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machines & automation


This sequence shows how the Landis LTT moves the wheel along the length of a part using only rotation. The rotary axes are mounted in granite and extremely stiff. The distances from the center of each turret to the wheel/part never change in this move and the mechanical and thermal loops are small, resulting in optimal surface quality and form accuracy.


a chuck, or whatever tooling suits the application. The B axis is the “spindle turret,” and it can be configured with a variety of OD and ID grinding spindles, turning/milling tools, and a probe. It’s the two rotary axes that provide relative motion between the part and the cutting tool or grinding wheel. The linear axis, which is short, controls the depth of cut and profile shape of the component being machined. It’s a bit difficult to wrap your head around, but in a linear grind down the side of a part it looks like the component is caught between two swinging doors.


Single-Digit Nanometer Surface Finish There are several advantages to this unusual layout. First, all things being equal, a circle is stronger than a rectangle. So relying primarily on the interface of two large circular bearings is inherently stiffer than the interplay of similarly sized linear guideways. Second, in a conventional design, the linear axes must be stacked to enable their independent movement. Each interface between a linear bearing and the supporting structure is more compliant than the machine base, so each such interface reduces the stiffness of the machine. Stacking linear axes also increases the length of the “machining loop,” the shortest path from the cutting tool, back through the cutting tool mount, then through the machine base, back up to the component mount and then back to the component. The longer and more convoluted the path (or loop), the less stiff the machine. The shorter the loop, the stiffer the machine. The LTT has a very short and very consistent path length, so the machine is very stiff and the stiffness is very consistent for all tool and component orientations.


Not only are the LTT’s rotary axes bolted directly to the granite base, the motors are direct-drive high torque motors with a high resolution encoder and minimal axial and radial error motion. Fives Landis also used hydrostatic bearings with a stiffness of between 2000 and 10,000 N/ μm (depending on the model) and incorporated glass scales plus hydrostatic guideways and linear motors in the X axis. Depending on the application, the spindles use hydro, aerostatic and/or rolling element bearings. The layout is so stiff and damped that the machine can grind and hard turn the most difficult materials to a surface finish in single-digit nanometers.


Temperature Control also Plays a Role In a traditional machine design, the coolant return path changes constantly as the grinding wheel carriage moves along the linear axis. So heat from the grinding process transfers to different sections of the machine bed, resulting in constantly variable machine distortions. Conversely, the LTT’s thermal loop is small and consistent and the twin turret design enables a simple noncontacting labyrinth seal, making the machine base almost immune to such distortions. The coolant touches the turrets at the top, above a steel


tray, and a chiller controls the temperature of this coolant to within 0.1° to minimize its effects. But the tray isolates the coolant from the machine base to which the turrets are mounted. Below the tray, the hydrostatic oil in the turrets controls the temperature of the lower part of the turrets. Together this greatly reduces any thermal fluctuations at the base, keeping the center distance between the two tur- rets essentially constant. That’s a key to the patent and the accuracy of the machine.


80 — Aerospace & Defense Manufacturing 2016


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