The tools and the processes at machine tools can be monitored by a wide variety of sensors for the detection of forces, vibrations, acoustic emissions and true power. In general, the most meaningful information is obtained from the monitoring of forces generated by the metal removal process. To ensure the reliable monitoring of tool wear, in particular, three forces must be measured with the utmost accuracy.
Many wear detecting mechanisms for turning machine tools demonstrate that the evaluation and appropriate weighting of all three force components is far more reliable than the monitoring of only a single force component. The latter may generally be adequate for the purpose of monitoring collisions and tool breakages, but even here the measurement of the forces in all three directions improves reliability.
Piezoelectric multi-component force sensors are being successfully used in lathes to monitor collisions, breakages and wear, due to their exceptional stiffness and compact dimensions. To permit their use under production conditions, in particular, they are now being fitted with integrated charge amplifiers for signal pre-processing.
These deliver voltages at the outputs which are precisely proportional to the forces applied to the sensor. The sensors themselves are welded to make them hermetically sealed and are provided with an integrated cable output or—for more flexible use—a plug connector. This way the sensor installation and machine wiring can be carried out separately.
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Multi-component force sensors consist of a package of quartz crystal discs, precision mounted in a steel housing. Through the use of a variety of crystal shapes and the appropriate positioning of the discs, up to three force components can be measured simultaneously. The force components at orthodox angles to the sensor axis are transmitted as frictionally engaged shear forces.
This means that the sensor must be axially preloaded. A prerequisite for the trouble-free monitoring of several force components is that the sensor must be precision made. Precise linearity, extreme sensitivity and the minimum of cross talk between transverse forces are essential for the trouble-free monitoring of up to three force components.
The forces must act on the sensor element under the optimum spatial conditions. To this end, the sensor must be generally preloaded in the machine tool structure. This can be accomplished without difficulty with annular sensors.
The requisite preload is achieved with a double wedge Fig. 25.11. In this way, the sensor can be directly installed and preloaded in a friction-locked fit in a recess in the structure or in the parallel plane of separation between two machine tools.
To permit the use of sensors with housings of standardised dimensions, many lathes are now made with recesses of standard dimensions (Fig. 25.11).
Force sensors can also be installed directly in joints, e.g. between the tool turret and machine slide of lathes (Fig. 25.12) or between the spindle head and machine tool of grinding machines. In this case, the sensor is mounted in a recess in the machine slide or in an intermediage plate.
The requisite preloading of the sensor is achieved through the slight projection of the sensor mousing the surface of the joint when the machine parts are bolted together. This form of installation possesses extreme rigidity. The depth of the recess or thickness of the intermediate plates must be at least 10 mm.
Properly evaluated, multi-component force monitors on lathes offer reliable monitoring for collisions, tool breakages and wear. For the latter, in particular, they are a must, since only by allowing for all the force components is it possible to arrive at reliable conclusions as to tool wear.
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At the same time, a sensor of very high quality is required, a sensor which stands out by virtue of its uniform sensitivity in all three axes, the minimum of cross talk it generates between the individual force components and the ability of its design to stand up to demanding factory conditions.