An Investigation Into Fixture Error Compensation in Micromilling Using Tool-Based Conductive Touch-Off

In micro-milling, decreased tool size leads to a need for tighter tolerances for fixture error in order to avoid excessive tool load and maintain machining accuracy. In 4-axis machining on a curved surface, fixture errors propagate cumulatively leading to a significant error at the tool tip. As a result a compensation approach is essential to successful microfeature production on curved surfaces. Tool stresses are shown to be highly dependent on the amount of fixture error. The scaling down of tool sizes is shown to result in an exponential increase in tool stresses. This paper proposes the use of a conductive touch-off method that utilizes the milling tool in its spindle to perform an in-situ registration mapping of positional errors. The fixturing errors are characterized using the Denavit-Hartenberg robotic linkage convention. A forward kinematic solution uses homogeneous transformation matrices to investigate the effects of fixturing errors on milling tool path errors in 4-axis micro-milling on curved surfaces. The touch-off registration measures the positional error in the tool axis direction allowing for axial tool position compensation. This results in decreased tool stresses and increased channel depth accuracy which is necessary for successful milling. A preliminary implementation of the conductive touch-off registration approach has demonstrated the efficacy of the technique when applied to production of micro-features on concave surfaces.