Cutting Temperatures and Their Effects on the Machining Behaviour in Drilling Reinforced Plastic Composites

Abstract

In many cases machining of fiber reinforced polymer materials is performed without cooling lubricant, since the coolant can lead to swelling of the polymer as well as inducing chemical reactions with certain functional groups of the macro molecules. This effect can lead to shape and accuracy errors not only of the drilled hole but also of the entire work piece, since a spreading of coolant can not be completely avoided. The latter effect causes a weakening of the composite material by reducing the material strength, which is a result of lower adhesive forces within the polymer material and between polymer and fiber. In contrast, dry machinining offers the risk of a thermal damage induced by high process temperatures. [1±3] Especially, the thermo physical properties of fiber reinforced polymers causes high temperatures at the tool tip, when machined. The thermal conductivity as well as the heat capacity is rather low compared to metal matrix materials, hence the energy balance between tool and workpiece is disturbed. Furthermore, the thermal expansion coefficient is also low, which causes accuracy errors when machining. Another reason which must be considered is the fact, that especially thermoplastic polymers have a glass transition temperature ranging from 150±250 C. This means, that heat formation during the drilling process leads to plastifying of the polymer matrix. In the presented research work, the temperatures which are generated when drilling fiber reinforced plastics are measured and the dependency of cutting parameters is discused. Furthermore, some general aspects are outlined including damages, tool wear and cutting forces, which can be attributed to the process temperatures and thermal overloads.Tool Temperatures: Within the presented work an initial approach was undertaken for measuring the cutting temperatures at the tool tip, in order to estimate the thermal load when drilling fiber reinforced polymers. In Figure 1 (left) the feed force is shown as well as the tool temperature, which was measured with a thermocouple at the drilling tool. It is notable, that the temperature increases instantaneously up to about 180 C at the beginning of drilling. After reaching the initial temperature level it increases through the whole process continuously. The maximum of 387 C is reached, when the tool exits the bore hole. The occuring temperature maximum is possibly caused by stronger friction, as the reinforcement plies are not cutted anymore, but pressed out by the tool tip. Because the temperatures were not directly measured at the cutting edge, it can be assumed, that the temperatures are much higher there. It is also important, that the temperatures are at unexpected high levels, which can be rated as crucial for the machining of plastics. Furthermore, increasing the feed changes the course of the temperature progression (Fig. 1, right). While a definitely progressive rising of the tool temperature is observed at lower feed rates, a nearly constant temperature course is evident when elevated feed rates a...

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