| Freeform surfaces are applied increasingly in production fields such as optical engineering,aviation,mold production for their specific functions in improving related devices' performance and reducing size as well as energy consumption.The manufacture of freeform surface normally depends on the grinding by forming wheel and 5-axis machining which lead to long machining time and complex calculation.Therefore,in order to realize the precision manufacture of optical lens,a virtual ball-end algorithm and the parameterized model of grinding process were put forward to guide the following ductile-mode grinding of hard-brittle materials.As for the fabrication of metal freeform surface,three kinds of cutters were studied in milling for their related machining surface quality.The key points of study are shown as follows.Firstly,virtual ball-end algorithm applied in 4-axis machining and the 3D model of abrasive-distribution were proposed.Then,a mathematic model of single-abrasive cutting depth and ground surface roughness in micro grinding was constructed including parameters such as grinding wheel concentration,abrasive-exposure and tool posture angles,spindle and feeding speed.Therefore,the theory of brittle-ductile grinding process in the following experiment was proposed.Secondly,an experiment of freeform grinding and polishing on optical glass was performed and related experimental parameters were studied based on the mathematic model proposed above.The results show that increasing tool posture angle can decrease abrasive grain cutting depth and surface roughness.Besides,the control of tool posture angle lead to stable forming envelop trace height that ensure high quality surface.In experiment,grinding wheel with 30 ?m and 3 ?m grain size were applied for BAK7 freeform surface' form-grinding and compensation-polishing within ground range of 54 mm×6 mm by 30 ° tool posture angle respectively.After compensation-polishing,high quality freeform surface with 14 nm surface roughness can be obtained and the surface's mean form error and extreme form error were decreased by 26% and 35% respectively.The results shown that,based on the virtual-ball end algorithm,normal grinding wheel can replace forming grinding wheel to realize precision freeform surface grinding.Finally,with the same 4-axis algorithm,different kinds of milling cutters containing flat-end cutter,torus cutter and ball-end cutter were applied in freeform milling experiment on 7075 aluminium alloy.The results shown that,machining-surface roughness decrease along with increase of tool radius size,number of flutes,spindle speed and decrease of feeding speed,tool posture angle.Besides,tool wear and tool contact force will affect the surface form accuracy and the tool tip deformation model based on the tool contact force can approximately predict form accuracy of flat-end cutter and torus cutter's machining surface.In experiment,the best surface quality could be obtained with small tool posture angle.Moreover,small dividing movement of machine tool and high speed milling can prevent material remnants caused by curved edge of cutters thus lead to low surface roughness.According to the results,torus cutter is more suitable for freeform surface precision milling.Therefore,precision milling freeform surface within the range of 40 mm×5.5 mm has been obtained by torus cutter with optimized milling parameters in the final milling experiment.The surface roughness reached 0.31 ?m and its mean form error was 1.63 ?m. |
PDF Full Text Request