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Helical milling is a hole-making process which has been applied in hardened materials. Due to the difficulties on achieving high-quality boreholes in these materials, the influence of noise factors, and multi-quality performance outcomes, this work aims the multi-objective robust design of hole quality on AISI H13 hardened steel. Experiments were carried out through a central composite design considering process and noise factors. The process factors were the axial and tangential feed per tooth of the helix, and the cutting velocity. The noise factors considered were the tool overhang length, the material hardness and the borehole height of measurement. Response models were obtained through response surface methodology for roughness and roundness outcomes. The models presented good explanation of data variability and good prediction capability. Mean and variance models were derived through robust parameter design for all responses. Similarity analysis through cluster analysis was realised, and average surface roughness and total roundness were selected to multi-objective optimisation. Mean square error optimisation was performed to achieve bias and variance minimization. Multi-objective optimisation through normalized normal constraint was performed to achieve a robust Pareto set for the hole quality outcomes. The normalized normal constraint optimisation results outperformed the results of other methods in terms of evenness of the Pareto solutions and number of Pareto optimal solutions. The most compromise solution was selected considering the lowest Euclidian distance to the utopia point in the normalized space. Individual and moving range control charts were used to confirm the robustness achievement with regard to noise factors in the most compromise Pareto optimal solution. The methodology applied for robust modelling and optimisation of helical milling of AISI H13 hardened steel was confirmed and may be applied to other manufacturing processes. KeywordsHelical milling; AISI H13 hardened steel; Multi-objective robust optimization; Robust parameter design; Normalized normal constraint method. R 2 coefficient of determinationRadj 2 adjusted coefficient of determination Radj 2 prediction coefficient of determination

In the manufacturing of a medical device, may occur the need to make a hole with a specific function. Among current methods, conventional drilling (CD) referred in this work as drilling (D) and helical milling (HM) are two options with different potential.When making the hole, it is important to choose the most suitable method to obtain the desired geometry and ensure the functionality of the device. This work aims to analyze surface parameters as, arithmetic average height (R a ), the maximum height of the profile (R t ), the average peak to valley height (R z DIN), chip formation and the geometric deviation of holes obtained by the previously referred manufacturing processes. The specimens, with cylindrical geometry, were made of titanium alloys, Ti-6Al-4V and Ti-6Al-7Nb, currently used in the manufacture of medical devices. For this purpose, holes were made in a machining centre with different feed rate (F) for both methods and in the value of vertical step (a p ) in HM. The results obtained demonstrate that, at lower F and a p , HM presents better results. The Ti-6Al-7Nb alloy presents better roughness results compared to Ti-6Al-4V, validating it as a material able to be used in medical devices according to the fact that, a lower roughness is associated with higher corrosion resistance and fewer fatigue problems derived from it in components. By the work carried out, can be concluded that the roughness values obtained in HM are lower to those obtained by D making HM as a better option in hole making.

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