March 2008 Edition

MACHINING EXOTIC MATERIALS

Polishing Off the Work

Using magneto-rheological fluid cutting and polishing of materials ranging from exotic sapphire to mundane acrylic, means having a tool that never dulls. Pioneering MRF machining is a company applying it to the shop environment.

In Latin, Quod Erat Demonstrandum — QED — literally means "that what was to be demonstrated." The phrase is used as the last statement in a philosophical or mathematical argument to mean the proof is complete. There's not much call for Latin in the machine shop, but for QED Technologies Inc., Rochester, NY, it means the proof of its quality is self evident.

The company deals in high-precision finishing and polishing machinery for the optics, telecom, and semiconductor markets. Achieving these finishes in significantly lower production times by developing advanced precision manufacturing technology, aided by CNC platforms, has been the company's goal since it was founded in 1996.

But, there were obsticals to be conquered before the company could put QED at the end of its proof.

The first challenge was processing the materials. Typical applications for the QED machines involve brittle and hard substrates such as optical glass and glass ceramics such as fused silica, borosilicate, and Zerodur; single crystal materials such as sapphire, calcium fluoride, and silicon; and polymers like acrylic.

The applications can involve optical or electro-optical systems, including miniaturized micro-optoelectromechanical systems, megapixel recording devices, optical communications, computer storage devices, integrated circuit fabrication, and other emerging technologies that benefit from cost-effective production of precise optical components.

Exotics on Exotics

QED Technologies uses CNC-based polishing machines that use a magnetic- or magneto-rheological fluid — MRF — to finish workpiece elements in minutes, replacing manual technologies that required hours, weeks or even months. MRF-based manufacturing provides rapid, deterministic, and repeatable surface-shape corrections in minutes to levels of precision and surface finish previously considered impossible.

MRF machining was created in the academic environment and QED developed it into an industrial platform. At QED headquarters, machines are designed, built, and assembled. QED also conducts MRF research and software development at its customer and user facility which has precision metrology capabilities for prototype lens production. It also provides machine demonstrations there, as well.

The MRF process uses a magneto-rheological fluid as a polishing tool. The workpiece is installed at a fixed distance from a spherical wheel which rotates about its horizontal axis.

An electromagnet located below the wheel surface generates a radiant magnetic field in the gap between the wheel and the workpiece. The MR fluid is delivered to the rotating wheel, pulled against the wheel's velocity and becomes a subaperture polishing tool.

The computer program controls the dwell time — how long the optic is immersed in the MRF — and determines a schedule for varying the position of the rotating workpiece through the fluid.

Before beginning a polishing sequence, both the surface to be processed and the material removal function of the MR fluid tool are precisely characterized. The surface is characterized using various types of interferometric metrology, producing an accurate map of the surface.

Errors Less than Two-millionths of an Inch

The removal rate of the MRF correlates to its viscosity, which is controlled to ±1 percent. A key to the MRF process is that the tool does not change, creating a machine tool that never dulls. A spiral toolpath for round parts, or a raster toolpath for rectangular parts are created by de-convolving the MRF with the desired removal function.

"Our process uses MRF to polish optical surfaces to extreme precision," Steve Hogan, QED alliance manager, said, "where the form error on a surface of 8" in diameter is typically less than one- to two-millionths of an inch. We accomplish and maintain this accuracy by optimizing the servo tuning on the machine tool and taking advantage of the many extended functions available on the Siemens control package, such as beam sag, friction, backlash and other compensations."

The process self-programs to compensate for any variables or machine misalignment to achieve the desired outcome of surface finish and shape.

The stability of this polishing tool creates a deterministic process which lets the user accurately predict the results of the polishing run before the cycle begins.

The fluid stability, combined with a robust machine platform, lets the MRF tool be compliant, creating a high level of deterministic polishing on precision surfaces.

The company relies on its core competencies for design and application engineering, while outsourcing large component manufacturing, system integration, and the CNCs used onboard QED machines. The company has partnered with Siemens Corp., Elk Grove Village, IL, to supply much of the critical motion control components and CNC equipment.

"The Siemens control package provides the critical flexibility necessary for QED's software to allow our machines to self-program, based on the advanced knowledge gained during the characterization phase," Hogan said.

On several of its machinery lines, QED buys or retrofits a standard grinding machine or machining center, modifying the original design to accommodate the unique needs of its customer applications. Standard QED Q22 Series machines now polish parts or material workpieces up to 750mm as spheres, flats, aspheres, mirrors, windows, prisms, cylindrical surfaces, and more. The latest innovation is QED's Subaperture Stitching Interferometer, a 6-axis, CNC interferomatic workstation enabling the capture of precise metrology data for larger — up to 200mm diameter — parts, or convex or concave parts.

Partners in Production

The CNC most often found on QED machines is the Siemens SINUMERIK 840D, with HMI Advanced Machine Monitoring and Control — MMC. Proprietary C++ application data from QED software passes information onto the HMI Advanced MMC.

Typically, a 5-axis CNC with Siemens 611 drive packages is used with the company's 1FT6/1FK6 motor on each axis. An auxiliary 611U is often used to control additional servomotor components. These components can be controlled through the CNC programs or the integrated PLC provided with the 840D. All machine digital and analog I/O are remote and connected via Profibus to the PLC for machine safety, monitoring, and control.

"The open architecture offered in the Siemens environment provides us with substantial possibilities in setting up our machine controls," Hogan said.

"Our user interface is created for easy use in the optics industry, so our customers don't need to learn machine tool language," Hogan said. "Several CNC subprograms are selected and run via this custom program. They're used to position the axes for easy part loading or probing, prior to process running. After entering all the critical lens information, a part program is automatically created which generates the tool path of the lens or workpiece as it moves through the MR fluid." Siemens Corp.

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