Diamond wire sawing is a cut-off grinding process whose source is found in rock extraction from quarries . Today, mobile diamond wire sawing is used for numerous applications like the dismantling of nuclear facilities and steel constructions [2,3,4]. The process is also widely used in the construction industry . Compared to processes (e.g. wall saw), diamond wire sawing offers a high degree of flexibility in terms of component geometry/volume and the range of materials that can be separated. The low space requirement, low set-up effort, and low noise emission are also advantages of diamond wire sawing [6, 7]. The schematic construction of a mobile diamond wire sawing machine is shown in Fig. 1. During the grinding process, the material is continuously removed from the workpiece. The drive roller generates the relative movement between the tool and workpiece required for the grinding process (up to 30 m/s ). To ensure the required contact between tool and workpiece, the diamond wire is pulled into the wire sawing machine by the force-controlled feed unit. Caused by the continuous cutting process, the engagement length of the diamond wire in the workpiece is shortened. The released diamond wire is drawn into the wire storage unit by the feed unit.
Figure 2 shows the structure of common diamond wire grinding tools. The grinding segments with the diameter ds and the length ls are threaded onto the flexible carrier wire. In between each grinding segment is a defined distance lt [1, 9]. The additional steel springs are used to prevent the segments from displacing. The applied rubber coating serves to protect the carrier wire and the springs. It also increases the adhesion of the individual components on the carrier wire. An endless wire tool is achieved by joining the ends of the wire. The structure of the common grinding segments is also illustrated in Fig. 2. Here, the diamonds required for grinding are fixed to the metallic sleeve by a bonding matrix. Thereby, a distinction is made between galvanic, vacuum-brazed, and sintered bonding matrices. The bonding matrix is an important component in the tool structure. It must have sufficient strength to absorb and transmit the machining and centrifugal forces. For sintered tools, it is also important that sufficient bond removal is achieved so that self-sharpening occurs. Furthermore, all bond types must have sufficient thermal conductivity and chemical resistance .
Diamond wire sawing is characterised by constantly changing engagement conditions. The reasons for this are the grinding progress (change of the engagement length) and unpredictable material changes in the workpiece (cavities, concrete to steel, etc.). To ensure high productivity and avoid process errors, the operator must continuously adjust the wire tension and wire velocity depending on the process progress. Insufficient adaptation of the process parameters leads to process errors such as unilateral grinding segment wear or tool failure. In contrast to the unilateral grinding segment wear, tool failure represents a significant safety risk (e.g., whip effect). This can lead to serious and deadly accidents [11,12,13].
Tool failure can be subdivided into carrier wire breakage and displacement of the grinding segments on the carrier wire. Regardless of the reason, carrier wire breakage is generally defined as the loosening of the closed wire. Caused by the whip effect, grinding segments and the springs can detach from the carrier wire and fly through the air like bullets. In the natural stone industry, a breakage is often caused by fatigue of the carrier wire . However, in the construction industry and the dismantling of power plants , which are the main application areas, the carrier wire breakage can be neglected due to the significantly shorter service life of the grinding segments. In the last mentioned application areas, the pull out of the carrier wire end from the connector is the main cause of a breakage. The reason for this is often incorrect crimping of the connections. Another reason is the clamping of the tool with the workpiece, which causes the end of the carrier wire to be torn out of the connector. Such rope breaks are not predictable and unavoidable .
A rope break, however, can be predicted because of because of the displaced grinding segments. The reason for the displacement of the grinding segments is the damage of the rubber coating [6, 14]. As a result of the mechanical work (contact between tool and workpiece), the rubber coating detaches from the carrier wire. The wear of the rubber coating is supported by the thermal stress as a consequence of the cutting process . Consequently, the rubber coating can no longer ensure the adhesion of the components . Friction or catching of the tool on the workpiece (cavities, edges, …) causes the grinding segments to slide. Accordingly, the end of the tool's usability is reached. If the displacement of the grinding segments remains unnoticed, the rope will break.
Since the displacement of the grinding segments is a continuous change in the tool structure, it can be monitored, unlike an abrupt wire break. To detect this process failure an intermittent manual inspection based on predefined operating times (1–2 h intervals ) is recommended. For this purpose, the process is stopped and the operator visually controls the accessible tool sections. The disadvantage of this procedure is the absence of process reliability, reduced productivity, and the fact that it is not possible to carry out a complete inspection of the entire rope. Tatzig presents the only known approach for the detection of displaced grinding segments. He showed that it is possible to detect displaced grinding segments in the frequency spectra of the force signal . However, the approach is not suitable for use in mobile diamond wire grinding. This is because the workpiece was mounted on a force measurement platform. An adaption of this approach for different workpieces was not shown.
As can be seen from the state of the art, there is no approach to monitoring the diamond wire tool for use in mobile wire grinding. The manual and intermittent control through the operator does not allow reliable monitoring of the tool. Additionally, the detection of damage is significantly dependent on the experience and attention of the operator. The force-based monitoring approach presented is also not suitable. Hence, the paper presents a measurement approach that enables in-process monitoring of displacing grinding segments.