Analysis of Drilling Processes

For developing an operating process monitoring it is rudimentary to understand the drilling process in detail. Generally drilling processes can be divided into two steps, drilling stroke and return stroke. A generic drilling process is shown in figure 6.

Figure 6: Generic drilling process according to [1].

The initial step of drilling processes is the idle task. In this position only the no-load current and the no-load voltage of the drilling module can be recorded. As shown in figure 4 these two values can be affected by air motions of motor, thermal energy and the system inertia. The proper drilling process starts with a so called drilling stroke which can be divided into three steps, acceleration phase, phase with constant velocity and lag phase. It is

important to keep the period with constant velocity as long as possible to reduce disturbing acceleration effects. Realization of short acceleration and lag phases submitting the possibility to change process parameters during the short duration of drilling processes and ensures a data acquisition without influences of acceleration effects. To realize a long phase with constant velocity it is necessary to shift the acceleration phase out of the workpiece so that the main drilling process (chip removal with main cutters) can start at the beginning of constant velocity phase.

The first contact of tool and workpiece is a crucial point during the drilling process. The main quality criterion of drill holes is the quality of their edges on surface of workpiece. Due to the commonly very brittle coatings of wood based panels these edges can be easily spoiled by using worn, damaged and wrong tools or wrong process parameters. In this point it is necessary to ensure that spurs of tool scratching the coating of workpiece one complete turn before main cutters starts to work. This can be realized by the use of eligible combination of process parameters (feed speed and rotational speed) and tools. After the phase with constant feed the tool has to be decelerated. This phase is called lag phase. The lag phase occurs during the intervention of main cutters in fact of changing feed direction and leads to an elongation of the cutting length. These elongations in correlation with high feed speeds induces longer acceleration and lag phases [1] and are a reason for wear out of tool. To ensure a longer tool lifetime it is important to reduce the rotational speed during lag phase. The return stroke works in the same way. After the deceleration of the feed, acceleration in opposite direction will be performed followed by a phase with constant velocity and a lag phase until the idle position of the tool. Also during the tool leaves the hole it is important to control the rotational speed. A high rotational speed in this situation signifies waste of energy. If the rotational speed is equal to zero tilting of tool is possible. The edges of the hole can be spoiled if the tool tilts.

Recording of torque, feed force and controlling of feed is necessary during the whole process. Due to changing density an inhomogeneity’s in wood based materials, process monitoring and self-optimization of spindle are able to avoid unnecessary waste of energy and wear out of tool. Therefore it is important to analyze the shapes of feed force and torque. In these curves different points of interest can be found. Figure 7 shows curves of torque and feed force which were recorded on a machine center equipped with the self-developed powertrain and a Wittenstein drilling module. A typical eight millimeter twist drill with center point and spurs was used. The process was performed with a feed speed of 8 [m/min] and a rotational speed of 12.000 [1/min]. The depth of the final hole was 15 millimeters.

Figure 7: Analysis of feed force and torque curves.

Point (A) shows first contact of tool center point on workpiece surface.

Result is a strong increasing of feed force (B). This point can be used to identify the start point of drilling process. During the increasing of feed force the contact of spurs (D) and main cutters (E) can be seen in the curve of torque. Based on contact of main cutters with workpiece and the chip removal, a strong decreasing of feed force can be seen (C). After the contact of main cutters, the torque reaches the maximal value (F). Due to density of the material a decreasing of torque can be measured (G). The material which was used in figure 5 is a standardized particle board with a typical density profile. Density profile means a deviation of density over the thickness of particle- or fiberboards due to their manufacturing. To increase the strength properties of particle- or fiberboards the surface layers get a higher density as the core layer [5]. Due to the deceleration of drilling process a decreasing of feed force in point (H) is measureable. This point should be determined before the start of drilling process. After this point process parameters like rotational speed and feed speed shall be reduced to save energy and ensure a longer lifetime of tools. In the end of the lag phase the drilling stroke is finished and the return stroke starts. During the return stroke (point (I) until the idle task) only a decreasing torque can be measured. Between the points (I) and (J) the torque is measureable due to the friction between tool and wall of drilling hole. Outgoing of the deepest

point of drilling hole the torque decreases over the drilling depth. In point (J) the tool leaves the drilling hole.

The prior shown graph is only an example of a drilling process. The Measured values variegating in dependence of different parameters like density, structure and kind of workpiece etc. and can be influenced by controlling of feed per revolution.

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