The SSVF horizontal CNC milling machine features an aggregate stiffness of 2 million pounds per inch at the spindle face. It was built with the help of Dial Machine in Rockford, Illinois, and currently performs test cuts on Dial’s shop floor.
Much of the “super stiffness” is the result of membrane dual restrictor control of the guideway systems. The two long parts seen here are components of the small hydrostatic guideway test system used to prove this theory. The upright piece is the membrane dual restrictor unit, which maintains a constant oil film thickness in the guideways despite changes in force.
(Photo courtesy of Heidenhain.) Rockford Engineering Associates is led by Kanwar Singh, John Hurd...
and John Osborn.
Various companies contributed to the SSVF project. At Dial Machine, this sign sits beside the super-stiff prototype.
Vibration is a worse problem for machining titanium than it is for machining most other metals.
Shops milling titanium understand that the challenge is heat. Titanium is a poor thermal conductor. The heat of machining that might otherwise leave with the chip or dissipate into the workpiece instead gets concentrated in the tool. Microscopic thermal fissures develop in the tool’s surface. This is the start of the rapid tool breakdown that can characterize titanium machining.
However, the vibration of the machining process is what causes these fissures to propagate and deepen. In titanium, heat starts the tool toward failure, but vibration takes it the rest of the way.
NIST, the U.S. government’s National Institute of Standards and Technology, recently funded a $2 million project aimed at developing a “Super-Stiff Vibration-Free” (SSVF) machine tool that could mill titanium more productively than existing machines. The funding went to Rockford Engineering Associates LLC, or REALLCo, an Illinois firm led by machine tool engineers formerly with Ingersoll Milling Machine Co. During the course of three years, REALLCo performed tests to validate a theory for obtaining a stiffer machine-tool bearing system. Then—with the help of Rockford, Illinois-based Dial Machine—the company built a working CNC machine tool based on this principle. The machine, which is now running on Dial’s production floor, has demonstrated the ability to consistently double either tool life or material removal rate during titanium milling. According to the machine’s developers, the key is hydrostatic guideway technology providing stiffness that is (literally) infinite.
REALLCo is led by John Hurd, John Osborn and Kanwar Singh, three men whose combined machine tool experience amounts to more than 120 years. The technology they applied to the super-stiff machine involves regulating the hydrostatic guideway systems through “membrane dual restrictor” control.
Hydrostatic guideways use a thin film of oil to eliminate metal-to-metal contact along the machine’s axes. The membrane dual restrictor devices developed by the Rockford team essentially provide a control loop that maintains this oil film’s thickness. The devices respond mechanically (they have no electronics) to changes in fluid pressure. The principle is not new, but the team says past versions of membrane dual restrictor control have been used to reduce guideway compliance to a point at which stiffness was very high, but still some measurable value. The innovation for the SSVF machine involved creating a system so responsive that it maintains a fixed oil film thickness regardless of the change in force. Thus, the force against the way system can increase without the change producing measurable deflection. Zero deflection per unit increase in force is the mathematical definition of infinite stiffness.
To be sure, the machine tool as a whole does not have stiffness that is infinite. A machine’s assembly consists of large, solid masses that individually distort and comply. Plus (as in any assembly), the compliances of these components are cumulative. However, the stiffness of the guideway system alone removes much of the compliance that is typical of even precision machines. Additionally, the Rockford team engineered the shape of the machine to enhance stiffness, and designed a spindle with hydrostatic bearings that maximizes stiffness as well. Mr. Singh says the resulting aggregate stiffness of the complete machine is 2 million pounds per inch at the spindle face—several times the stiffness of even a rigid standard machining center.
This high stiffness also contributes to accuracy. Linear motors from Siemens drive XYZ motion on the SSVF machine, and the machine’s control system applies a servo gain of 10 for tight precision at high feed rates. A gain of 2 might be unstable on a standard machine. To provide for this responsive control, Heidenhain contributed to the project by providing fine-resolution linear scales (described in the grey box below).
Cutting tool companies joined the project, too. Ingersoll Cutting Tools, Kennametal and Sandvik Coromant all provided tooling for the sake of measuring the machine’s performance. From each of these companies, the team requested tooling engineered for titanium, along with recommendations for speed, feed rate and depth of cut. In test cuts run at these parameters, tool life consistently doubled what would be expected on standard machines. In test cuts run at twice the recommended speed (with advance per tooth kept constant), the super-stiff machine was consistently able to achieve the expected tool life while doubling the rate of material removal.
Since completing the NIST project, the team in Rockford has been performing test cuts like these for various manufacturers with an interest in machining titanium more productively. The machine might make it to market in one of two ways, Mr. Singh says. One possibility is that a machine tool builder might pick up the design and commercialize it. The other possibility is that a contract manufacturer heavily involved in titanium machining might work with a custom builder to produce machines for its own use that are based on the SSVF research model. Indeed, in addition to titanium, the machine’s stiffness would also be effective for machining other heat-resistant materials, including ceramics.
During one demonstration of the machine, Mr. Singh says an observer raised the objection that the machine’s solution to vibration was incomplete. Compliance had been dealt with, the person said, but what about damping?
Mr. Singh says his first response was that the oil film itself provides damping. Hydrostatic bearings are often specified for their damping advantages. In the super-stiff machine, the solution to compliance provides damping for free.
But he had a second response, too. Dealing with compliance actually deals with the need for damping. Low compliance means there is less amplitude for vibration, reducing the vibration’s severity. When the system stiffness is high enough, says Mr. Singh, there is nothing left to damp.
System for High Servo Gain
Siemens’ contribution to the SSVF project included linear motors and the control unit. Heidenhain donated its LC 183 absolute-position linear scales for the X, Y and Z axes. Mark Tingley, chief design engineer for Rockford Engineering Associates, says this combination, along with the machine structure’s high stiffness, enables exceptionally high position-loop gains on the axes.
The engineering advantages of the linear scales go beyond their fine resolution, he says. He explains that the proximity of the Heidenhain encoders to the load was chosen to minimize compliance outside of the position control loop. The linear scales’ serial output eliminates the need for any additional interface. As a result, the team could install the encoders at critical points in tight spaces, interfacing them directly to the control.
Thanks to this motion system, the SSVF machine now achieves position loop gains of 10 mmpm per micron (or 10 ipm per 0.001 inch). The machine can circular interpolate a 25-mm-diameter circle at 200 mmpm within a total error of 1.1 micron, including reversal errors at transition points. This compares to position loop gains of 0.5 to 1.5 in most machine tools, along with geometric error of 10 to 25 microns. The team’s John Hurd says, “We have never seen performance this good.”
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