Mating a non-cogging motor with an ultra-precision air-bearing spindle enabled a micro-grinder spindle/indexing system for outside diameter (OD) grinding and profiling of parts with diameters as small as 25 microns. The non-cogging motor eliminated flat spots caused by conventional brushless motor cogging, while improving accuracy, repeatability, and surface finish.
A customer of Nelson Air Corp. developed a computer-controlled micro-grinding system to grind extremely small parts in a proprietary manufacturing process. The parts are microscopic so the tolerances are an order of magnitude tighter than can be achieved with conventional equipment.
The ThinGap motorized air-bearing spindle is the key component of the solution. Three Nelson Air motorized air bearing spindles are mounted on ultra-high-resolution positioning stages and the grinding and dressing operation is monitored using a high-powered video microscope with machine vision system. The air-bearing spindles include the grinding, dressing, and tool spindles. The grinding spindle spins a 3-inch diameter diamond-impregnated grinding wheel at speeds up to 13,000 rpm. The dressing spindle uses another abrasive wheel to dress and profile the grinding wheel. A tool spindle with a non-contact optical encoder rated at 8192 counts per revolution holds the part being ground. The tool spindle with encoder is used for grinding part profiles down to 25-micron diameter, which is too small to be seen without a microscope. The tool spindle allows multi-tasking, spinning the part for concentric grinding, and indexing the part to a precise position for grinding flat profiles at different angles. The micro grinder reduces cycle time and part handling.
While the grinding and dressing spindles have a runout tolerance of ±20 µinches, the tool spindle needed to achieve a tolerance of ±2 µinches. "When we first built the tool spindle, we used a conventional brushless dc motor to drive it," says Brad Engel, president of Nelson Air. "But when we looked at the parts ground with the conventional brushless dc motorized air-bearing spindle under a microscope, we saw it had created a part that was not round but had flats where the cogging occurred." Magnetic attraction between the iron in the motor stator and the magnets in the rotor was causing a radial deviation of the spindle as it rotated past each motor pole. When the part in the spindle moved radially, the grinder took a deeper cut and it created a flat. A ThinGap brushless motor achieves zero cogging with a coreless circular copper coil replacing the iron core and wire windings used by conventional brushless motors. The coil is a thin, freestanding composite structure made of copper sheet, glass fiber, and polyimide. Since there are no magnetic materials in the air gap, there would be no magnetic attraction to cause radial motion of the spindle. The ThinGap motor also eliminates cogging due to the moving magnetic field. Generally, most brushless motors (iron core and ironless) rely on a rotating magnetic field, which causes hysteresis losses. ThinGap brushless motors eliminate relative motion between the rotor and housing return path by using a fixed magnetic field with a rotating return path, which results in hysteresis losses that are barely measurable. This eliminates cogging, which enables very precise control. The unique geometry of the ThinGap motor allows the rotor to be mounted directly to the air-bearing shaft, eliminating the use of ball bearings, which would degrade the accuracy of the spindle.
Because the inside and outside surfaces of the coil are exposed to moving air, it quickly dissipates. The maximum stator winding temperature is 100C. By eliminating the iron core, eddy current losses are eliminated and efficiency is improved.