A Concrete Solution to a Cast-Iron Problem
Category: AMT • Dec 8, 2020
When Oak Ridge National Laboratory (ORNL) researchers at the Department of Energy’s Manufacturing Demonstration Facility (MDF) set an audacious goal—such as 3D printing the Strati electric car live at IMTS 2014—they call it a moonshot. IMTS spark and the IMTS Network have been chronicling the research team’s current moon shot, a project that can change the trajectory of the U.S. machine tool industry: creating a machine tool base from a 3D-printed polymer mold and concrete.
Nearly all manufacturing machinery has cast components, but unfortunately, the number of cast iron facilities in the U.S. declined from 774 facilities in 1987 to 330 by 2017. In short, the U.S. lacks sufficient capabilities to produce bases for “mother machines”—the machine tools that make other machines. In the absence of a WWII-level crisis, the chance of restarting hundreds of U.S. foundries is non-existent, especially given current economics and environmental pressures. Fortunately, the U.S. Department of Defense recognizes that secure and local production of machine tools is essential for national security.
“When you don’t have sufficient capacity to produce machine tools domestically, then the supply chains for the things that we need start to become long. Certainly that is a problem for defense—we can’t make what we need when we need it—but it’s true across the rest of economy as well,” says Scott Smith, Group Leader, Intelligent Machine Tools at ORNL. He cites our inability to surge PPE production capacity as a prime example. Fundamental machine components (e.g., ball screws) often have a lead time measured in months.
A New Base
In 2018, the DoD initiated a partnership with ORNL as part of its Industrial Base Analysis and Sustainment Program (IBAS, pronounced I-bass). Others in the partnership include the Institute for Advanced Composites Manufacturing Innovation and the University of Tennessee, Knoxville. The entire program falls under a suite of projects known as ACE, or America’s Cutting Edge, which represents a collaboration between the Department of Energy (ORNL operates under the DOE) and the DoD.
Much like choosing to go to the moon before the end of the decade accelerated results, the current moonshot project has yielded fast results. Less than a year after commencing, the team working at the MDF has produced the base for a 3-axis vertical CNC mill with a 30 x 20 x 16 in. work volume.
To create the mold for the base, ORNL used its Big Area Additive Manufacturing™ system (BAAM) and 20% carbon fiber ABS pellets and a standard, ready-to-mix concrete formula to create a 15,000-lb. base.
The concept of using concrete for machine tool bases is not new. American Machinist magazine explored the topic of using polymer concrete in a 2008 article. What is new is using AM to create the molds and embedding 3-axis accelerometers and temperature loggers to digitally capture data. Together, they allow rapidly evaluating performance, exploring base geometries never before possible and adjusting concrete formulations to optimize damping capabilities—all at a comparatively low cost.
“Machine tools require a base that offers some special characteristics,” says Tony Schmitz, Ph.D., Joint Faculty at ORNL, Professor, University of Tennessee, Knoxville. “You want them to be stiff, so they don’t deform under the machining loads. You want them to have high damping, so they dissipate energy. You want them to be thermally stable, so they don’t change shape as the temperature changes. So in this project, we’re looking at addressing all of those issues using a concrete base.”
To measure damping, the ORNL team used a technique called tap testing. The test involves striking the base with an instrumented hammer on one side and recording the vibration using an accelerometer placed on the opposite side (think of it as a tuning fork for machine tools). The resulting frequency response function (FRF) enabled the team to explore the base’s natural frequencies, modal stiffness values, and modal damping ratios.
Schmitz explains that concrete works well for machine tool bases because of its inherent vibration damping capabilities. Compared to cast iron’s crystalline structure, which is comparatively homogenous, concrete’s microstructure offers more possibilities to absorb energy as the vibrations pass through the cell boundaries between the cement and aggregate mix.
“When you perform these type of tests on cast iron, a typical damping ratio is about 0.25 percent,” says Schmitz. “Within our measurements on the concrete/polymer structure, we see values that are 2.8 to 68 times higher than what we might expect from cast iron. Immediately, we were excited about the possibility to add damping to our structure through the replacement of cast iron with concrete.”
Natural vibration frequencies have a corresponding mode shape, or deformation profile, that the structure takes when vibrating. Machinists understand this phenomena. Just by the sounds emanating from the cutting zone, they know what’s happening. They know that certain combinations of feeds and speeds lead to smooth surfaces, while others create frequencies that lead to tool chatter, ruined parts, and broken tools.
Aware of the need for good damping, the ORNL team measured mode shape, or the direction in which the base moved. Schmitz covered some of the team’s results in the second of four IMTS spark sessions devoted to the moonshot project. Not surprisingly, different frequencies produced different mode shapes. At 250 Hz, the top of the vertical columns of the base moved inward (cantilevered) towards each other. At 356 Hz, the columns twisted along the Y axis (for an animation of these mode shapes, as seen in this IMTS spark session, “ETC Talks - Moonshot: Structural Dynamics for Concrete Base Machine Tool”).
After tests without the cross rail, they performed the same tests with it. In “mode 4,” an impact on the top of the column just beneath the cross rail, adding the crossbeam improved the stiffness of the base about five times. The damping ratio decreased slightly, from 0.97 to 0.88, meaning that performance of the overall structure improved with the addition of the cross rail.
As part of its next steps, the team is continuing to explore frequency response functions and variations in concrete formulations and mold designs. Emma Betters, a Ph.D. student and ORNL R&D Assistant Staff Member in the Intelligent Machine Tools Group, notes that while much is known about predicting the elastic modulus of concrete, prediction of its damping ratios remained largely unstudied.
The team will also explore adding internal features and options to stiffen the base besides the steel rebar currently being used. Because of the embedded digital sensors, data collection will be much easier. Ultimately, the ORNL research team will work towards creating a digital twin of the base.
“We have this large, green landscape of design freedom that’s enabled by additive manufacturing. We’re really excited about how we could change the way machine tool bases are designed and implemented,” says Schmitz.