Dynamic parameter adjustment during drilling in fiber-reinforced plastics with additional consideration of the clamping situation

Fiber Reinforced Plastics (FRP) open up new possibilities in weight reduction for highly loaded parts in numerous industrial sectors. For allowing the extensive application of FRP, the whole process chain needs to be controlled. FRPs are mostly manufactured near net shape. Nevertheless further post processing methods are necessary. Major problems of machining FRPs are damages like delamination or burr which are induced by the machining process. Especially drilling damages around the drill hole are undesirable problems which need to be avoided. They increase with increasing wear of the drill caused by a change of cutting condition between the cutting edge and the CFRP part. Furthermore clamping of the FRPs causes high costs by the rising complexity of the parts and the needed stiffness to avoid damage bred by the clamping system. This work deals with the development of a new machining process which reduces the damage when drilling CFRP and additionally increases the tool lifetime. To realize this aim the feed was dynamically adapted depending on the current wear status of the drill to hold a predefined contact ratio between the cutting edge and the workpiece constant. Different contact ratios are examined in this work. The results show that the damage of the workpiece is significant lower if contact ratios are adjusted at which the feed per tooth perpendicular to the cutting edge is two or four times higher than the current wear of the tool. If a contact ratio was used at which the feed per tooth is lower than the current tool wear the damage increases. When changing the contact ratio during the lifetime of the tool the wear behavior at the cutting edge can be influenced but in the investigations only a constant progression could be reached. A reduction of tool wear could not be measured wherefore further investigations in this field are needed. With the proposed method, the feed rate can be increased during the cutting process without the loss of quality and a simultaneous tool life increase is achieved. Within the investigation of a four point clamping system the dependency of the damage of the workpiece to the influence of clamping distance shows that there was a boundary below which no decrease of quality could be measured. When this boundary was exceeded a catastrophic breakthrough of the drill occurs and the damage rises drastically. Further studies are conducted to show the variation of drilling parameters caused by the bending behavior of the workpiece. At the beginning of a drill the feed per tooth decreases and when the cutting edge is in full contact to the workpiece the feed per tooth reaches roughly the predefined value up to the time the chisel edge exits the workpiece. At that point the feed per tooth was much higher than the predefined value. When combining the method of dynamically adjusted feed depending on the tool wear and the influence of clamping system a method could be shown which allows to ensure the compliance of the preset feed per tooth independent of the actual bending behavior of the workpiece. With this method an economical enhancement for drilling could be reached caused by enabling of greater clamping distances and the possibility to ensure the preset drilling parameters which helps to predict the tool wear progression and, therefore, to enlarge the tool lifetime.