Instrumented Spindle Improves Manufacturing of Optical Materials
- Monday, 01 January 2007
Sensors and data acquisition system provide real-time visibility of grinding system operation.
Lion Precision and Professional Instruments, St. Paul, Minnesota
High-performance materials such as ceramics, optics, and alloy steels are manufactured using abrasive grinding technology. Until now, the grinding wheel and process conditions have been difficult to measure in production.
A new, instrumented spindle provides real-time force information that allows adaptive control of the grinding process. This approach provides information to the machine controller that is used to maintain quality in the presence of changing conditions. Previously, the time required to make a part was kept constant while quality varied. Now, the machine controller makes adjustments to maintain quality in the shortest possible time. The quality remains constant, but the process time varies.
The instrumented spindle has embedded, non-contact sensors that measure the time-varying gap between the rotor and stator. This gap is converted to a force using the known stiffness of the air bearing spindle. The instrumented spindle may be configured as a smaller, higher-speed spindle with milli-Newton force resolution at speeds up to 20 kRPM. Larger configurations also are available with much higher load capacity.
The bandwidth of the instrument is currently 100 Hz, with future improvements planned to increase the dynamic range. Data acquisition hardware and a custom software interface from National Instruments of Austin, TX, provide real-time feedback to the operator or to the machine controller with the appropriate digital or analog inputs.
Several grinding phenomena are of particular interest to the precision grinding community, including workpiece contact detection, workpiece defect detection, grinding wheel loading and dressing condition, and coolant issues. The instrumented spindle provides the information required for the manufacturing engineer to pursue a deterministic approach to process control. Individual process variables may be studied systematically to obtain a clear understanding of the key operating parameters.
For example, the instrumented spindle approach can be used to explore the fluctuations in force during ultra-precision grinding of silicon wafers. The crystal anisotropy of silicon results in significant variations in the material properties depending on how the grinding wheel is oriented with respect to the crystal lattice. In the waterfall plot in Figure 1, we see a series of grinding force spectra calculated from force data collected during a single pass of the diamond grinding wheel over a rotating (100) silicon workpiece. The workpiece spindle speed is 500 RPM (8.3 Hz) and appears, along with many harmonics, in the figure during the contact of the silicon workpiece with the wheel. The fourth harmonic (33.3 Hz) is particularly strong because of the cubic (100) crystal lattice structure.
Also seen is the significant fluctuation in the grinding force (Figure 2) at the frequency of the grinding wheel speed (3,600 RPM or 60 Hz). As additional parts are ground with this wheel, the 60-Hz wheel speed component will continue to grow as the wheel wears and breaks down. Eventually, the wheel will have to be redressed, at which point the 60-Hz component is again small compared to the workpiece rotation and its harmonics. The spindle provides information vital to proper wheel selection, wheel preparation, and other grinding parameters.
The instrumented spindle is a product of a university/industrial consortium including partners at Penn State University, George Washington University, Professional Instruments, and Lion Precision.