Motion Enhances Machine Tool Performance
Producing high-quality, tightly toleranced parts require well-controlled 3-D motion.
by Kristin Lewotsky, Contributing Editor
Machining highly accurate parts presents extremely tight performance specifications. There was a time the technology was more art than science and users were prepared to be patient for the results. No more. Today, a wide range of commercial applications put job shops under pressure for high-volume production even as performance requirements tighten. Motion control provides the best solution for economically producing sophisticated components from a variety of materials ranging from aluminum, brass, copper, and nickel to optical crystals and other exotics.
Machine tools typically feature a cutting tool on one axis and positioning equipment on the other two. In a lathe, typically used to produce rotationally symmetric parts, a high-speed spindle rotates the work piece while the cutting bit is translated on additional axes. The cutting tool can translate parallel to the spindle to remove material from the outside of the shaft or cut threads into the shaft. It can move perpendicular to the spindle to cut material from the face of the work piece, or in line with the spindle to drill holes. A milling machine can produce non-rotationally symmetric parts by combining a rotating cutting bit on the z axis with a positioning table along the x and y axes.
Computer-numeric controlled (CNC) machine tools leverage motion control to produce far more sophisticated three-dimensional profiles rapidly and repeatedly. A CNC diamond-turning machine, for example, can rapidly and economically fabricate spherical metal lenses for infrared imaging applications. CNC designs can include horizontal and vertical spindles, as well as multiple-spindle versions. The machines can produce parts with tolerances ranging from a few hundred micrometers to a few nanometers RMS. Because of the emphasis on accuracy, they leverage speed rather than torque to remove only small amounts of material at any one time.
With these designs, axes counts can mount up quickly. CNC machine builder Murata Machinery USA, Inc. integrates the basic turning machine with an x, y, z gantry loader, also driven by motion control. A typical twin-spindle machine, for example, will feature x- and z-axis cutting motions on each spindle, plus a gantry loader with x-, y-, and z-axis capabilities for a total of seven axes, all positioned by servo motor. Add a second gantry, and the total number of axes jumps up to 10.
The load/unload capabilities are essential because today machine tools are part of a production environment. “The diamond-turning industry has basically evolved over the last 10 or 20 years from a laboratory environment where time really did not matter to a situation where many of these systems now are used in a production setting," says Pat Hurst, Engineering Manager at CNC diamond-turning machine builder Moore Nanotechnology Systems LLC.
“The customer is not making money if the cutting spindle is not moving," says Jeff Kalmbach, Engineering Manager, machine tools division, at Muratec. Cycle times can range from eight seconds to several minutes, depending on part size, which can run from a fraction of a kilogram to more than 100 kg. “If we have a cutting time of 30 s on a single-gantry machine, the gantry is constantly moving during that cutting cycle. So the focus is to optimize the gantry loading with the cutting spindle.”
Part of the optimization includes safety technology, especially for systems featuring more than one gantry. "There are safety zones that can be set up and customized depending upon what is being done," says Kalmbach. "There is quite a bit of communication done internally with the control system to make sure that there is never any overlap between the two gantries.”
Designing for tight tolerances
Torque ripple or cogging effects in motors can compromise quality of parts produced by CNC machines. Varying spindle speed can help with this, as can high-pole-count motors.
For truly demanding applications, controls and feedback may provide the best solution. Moore produces machines that are used, among other things, to machine optical components of glass, for visible-wavelength applications, and more exotic materials like zinc selenide and germanium, for IR applications. The machines can deliver parts with surface finish requirements as tight as 1 nm rms. Achieving that level of results requires ultra-smooth motion on all axes, coupled with high-spatial-resolution feedback. For x, y, z motion, the machines use air bearings or hydrostatic bearings driven by linear motors. The air bearings help smooth out any cogging introduced by the motor and the feedback loop does the rest.
“The linear motors have some cogging but they’re sinusoidally commutated, which means we map out the theoretical force curves in the motor very close to what is provided," says Jeff Lowe, senior controls engineer at Moore. “Beyond that, it’s up to our servo loop to close the loop and linearize a nonlinear system.”
The team uses interpolation to convert a 137-nm signal pitch to a measurement resolution of 34 pm. “In the world of linear motors, it’s very hard to get a good velocity signal for loop stabilization," says Lowe. “The way we work around that is by interpolation of the feedback encoder to derive a velocity signal. This improves system damping and disturbance rejection, and helps us to get measurements in the 1 nm range.”
The machines typically process parts ranging in size from less than a millimeter to 450 mm. Fabricating spherical components requires purely rotational motion. Fabricating spheres, which reduce optical aberrations, requires not just rotation but an oscillatory motion. In the case of a large spherical part, fabrication can take as long as 10 hours.
All that oscillation tends to introduce vibration which can create significant problems for jobs with ultra-high smoothness requirements. The starting point to address this is striking a balance between rigidity and compliance in the initial machine design. Here, mechatronic modeling techniques can provide a big assist. Various system elements such as hydrostatic bearings and even the chassis material can act to damp any surviving vibrations, as can the control system. Mounting the machine on vibration-isolation components further improves performance.
Machine designs can also be adapted to grinding applications, primarily to produce molds. When it comes to grinding, the amount of material removed is directly proportional to the force applied. Depending on the material and the peak-to-peak surface variations being tackled, a surprisingly low amount of force can be used to adjust surface figure and finish. A skilled optician, for example, can remove a ridge from the edge of an optic just by pressing lightly with a fingertip as the glass turns. In the case of millimeter-scale optics, the sensitivity to applied force jumps dramatically. Mounting the 3 mm to 10 mm grind wheels on air-bearing spindles allows the degree of force to be tightly constrained and adjusted with each pass.
Heat is a perennial concern in motion control applications. Given the tolerances involved in producing these parts, thermal management becomes essential. Motors have to be undersized so that they do not generate heat that might change the physical parameters of the part. “When you're trying to maintain 50- to 100-nm surface profile - ?/10 [or you can say “a 10th of a wavelength” if you can't do Greek symbols] - on parts that are up to a couple hundred of millimeters, thermals are the killer in the whole process,” says Lowe. “If we can avoid heating [by over driving] the motors, we do that. In addition, maintaining a thermal envelope in the machine becomes critical.”
Motion control becomes ever more sophisticated, providing the machine tool community with a variety of sophisticated platforms to easily produce multi-functional machine tools (see figure 2). As CNC fabrication penetrates into more and more application areas, demand for performance and throughput will continue to rise. With the help of motion control, OEMs and end-users will have no problem keeping pace.