The distortion of thin-walled aluminum ribs under milling operations places a constraint on the achievable rib thickness and surface quality.  End milling is the most common way of producing thin-walled aluminum structures such as ribs and webs.  The cutting tools for this process could be significantly long, slender and deficient in stiffness.  Hence, the combined elastic compliance of the thin-walled rib and cutting tool produces significant distortions, in terms of deviations in the thickness and surface flatness of the machined rib.  This paper presents a study of such variations in the distortion of aluminum ribs machined by end milling at low and high spindle speeds.  Unsupported thin-walled aluminum structures were end-milled to thin-walled sections at low and high spindle speeds, respectively.   The resulting variations in thickness and flatness at both speed regimes were compared.  The same machining conditions, in terms of feeds, depth of cut, and cutter tool paths were maintained.  Also, the use of mist and flood coolant was respectively investigated to demonstrate the effect of environmental conditions on the achievable rib distortion.