Smart Technology in Machine Tools | Industrial Engineering

Smart technology in machine tools will enable quality products through introduction of six sigma and zero defects technique. It requires lot of research in development of suitable sensors and appropriate software.

Sensors are embedded in hard to reach places and provide real time information to enable control of speed, feed and depth for optimised conditions with a view of achiever either speed of operation, or accuracy of product, or finish as per requirements.

A major issue often encountered in metal cutting is the tool vibration and chatter. A usual practice is to control them by adjusting spindle speed or feed or depth as per experience of operator. But that may not yield most optimum results to achieve highest productivity.

In one approach, the tool point vibration response is gathered using impact testing. An instrumented hammer excites the free end of tool point and the resulting vibration is measured by a low mass accelerometer mounted at the tool point.

The complex ratio of the frequency domain vibration and force signals, or frequency response function, is used to generate the corresponding stability lobe diagram, which separates unstable combination of chip width, or axial depth of cut in peripheral end milling, and spindle speed from stable combinations.

Stable cuts occur in region below the stability boundary and unstable cuts above. It is thus possible to increase depth of cut without chatter by increasing the spindle speed instead of slowing it. It is thus possible to increase productivity making use of stability lobe diagram.

A new approach is to use active vibration monitoring to measure vibration at spindle/workpiece. New sensors based on ultrasonic technology have been developed and these allow non-contact vibration testing, monitoring, and control.

The compact size of sensor enables it to be used for multiple points including cutting tool vibration. This sensor uses a fluid jet from a compact nozzle and delivers ultrasonic wave to the point of interest. After contacting the point, the ultrasonic wave is modulated by vibration and reflected back. Demodulation techniques can be used to yield a spectrum of vibration.

There are a large number of smart sensors and software being developed which make machining operations really smart and enable quality components of best accuracy and still ensuring high productivity. Systems are available to monitor life of all components and replace them as and when called for instead of taking shut down for maintenance of entire machine at a time.

Smart machines offer the advantage of enhanced cutting capability at higher accuracy and also offer higher availability and flexibility.

Identification of wear condition makes it possible to evaluate the life of bearing. To improve accuracy, compensations are made for displacements between shaft and housing due to speed, temperature and load. Early warning systems are designed and installed on machine tools.

These allow for service and maintenance measures to be taken in advance and preventing potential machine breakdowns. Parameters like friction conditions, rigidity of mechanisms, loose play, etc., are monitored to give early warnings.

Workpiece and Tool-Monitoring Systems:

Unattended machines call for extensive use of monitoring system for workpiece and tools. Workpiece monitoring systems monitor the presence of right workpiece, automatically setup and gauge it. Touch trigger probes are used to identify various predetermined features on either the pallet, its fixture or the component.

A machine vision system can also be used to recognise a workpiece by using the image-comparison technique. After the desired workpiece is positioned, it is correctly aligned w.r.t. machine axes and datum point of part program adjusted.

Touch-trigger probes are of great help for this purpose. Workpiece can be gauged during in-process, in-cycle gauging, and post-process gauging. Similarly there is need to monitor the tool while the machine tool is cutting.

Important steps are tool identification, measuring tool offset, monitoring tool life and detecting tool breakage. To achieve accurate and speedy tool setting, a variety of on-machine tool sensing devices based on touch trigger probe and vision based sensors are available.

Tool wear rates can be measured by measuring the force increase. Acoustic emission principle is used for detecting tool breakage. It is based on a piezoelectric sensor. The amplitude of acoustic emission increases considerably as the tool is about to break.