasingly stringent, particularly in the area of five-axis machining," notes Chris Weber, Heidenhain product manager. "Complex parts are required to be manufactured with precision and reproducible accuracy even over long periods."   

With the new iTNC control's KinematicsOpt function, machine tool operators can insert a Heidenhain touch probe, and a 3-D touch-probe cycle measures the machine's rotary axes fully automatically, Weber says. "The results of measurement are the same regardless of whether the axis is a rotary table, a tilting table, or a swivel head," he notes.

"To measure the rotary axes, a calibration ball is fixed at any position on the machine table and probed with the Heidenhain touch probe," Weber says. "But first you define the resolution of the measurement and define for each rotary axis the area that you want to measure. From the measured values, the TNC calculates the statistical tilting accuracy. The software minimizes the spatial error arising from the tilting movements and, at the end of the measurement process, automatically saves the machine geometry in the respective machine constants of the kinematic table. Of course, a comprehensive log file is also saved with the actual measured values and the measured and optimized dispersion [measure for the static tilting accuracy], as well as the actual compensation values."

The application enables measurement of a calibration standard in space with a workpiece touch probe (OEM cycles); measurement in a tilted coordinate system; and it permits very accurate conclusions about the machine's accuracy, Weber adds. The system compensates for the machine's inaccuracies via automatic compensation within the machine kinematics, and a recalibration is performed during maintenance of the machine. It also performs back-up and restoration of kinematics, initial measurement and optimization of the active kinematics, and static kinematics compensation in selectable axes.

Flexible, affordable controls that are easy to use and adapt to many applications are in demand by today's companies. "Manufacturers are looking for a simple CNC system that an operator can easily adapt to regardless of their skill level," says Todd Drane of Fagor Automation Corp. (Elk Grove Village, IL). "Customers across the country have had difficulty in finding experienced people to run their CNC machines, and thus need the CNC systems to continue to get simpler to use. Fagor has continued to refine its user interface with this in mind."

         Volumetric Compensation System (VCS) software gives the Siemens Sinumerik 840D sl control greater precision in five-axis machining.

With Fagor's new 8070 ETX CNC control system, manufacturers get an operating system that contains the ability to program in either traditional ISO G-code language or conversational, Drane notes. "The customer may toggle from one method to the other by simply pressing Shift-Esc," he says. "We utilize an advanced ICON key conversational system in which the operator does not require any knowledge of G-code at all; they simply fill in the blank based upon the graphical example presented to them. This flexibility gives the customer the option to approach a particular job in any way they choose." 

Designed to be an affordable solution for high-performance applications, the Fagor 8070 ETX CNC is based upon an industrial hardened Windows operating system running on an XP embedded platform, thus allowing the integration of a host of online software configurations for a true open-architecture system, Drane notes. In addition, the 8070 is built with a solid-state disk, thus ensuring maximum reliability to work in harsh industrial environments.

The operating system includes the FBWF (File-Based Write Filter) feature, which provides protected disk access that shields the 8070 CNC against viruses, hard shutdowns, and undirected software installations. The system increases productivity while maintaining part-finish accuracy, Drane adds, optimizing block processing time (lower than 0.5 msec) and analyzing the toolpath in advance with high-speed block look-ahead that results in optimum part finish. "Toolpath transition gets smoother thanks to the predictive-control feature, which applies the most efficient acceleration control for each machining operation," Drane says. "Mechanical stress is further minimized by applying post-interpolation filters that eliminate the resonance frequencies common to high speed machines, thus achieving smoother machining, better part finish, and longer life span of the machine components."

More powerful CNCs from Bosch Rexroth Corp.—Electric Drives and Controls (Hoffman Estates, IL) include the MTX Advanced control unveiled at IMTS 2008. Part of the Rexroth IndraMotion family of PC-based CNCs, the MTX Advanced CNC is aimed at high-end applications and features a higher-performance Intel-based dualcore processor with up to 4GB of memory, two LAN ports for data traffic, and USB 2.0 for connecting peripherals.

The MTX Advanced offers extremely short PLC and CNC cycle times, making it suitable for demanding applications such as multiaxis machining and rotary indexing machines with multiple stations. The Sercos interface provides connectivity to digital electric drives in the Rexroth IndraDrive product range. Fieldbus nodes and the I/O peripherals are attached to the integrated onboard Profibus DP interface, and a high-speed Ethernet port on the CPU unit links the MTX advanced into the controller network.

"Our MTX Advanced is capable of doing 64 axes of motion logic and 12 CNC processes concurrently," notes Dan Throne of Bosch Rexroth. "It has a lot of advanced grinding and specialized cutting algorithms built-in. The CNC controller plugs into a new PC box on which we've increased the performance with a processor and operating system that allows us to run the CNC fast, and the motion control board is at the very high end of CNC applications. It scales with the same software as the lower end or standard type of CNCs, the MTX compact."

         The 8070 ETX CNC from Fagor Automation offers a high-resolution TFT LCD and custom touch-screen interfaces.           

The Linux-based Power PMAC real-time motion-control system from Delta Tau Data Systems Inc. (Chatsworth, CA) offers vastly improved performance with 64- bit floating point calculation capability, states Vince Burokas, Delta Tau CNC product manager. The new Power PMAC control features a 1-GHz PowerPC RISC processor rather than the previous versions' Motorola DSP chips, Burokas notes, and it is also the first Delta Tau controller to offer the Linux operating system.

"We really drew a line in the sand, and it's completely different than the old architecture," notes Burokas of the new system. "We don't use the Motorola DSP anymore. The PowerPC chip is embedded, with real-time Linux, and it's many magnitudes faster than the old generation. It has its own Integrated Development Environment [IDE] software environment as well, which is completely different. You don't have to memorize—it's a character-recognition type of environment, and it offers you a list of options."


This article was first published in the January 2009 edition of Manufacturing Engineering magazine. 

Published Date : 1/1/2009

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