Error models are available for individual error sources for specific components in a machine tool. However, proper error budgeting requires an understanding of the relative importance of various error sources. This paper presents an analysis of the relative contribution of process and geometric errors on the dimensional error produced in micro scale milling. The error contributions at the micro scale are compared and contrasted with those in macro scale milling. The analysis makes use of an error model that uses homogeneous transformation matrices (HTMs) to model the effect of each error source in each component of the machine tool on the final position of the tool with respect to the workpiece. The model is developed for a generic milling machine and used to compare the relative importance of errors due to misalignment, thermal growth, tool deflection and wear of the tool on the size of the machined feature at the micro and macro scale for slot milling using standard length tools.