The machining of complex surfaces such as those present in rotors, impeller blades, thin-walled parts among other engineering components has been commonly carried out in multi-axis CNC machines programmed by CAD/CAM software. Even with a significant aid of the CAD/CAM systems, the users are responsible for defining an optimal set of cutting parameters, relying on their practical knowledge. This task gets harder when thin-walled parts have to be milled, because besides the problems caused by the limit of rigidity of the cutting tool, there is also the limit of rigidity of the thin-walled parts. Few works can be found on current literature concerning this issue. Then, the current work aims to analyse the influences of the cutting parameters: feed per tooth (f z); radial depth of cut (a e); and cutting direction (up and down milling), on the form errors and surface roughness of thin-walled parts manufactured by 4-axis milling using AISI H13 steel. The results show that the cutting direction (up/down milling) was the major variable to affect the form errors. Down milling left more than 0.2 mm of material on the machined part. On the opposite, up milling removes up to 0.2 mm more material than it should be. ANOVA shows that the influence of the f z on the form error is slightly higher than a e influence. In extreme cases, it can be about 1.5 times more influent than a e. Down milling and the highest values of a e and f z tested proved to be a good set of cutting conditions for milling free-form thin-walled parts, because with these conditions a good surface roughness, a positive form error and higher productivity were obtained.

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