Tool Wear Monitoring

Tool wear is a phenomenon whose behaviour can be explained qualitatively but not quantitatively. Though some tool life equations do exist, their universal adaptability or their utilisation even in restricted work-tool material zones for all parameter ranges are doubtful. Further, direct in-process measurement of tool wear is difficult in view of the location of the wear and the measurement techniques employed.

An important process parameter is the tool wear, which may be measured directly or indirectly. The following principles are generally used for direct measurement of tool wear.

1. Tool wear is measured by relating it to changes in the resistivity of a resistor embedded in the tool tip. In this method there is no need to interrupt the process.

2. The profile of the tool tip is recorded periodically using optical methods and tool wear is determined from the variations.

3. Opto-electrical methods using TV cameras and photodiodes etc. are employed to record variations in the cutting edge to measure the width of the worn edge.

All the above methods are complex and are more expensive to implement on a regular basis on the production CNC machine tools. As a result, tool wear measurement has to be indirect. Some parameters used for measuring tool wear are:

• Cutting Power
• Cutting Forces: By measuring cutting force using force measuring sensors. The cutting force increases with the increasing dullness of the tool and can therefore be related to tool wear.
• Ibrations: By measuring vibrations of the tool edge, i.e. tool chatter wear can also be indirectly estimated.
• Acoustic Emission
• Tool Temperatures: By measuring the tool tip temperature and relating it to the wear of the tool.

Of these variables, mainly the cutting forces and power based tool monitoring systems are commercially and widely available, whereas the others are still not proven to be widely used in practice.

The power consumed during a machining process is a function of the forces acting. Further, the cutting forces depend upon the quality and condition of the cutting edge. As time progresses, the power consumed by the tool for the same material removal increase with increase in tool wear. Thus power measurement is an indirect way of monitoring the life of the tool. Power may be measured by a power meter (watt hour meter) installed in the spindle motor circuit. However, a more effective method is to determine the resistance offered to the tool motion, which can be measured in terms of the power consumed by the axes motors. It was found that in applications like drilling, measuring the current consumed by the feed motor in the spindle (Z-axis) direction would be a good indicator for tool condition.

Another system of tool condition monitoring is by the measurement of the torque on the main spindle as used by Maho. In this system, the spindle torque is measured in terms of the differential twist separated by a small distance.

Direct measurement of cutting force is a better method for tool condition monitoring, rather than power. Hence a number of systems based on force measurement are available commercially. The problem encountered often is that the force sensor should be located close to the source of power, i.e. the tool. However, with the ability to change tools during the machining, it is necessary to have a force sensor located in the tool holding structure rather than with the tool itself. Thus Sandvik has come up with a plate sensor which can be conveniently mounted under the tool turret in case of turning centres.

The plate sensor consists of a structure, which is simple and has strain gauges to directly measure the feed forces. The simplicity of the construction of the plate sensor helps in its easy adaptability to any machine tool. The threshold signal for limiting tool wear from the plate sensor is linked to the CNC controller which automati- cally can call up a sister tool for replacement of the worn out tool, before a catastrophic failure. This also allows for uninterrupted cutting, and reduces the costly breakdowns of the CNC machine tool.

Another possibility of tool condition monitoring is through the measurement of vibrations of the cutting tool. The vibration signature of a cutting tool is a good indicator of the quality of the cutting edge. The vibration spectra at the beginning when the tool is sharp can be compared with those at each time, and the shift taking place in amplitudes and dominant frequencies can be measured. These are useful for identifying the tool failure criterion. The failure criterion used is the power spectral density of the cutting tool vibrations, which is a good indication of the energy consumed for cutting by the cutting tool.