The concept of using sensors, outside of those integrated within the machine tool, to monitor tool condition is not novel and there are several methods which can be deployed, and some are outlined below. The challenge, within the area of sensorisation, lies in the size of the tooling to be analysed, specifically micro boring bars which are typically removing very small amounts of material.

Acoustic Emission. This technology measures sound vibrations generated during the machining process and different tool wear would generate different sounds vibrations which could be analysed. We discussed this option with a research partner who had successfully measured tool failure in small milling tools. However, this technology was dismissed for this application as the recording hardware would need to be placed very close to the machining area and results can be significantly impacted by ambient noise such as coolant. Also, the use of this technology, while suitable for a laboratory environment, is not currently suitable for a machining environment.

Power monitoring. As cutting tools begin to wear, the cutting action of the tool becomes less efficient and therefore requires more power. Measurement of power consumption can be a clear indication of tool wear and was considered. However, for using on a micro tool and on a lathe machine where the machine spindle requires a large amount of power, there was a concern that any power changes due to tool wear would not be very minute and therefore deemed not suitable.

Figure 1 Strain Sensor in Tool Holder

Strain sensors are another successful method of measuring tool wear and is successfully being used to monitor micro tools. When considered further, the sensor is integrated into the tool holder and then set up on the machine. The tool holder is modified, the sensor is integrated, and it is then specially calibrated.

Vibration. One of the more common methods of measuring tool wear is through vibration. This is widely adopted by companies looking to measure tool wear and is also now integrated into some high-end machine tools to enable deployment of adoptive machining in reaction to monitored vibrations.

Examples of vibration test results

Using Vibration as a method of monitoring micro tools can identify multiple interesting results. Figure 2 displays vibrations recorded using fixed surface speed G96 and  fixed spindle speed G97. Without this capability it would be almost impossible to identify the differences.

Figure 2 displays the difference across the same tool paths as figure 2 but using different chucks. It is clear from this test that the standard 3 jaw chuck displays much more vibrations than the precision Hainbuch chuck.  

Figure 3 3 Jaw Vibration v Hainbuch

Once identified as a suitable method of monitoring tool vibration it is then possible to apply different tool paths and see the impact these have on the cutting tool. For example, Figure 4 shows the vibration signature for the same part but two different toolpaths. Series 3 clearly displays much larger vibration spikes than Series 4.

Figure 4 Full vibration profile for two parts

Figure 5 Hypermill Contour parallel toolpath

Figure 6 Straight Line roughing

Figure 5 and 6 above show different toolpaths and their correlated vibration signatures in Figure 4

Summary

Vibration analysis equipment can be used to monitor not only different toolpaths, but the impact different parts of the complete machining process can have on tool life. It is possible to observe the impact of different chuck types can have on turning with small tools and the impact of increasing surface speeds on very small tools and how the type of initial cuts and tool entry can be detrimental to tool life. 

The area of cutting tool condition monitoring in machining with micro tooling is still relatively new and while there are numerous technologies available, the challenge for industry presently is in identifying and testing the correct solution. Most solutions, such as this one, have been successful in a lab environment but a major challenge still is with the companies to deploy these tools in a production environment.