Cutting curved parts is an extreme challenge for synchronized motion.
Automation has expanded the average machine tool's functionality: Sensors, HMIs, and most importantly, coordinated axes and controls are making processes — particularly the cutting of curved paths — more exact. Where maneuvers are optimized to the limits of physics, automation can still make operations more efficient.
Nowhere is its integration more critical than in 2D shape cutting (in which plates or slabs are cut with noncontacting plasma, laser, water, and oxyfuel) or 3D part contouring — in which a traditional miller or router cuts material to make 2D or 3D parts.
These machines require more integration than most in other industries, because the motion isn't prescribed. Instead, paths continually change directions, and the cutting instructions for them (typically by CNC) must be meshed with the motion controls. Case in point: A fairly common contouring setup leveraging integration moves a workpiece-holding table as a separate spindle jogs over the part on its own trajectory.
Connecting the dots
Directing a machine to cut shapes and contours requires interpolation — the mathematical filling in of points on a curved path between those known. Twenty years ago, interpolation for contours was executed through CNC by tiny successive moves in X and Y directions — a bit like drawing with an Etch A Sketch — to cut arcs by stepped approximation. This approach requires abundant computing power; it is slow where curves are tight or require particularly small “bites” to meet higher accuracy or surface-finish requirements; and the finished product is left with ridges.
Many newer contouring machines now leverage the power of CAD files and the curves that they mathematically define so economically. One such tool for cutting 3D parts is CAD/CAM software called Mastercam X5 from CNC Software Inc., Tolland, Conn. It executes dynamic milling that constantly adjusts the toolpath to make full use of flute length; a dynamic contour function uses an intelligent strategy to remove material along walls in multiple passes. Cutting motions are commanded in steady position-velocity-time sweeps for a smoother final product.
Another technique executed with the software, hybrid finishing, evaluates model shape and smoothly switches between constant Z cutting (in which material is roughed out in horizontal layers) and constant scallop machining — a kind of finishing that moves the cutting tool in regularly spaced passes, and lifts and plunges it vertically over raised and cutout features, to hold the small “ridges” of the final surface finish consistent.
Instead of defining arcs by successive X and Y moves, newer software drives along splines. These are curves mathematically defined with anchors and weighted handles that define how sharply the paths swerve to one direction or another. SigmaNEST (published by SigmaTEK Systems LLC, Cincinnati) is another CAD/CAM nesting software that defines moves in such a manner for sheet metal fabrication and profile cutting.
By default many of these software packages are written for PCs. It's characteristic of the industry: Many OEMs in shape cutting are smaller companies choosing to make their own controllers (usually PC-based) or buy controllers with an open network interface. It's no wonder then that there's a higher concentration of PC control than in other industries: PC-control specialist Beckhoff Automation LLC, Burnsville, Minn., for example, supplies a controller (and TwinCAT CNC automation software) to Messer Cutting Systems Inc., Menomonee Falls, Wis., for plasma cutting machines that execute Messer-written software called Global Control.
Another setup is integrated CNCs that only work with servos from the same manufacturer. Elsewhere, many shape-cutting and contouring machines are still controlled with separate PLC, CNC, and robot programs. However, their isolated programming can't be fully synchronized or even summarized on one HMI, which makes it cumbersome to integrate multiple high-speed axes — and the sensors, actuators, and handling robotics that accompany them. Yet another “hybrid” setup (increasingly common) is open architectures that strike a balance between ease of use and flexibility — allowing OEMs to choose individual components from different vendors. Yaskawa America Inc., Waukegan, Ill., caters to these OEMs: The company sells no motion controllers or CNCs for shape-cutting applications, but offers servomotors and drives. Here, the OEM buys a CNC that works with multiple servos and chooses the best to meet a design's requirements.
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Motor and controls manufacturers are also getting in on the act, embedding interpolation functions in their programming languages, and preloading it on hardware. Baldor (see sidebar on granite cutting) includes contouring motion commands in its MINT language (which shares similarities with Basic) loaded on its controllers.
To the same end, one offering from B&R Industrial Automation Corp., Roswell, Ga., is Automation Studio development and runtime software, which provides one interface for visualization, PLC functionality, motion control, CNC, and robotics. Machine developers can generally determine where transitions between functions occur and arrange them in a way that's most logical.
Newer Quick Start automation software for CNC machines from B&R also allows engineers to build contouring cells without a development department — as it doesn't require programming from the ground up. The software environment provides CNC, hardware and simulation tools, and visualization; the latter contains preprogrammed modes, including automatic, single-step, and simulation. Finished Main, Program, Data, and Diagnostics pages shorten time-to-market.
Angles mean more axes
Imagine a traditional machine cutting a steel plate to be folded and then welded into a box. Cutters slice through the plate thickness at a right angle, so pieces require intermediate grinding before proceeding to the welding department.
According to Chris Knudsen of Yaskawa, the newest plasma and oxyfuel cutters can cut at 45° or more extreme angles, which eliminates this intermediate edge finishing. Torch heads are attached to what are essentially robotic arms with multi-rotational wrists that allow the machines to automatically cut a bevel on edges that need it.
“In fact, many OEMs across the country are now working to incorporate this capability into their machinery,” explains Knudsen. “The only catch is that it's mechanically complex: A typical shape-cutter's end effector has an X and Y axis driven by three servos. In contrast, beveling cutters have X, Y, and the cutting-head wrist requiring two more axes.”
Knudsen indicates that Yaskawa designs drives for these increasingly complex cutter arm with wrist assemblies. ?
Stiffness and accuracy
On cutters that make physical contact with materials, force vectors continuously change. This is enormously challenging for controllers tasked with maintaining accuracy. One solution: Tighten machine stiffness.
The HyperMach GTi series 5-axis titanium aerospace profiler from MAG Americas, Erlanger, Ky., delivers stiffness through a fixed-column, traveling table arrangement. Other T-type machines lose up to 80% of their stiffness and require frequent realignment to maintain accuracy; this profiler's structural link locks up between the column and X-axis table base to provide stiffness that's sufficient even for aggressive cuts. The four-point column link gives closed-loop stability and controlsgeometric changes caused by thermal growth.
One sub-mechanism, an AGS1500 gantry from Aerotech Inc., Pittsburgh, includes linear motion guides — with preloaded bearings mounted for dynamic stiffness and load distribution during dynamic contouring. Similarly, angular contact bearings maximize moment stiffness and minimize rotating friction on AccuRing rotary tables from InteLiDrives Inc., Philadelphia. Here, a machined rotating shaft further minimizes wobble.
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Fast feedback also ensures accuracy. Dual linear motors and noncontact optical linear encoders on the lower axis of Aerotech's design maximize precision — offering resolution to one nm. The tables from IntelLiDrives include motors with encoder resolutions to 0.16 arc sec.
Some profilers can also be calibrated with an approach developed last year with Automated Precision Inc. (API), Rockville, Md. Called volumetric error compensation, it leverages a laser tracker to detect errors on five axes, including those arising from kinematic interactions.
Need for speed
Shape cutters, like most other applications, have benefitted from ever-faster controller processor speed and network bandwidth for information exchange. Another improvement is reduced the time wasted on interpolation and setup tasks, such as workpiece clamping.
Reconsider the MAG profiler: It quickly processes monolithic parts (such as spars, door-edge frames, and nested part groups) with a five-axis HSK100A 6,000 rpm/850 Nm geared spindle moved by water-cooled servomotors on X, Y, and Z axes that eliminate motor-related thermal influence and maximize feed rates. The spindle carrier rotates at 60°/sec (10 rpm) in two axes, powered by worm-and-wheel drives. The Y axis reaches vertical speeds up to 35 m/min. to minimize parasitic time out of the cut.
Another approach to expediting cuts is to eliminate power-transmission components. While worm-and-geardrive-based rotary tables with comparable apertures and payload operate only to 10 rpm, IntelLiDrives AccuRing stages turn at 800 rpm. Similarly, Aerotech's AGS1500 direct-drive linear-motor Cartesian gantry outputs 3 m/sec to 5 g, and travels 500 × 500 mm.
Electrical controls also become faster when there are fewer points of transmission: In B&R's QuickStart automation package, CNC functionality runs on the same processor as the PLC logic (and not on a separate controller) to eliminate long transmission times and allow rigid timing of all processes with the shortest possible cycles … for sub-micron precision.
Add-on software further boosts speed: Mastercam X5 quickens contouring with a toolpath technique called OptiRough that makes contour roughing — the first broad swipes of material removal — faster and more aggressive by quickly removing lots of material with the cutter's full length.
The largest metal forming and machining trade show in North America is FABTECH, to be held November 2 to 4 in Atlanta. Visit fabtechexpo.com for more information.
CNC Software Inc.
Baldor Electric Co.
(479) 646 4711
B&R Ind. Automation Corp.
(215) 728 6804
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Yaskawa America Inc.
SigmaTEK Systems LLC
Cutting curved countertops
Contouring isn't limited to metal machining. Farnese Australia has been manufacturing stone cutting and polishing machines for 10 years. For its latest Quantum bridge saw, the company switched to an Ethernet-based controller from Baldor. It provides X, Y, and rotational motion for shaping stone kitchen and bathroom surfaces, with a low price-to-performance ratio. Four servomotors move the rotary saw tool (on a gantry) over a 3.7 × 2 m cutting area. Tool weight and rigidity are issues when sawing over such a large operating area, so two synchronized axes drive the gantry along the worktable. Two other axes provide transverse movement, and tool head rotation. The latter axis eliminates any need to reposition the workpiece or tool for changes of cutting direction, and can make angular and circular cuts to radii as small as 10 mm.
Baldor's free development environment Workbench is included and supports ActiveX, making it easy to interface with Farnese's existing PC/Windows-based user interface. Programming real-time motion control is in Baldor's Mint language, which offers keywords for even angular and circular cuts.
A range of preprogrammed shape cutting sequences (accessible through an HMI) cover common requirements, eliminating the need for skilled operator programming by the kitchen and bathroom surface suppliers that typically purchase these machines. The sequences include ready-to-use templates for major sink manufacturers, for example. The interface also supports more complex applications, allowing programming using G-code, as well as manual control.
Process and movements converge
CNC manufacturers are integrating motion axes and handling to increase efficiency, and software that mirrors this integration is put to good use. Consider machine-builder Fill: Their robot and kinematics-based processor Robmill handles robotics, PLC processing, and CNC with B&R's Generic Motion Control software on one hardware platform. The software includes drives, visualization and I/O handling, and movement and path control … which (with CNC functionality) works as an integral part of the automation. The soft CNC movements are programmed to DIN 66025.
The software combines CNC and PLC functions with motion derived from anything from stepper motors to hydraulics, so designers must only establish the software once. Then they are free to choose the most application-suitable hardware — whether it's a B&R servodrive, third-party drive, or even a stepper drive. This is helpful to engineers who decide to retrofit and upgrade controls, but keep their existing motors. It's also particularly useful for coordinating the proliferating axes of shape and contour cutters.
Speed versus quality
The main motion challenge in shape cutting is speed versus cut quality: At high speeds, cutter vibration can become so pronounced that it makes curved cuts rippled. Shape-cutting machinery does not benefit from the damping of embedding tools into workpiece material like millers do. Instead, laser and other noncontact cutters float above workpieces to provide higher accuracy. What's more, vibrations vary with location: At the edge of a plate to be cut, for example, where it's clamped to the worktable, stiffness is higher than at the plate's center.
When we call on OEM customers, mechanical design engineers on shape-cutting projects can often readily list the natural frequencies of their machine — because vibration is a big hurdle. We don't see this level of detail from most machine builders … except from builders of plasma cutters and more accurate laser cutters, which require utmost precision. In one case, an OEM machine builder actually knew all of the natural frequencies associated with each of their machine's axes. In fact, for this particular design, the pole count of one standard motor excited a machine natural frequency, so Yaskawa built and supplied another.
Such approaches for vibration mitigation are valid, but one new approach is simpler: We build vibration suppression into Sigma-5 servo amplifiers. In this setup, the servo itself has a precise encoder that bumps resolution to 20 bits with realtime feedback, which in turn provides information about vibrations and their changing frequencies at the machine. (Processors are now inexpensive enough to be economically installed in servos for this kind of operation.) Then the vibration-suppression algorithm applies torque on the motor shaft to cancel out the vibration at the frequency reported by the sensor.
Installed on the same servo-amplifier control switch as vibration suppression is a self-contained model-following function. Models describe the physical machine in terms of load inertia, stiffness of the mechanical transmission or rotary-to-linear component, and frequency response of individual axes. For the latter, a designer physically taps (impacts) the actual machine, and the encoder reports excited frequencies to the servo, which in turn adds them to its stored machine model. The servo then compares the stored model to actual dynamics and adjusts accordingly in realtime, for control that goes beyond simple motion to reduce position error — useful where a shape-cutting machine runs quickly, and through lots of tight turns.