Time is money in production operations, and machine productivity is often the determining factor in a manufacturer’s profit or loss. So companies sat up and paid close attention when Swedish tooling supplier System 3R International AB came up with a workpiece-clamping chuck that absorbs machining vibrations, enabling operators to increase metalcutting speeds while improving accuracy and lowering tool wear in milling machines and other machining centers.
Ordinary chucks cannot adequately control vibrations in the workpiece induced by dynamic forces from cutting-tool impact and rotation. So to avoid instability in the machining process, operators slow down cutter speed and feed rates to achieve the required machining accuracy. Even at these slower speeds, however, some persistent vibration still exists, producing gouges and ridges in the workpiece finish, thus necessitating a second or third pass to machine away the imperfections and achieve a smooth surface finish. These slower speeds and additional operations not only lengthen the time to complete machining operations but also increase the wear on cutting tools, which have to be replaced more often.
Benefits of a damped chuck
In the machining environment, operational speed and machining quality depend largely on the combined dynamic stability of three broad components of the elastic system: the cutting tool, the machine powering the tool and the workpiece clamping system. Extensive research has been conducted over the years by the machine tool industry to improve the stability of cutting tools and machining centers. Little research has been conducted on the dynamics of the workpiece clamping chuck, however, which has traditionally been a totally rigid component. But not any more.
After years of research into the dynamics of clamping systems, System 3R has developed and patented their revolutionary new Vibration Damped Palletization (VDP) chuck with a high damping interface comprised of a composite of viscoelastic polymers and metallic foils for structural rigidity. By absorbing workpiece vibration and a significant part of the cutting tool vibration, the VDP improves the dynamics of the entire machining process, enabling operators to run their machining centers at higher speeds with less tool wear and greater accuracy. Parts usually can be completed in a single pass to tight dimensional tolerances and surface finishes down to one micron – a millionth of a meter. As a result, interest in the VDP has been high and companies producing precision parts have achieved significant benefits.
Customers report impressive gains
At Swedish automotive component supplier Lidingö Produktion AB, general manager Bojan Stefanov reports that using the VDP damped chuck on their five-axis milling machines enables the company to achieve required surface finishes in one pass instead of two. “What’s more,” says Stefanov, “we found that we could double the machine feed rate, and tooling lasts twice as long.”
Similarly, Ingersoll Cutting Tools in Germany reduced machining time 32% and improved surface finish 72% using VDP in making titanium alloy aircraft components. MGM Moldes in Portugal shortened machining time 40% in making copper electrodes. Epic Yamada Corp. in Japan cut machining time in half and doubled tool life in the production of precision die for the semiconductor industry. A Swiss manufacturer of high-speed milling machines has improved stability of milling processes using VDP chucks, significantly increasing metal removal rates by enabling cutting depth to be tripled at a spindle speed of 10,000 rpm, for example, from 3 mm to 9 mm – the maximum allowed for the cutting tool.
Future business prospects for the VDP appear strong
According to statistics from Gardner Research, total worldwide investments in machine tools increased almost 70% since 2001 and jumped more than 10% this past year alone to reach an estimated $59.6 billion in 2006. This growing base of machine tools will likely increase VDP sales to first and second-tier manufacturers, as well as machine tool builders that have expressed interest in integrating the VDP into their product offerings.
Critical role of vibration testing
Dr. Amir Rashid, Manager of Research and Development at System 3R, was instrumental in developing the concept of the VDP and continues to lead the on-going work in designing the company’s expanding product line. In this work, engineers “tune” the compliance of the VDP by properly sizing the thickness, surface area and adjusting pre-load on the damping interface to damp the expected vibration loads, which change throughout the machining process as material is removed from the workpiece.
“The structure is designed with enough static but high dynamic rigidity to reduce machining vibrations in the expected frequency range while holding the workpiece in an exact position,” says Rashid. “The challenge is in accurately determining the level of damping required in the expected frequency range while retaining enough rigidity. In our development process, the LMS Test.Lab software is a key element in efficiently performing numerous vibration test cycles and analyzing the data as quickly as possible.”
The test system implemented at System 3R consists of two LMS SCADAS III data acquisition front-end units, each with eight data-acquisition channels and built-in signal-conditioning capabilities. The laptop-size units each connect to the LMS Test.Lab software for test control, measurement, result analysis, data management and report generation in an integrated platform.
Designing the compliant structure
Rashid explains that the development process begins with vibration testing of the workpiece and machining center without the VDP. With accelerometers mounted at several locations, engineers hit the structures with an instrumented hammer to determine their dynamic behavior. Operational speed-sweep tests are also performed to measure system vibration for a range of machine frequencies.
Based on data from these tests, LMS Test Lab generates animated mode-shape and operational deflection shape (ODS) displays showing deformation of the hardware. Also, Fourier Transform of vibration amplitude versus time data recording generate frequency domain maps for studying the system response at various frequencies. On these frequency plots, LMS Test.Lab uses colored spikes called “posts” to clearly highlight resonances that otherwise would take hours to spot on raw FRF data. Automation of this task saves time and also yields more accurate and consistent results.
These analyses of the vibration data enable engineers to calculate the energy being transmitted through the components and thus determine the level of damping required in the interface designed in the holding fixture . From previous empirical experience, engineers know the energy absorption properties of the polymer, so they size the interface to deliver the required damping.
Based on these specifications, a prototype VDP is built, and the same battery of modal impact and operational tests are run. LMS Test.Lab signature analysis plots of machining tests then show the level of reduction in vibration amplitudes measured on the workpiece held with the VDP system. Test data for vibration with and without the VDP are compared and the design of the damping interface is adjusted accordingly in an iterative process to damp out as much vibration as possible while maintaining workpiece stability and staying within the packaging requirements of the clamping system.
Value of an integrated test platform
“By providing one system platform, LMS Test.Lab gives us the speed and accuracy we need in performing the iterative vibration tests needed in the development of this new product concept,” says Rashid. “Previously we needed a separate software package for data acquisition with the modal and machining tests, another for modal analysis, and a third for generating machining test frequency domain maps.”
He explains that the systems were physically separated, so different people would be working with them and regular scheduling delays were caused. Also, exchanging data between systems was time-consuming and error-prone, since data many times would not convert completely and therefore had to be done manually.”
“Having all the vibration modules we need on a single, integrated platform lets us work faster with fewer mistakes. We run the test quickly and have results immediately without having to switch between separate systems. With LMS Test.Lab, we can complete tests and have results in a few hours instead of days. This increased productivity is critical for us in designing the various models to accommodate a wide range of machining applications and workpiece sizes, including some VDPs being custom-developed for particular clients.”
Rashid notes that worksheets and templates in LMS Test.Lab are especially useful in shortening the time to set up vibration tests. “There are numerous helpful worksheets for specific functions such as modal analysis, time data processing, and signature testing,” he says. Also, templates have instrumentation sensitivities and other information entered automatically instead of requiring engineers to manually enter the data. The test setup and required signal processing tasks are defined once, then they are saved in a template and procedures can then be performed automatically and quickly with highly consistent results. This lowers the risk of errors and is highly efficient, with test set-up time reported to be at least 30% less for System 3R.
“By enabling us to perform a wide range of vibration testing faster and more accurately on a single integrated platform, LMS Test.Lab avoids delays and mistakes that otherwise would slow down our operations. From a business perspective, the LMS system is an indispensable part of our continuing efforts in strengthening our leadership position in developing a steady stream of innovative products for the fast-moving global manufacturing industry.”
