The application of the acoustic emission (AE) sensor for monitoring can supply valuable information regarding the discontinuity in material. By Tim Wood, international sales manager, SBS
Changes in micro-stresses within CNC machine tool structure, caused by contact between tooling and work-piece, generate high frequency signals known as acoustic emission, or AE.
It’s best described as the phenomenon of radiation of acoustic waves in solids that occurs when a material undergoes irreversible changes in its internal structure.
It also generally refers to the generation of transient elastic waves produced by a sudden redistribution of stress in a material. When a structure is subjected to an external stimulus (change in pressure, load, or temperature), localised sources trigger the release of energy, in the form of stress waves, which propagate to the surface and are recorded by sensors.
Detection & Analysis
AE signals can be detected using an Acoustic Emission Monitoring System (AEMS), and used for advanced machine process control. With the right equipment and setup, motions on the order of picometers (10 – 12 m) can be identified. vDetection and analysis of AE signals can supply valuable information regarding the origin and importance of a discontinuity in a material. Because of the versatility of Acoustic Emission Testing (AET), it has many industrial applications (eg: assessing structural integrity, detecting flaws, testing for leaks, or monitoring weld quality) and is used extensively as a research tool.
In the case of grinding machines, this high frequency structure borne noise is created when the grinding wheel touches the part, or the diamond dresser. AE signals travel through solid materials, for example, tooling, with high velocity, meaning they are an ideal parameter for the detection of grinding wheel contact within milliseconds of time, or microns of axis travel.
Double disk grinding (DDG), a form of face grinding, is no exception. The recent application of the AE system to a DDG grinding process for rolling element IR (Inner Race) bearing faces resulted in 0.5 seconds saving on a 3.6 second grind cycle – a 14 percent reduction. In this case, the saving was made by using the system to detect contact between grinding wheels and work.
Eliminating The Gap
The process, also known as GapElimination or GE offer the ability to detect part contact in less than 1 millisecond. It also allows higher machine feed-rates, and less air grinding time, typically saving anywhere from 10 percent to 20 percent of cycle time.
Correct AE sensor location and mounting on the machine is critical. To achieve maximum sensitivity, best AE signal path and biggest cycle time reduction, a non-contact acoustic sensor was mounted on the rotary loader spindle. The corresponding stator was mounted on the guard door of the loader.
For Monitoring Purposes
Acoustic emission sensors can also be used for dressing monitoring on double disk grinding machines — either point or rotary diamond units — giving dresser touch detection, accurate machine indexing and monitoring of dressing profile.
For maximum efficiency gains the system can be interfaced with the machine CNC or PLC via hardwire, profibus or Ethernet protocols.
The SBS AEMS system is a permanent installation on a machine tool, and is available to machine manufacturers or as a retrofit package to end users. SBS can combine acoustic emission signals with other measurable machine parameters such as spindle power and work-piece RPM using a system called ExactControl.
Acoustic emission sensor rotor on loader spindle.
Double disk grinding process with rotary loader.