Nonconventional Technologies Review – no. 3 / 2008 51 STUDIES ON DRILLING OF FIBROUS COMPOSITES Maria OCNARESCU 1) , Paulina SPANU 2) , Aurelian VLASE 3) , Constantin OPRAN 4) ABSTRACT Though adhesive bonding is a preferred method of joining fibrous composite, sometimes bolting of components is essential for operation, maintenance or inspection purposes. It is necessary to drill holes in fibrous composites for joining them by bolting. Drilling of fibrous composite presents many problems like - delaminating, fibre/resin pullout, excessive tool wear. In the present work, drilling of glass fibre/epoxy and carbon fibre/epoxy laminates is studied using high speed steel and tungsten carbide coated drills. The effect of cutting speed, feed and number of holes on tool wear, thrust and torque is studied. KEYWORDS: composites, cutting, drilling, machining 1. INTRODUCTION Advanced Fiber Reinforced Plastics Composites are being increasingly used in modern aerospace and other engineering applications. This is mainly due to the high specific mechanical properties offered by these materials and the possibility of tailoring their performance. Though composites have excellent performance characteristics, but when machined they tend to develop the following flaws: I. Surface delimitation: separation of plies where the cutter enters and exits the material. II. Internal delimitation: separation that develops between plies as a result of improper machining and drilling. III. Fiber/resin pullout: tearing away of fiber/resin from the wall of the machined edge. IV. Higher tool wear due to abrasion by hard fibers. V. Lower flexural strength causes easy deformation of hole which subsequently leads to hole, shrinkage. VI. Presence of powdery chip which is a health hazard and is difficult to handle. VII. Lower thermal conductivity causing local heat accumulation leading to increase in tool drear. 2. STUDIES ON DRILLING Machining characteristics of composites vary from metals due to the following reasons; a. FRP is machinable in a limited range of temperature. Though fibers withstand higher temperature, the cutting temperature should not be exceeded to avoid softening of matrix. b. The low thermal conductivity causes heat build up in the cutting zone during machining operation, since there is only little dissipation by the material. The greatest part of heat is carried away through the tool. On the surface layer of composite there is a significant rise in temperature. c. The difference in the coefficient of linear expansion between the matrix and the fiber (carbon fiber has negative coefficient) gives rise to residual stresses and makes it difficult to attain high dimensional accuracy. Drilled holes show a smaller diameter than the drill used since, coefficient of linear expansion of plastics is more than metals, it increases friction between the tool and work and consequently the amount of heat produced. d. Reinforcing fibers and fillers can cause a highly abrasive tool, w ear. e. The change in physical properties by the absorption of fluids has to be considered while deciding to use a cool ant, Mackeyll [ 2], Boldt and Chanan [3] and Miller [4] have tried different tool geometries in drilling of composites. Sakuma [5] have studied the relation between tool material and