Analysis and continuous improvement of automated milling systems for eyewear production

The thesis project has taken place in one of the world largest eyewear company, Luxottica, specifically in production plant of Cencenighe Agordino, specialized in manufacture of components for metal eyeglass frames, such as temples (or rods), bridges, double bridges, rims, cores for plastic frames, end pieces, ornaments and decorations. The candidate has been put to work on a continuous improvement project about milling processing of eyewear metallic frame bridges on automated machining system YuMi FARO F1 TRS. YuMi Faro is a system composed of a "YuMi" 14000 ABB Robot and a CNC multi-spindle milling machine from the italian company FARO S.r.l. On these machines, the carried work is always the diamond cutting (“diamantatura”) of metal bridges, i.e., the milling machining starting from the raw piece in preparation of the surfaces which will then be welded to the rims of the metal frames. The criticality of the machining turns out to be in most cases the formation of unwanted chips which remain integral with the workpiece, leading to the occurrence of two main problems in the production phase: •??The chip formed prevents the milled metal bridge from being able to come out of the housing on the clamp just with the action of the blow of compressed air, during the discharge phase at 90° from the table, causing a jam situation; in this way, when the machine is automatically reloaded by the robot, the new piece collides with the one already machined, inevitably blocking production; •??The chip is sometimes so stiff that the subsequent tumbling process, specially carried out for several hours in order to eliminate any remnants of excess material, fails causing instead the flattening of the same chip on the piece, leaving “burr”, which will then be discarded during the inspection phase, often sending the entire micro-batch of milled bridges back to production. The first of the two problems causes a drastic decrease in the utilization of the machines, constantly monitored by control software, since every time there is a collision between the jammed piece and the piece not yet machined, there is an emergency pause in production with an associated system error. The objective of the problem analysis is to improve the use of the machines in the two daily working shifts, proposing various solutions to prevent jamming and machine downtime; broadening the view on the improvements that can be obtained, eliminating the criticality of chip formation from the outset, the general efficiency of the metal bridge manufacturing process would be significantly improved, reducing rejected pieces, machine downtime, the time required in the tumbling phase to obtain the same surface finish. During the analysis of the possible solutions to the problem in question, various types of possibilities were taken into account, , from modifications to the robot cycle or to the CNC command cycle, to the addition of tools on the end effector of the robotic arm to force the detachment of the worked piece, to technological solutions on the blow of compressed air, and finally, as an extreme solution, changes to parameters and milling trajectories. The objective of the discussion is to analyze which of these solutions are preferable and feasible in a high-pace production environment such as the Luxottica plants, and what the effects of the solutions actually adopted have been on the utilization statistics of the automatic systems in question.