Improving axial depth of cut accuracy in micromilling

In order to maintain an optimum cutting speed, the reduction of mill diameters requires machine tools with high rotational speed capabilities. A solution to update existing machine tools is the use of high speed attached spindles. Major drawbacks of these attachments are the high thermal expansion and their rapid warming and cooling, which prevent the achievement of a steady state. Several other factors, independent on the tool-workpiece interaction, influence the machining accuracy. The cutting parameter most heavily affected is the axial depth of cut which is the most critical when using micro end mills, due to the easy breakage particularly when milling on hard materials. Typical values for the errors on the control of the axial depth of cut are in the order of 50 microns [1], [2], while the aimed depth of cut can be as low as 5 microns. As a result, micromilling cannot enter yet normal workshops provided with conventional milling machines. This paper presents an investigation aimed at the reduction of the error on the axial depth of cut in micromilling operations, in a workshop environment. A method for tool length correction with sub-micrometer resolution by use of an inductive probe was developed, and a series of tests for isolation and identification of error contributions on the axial depth of cut was carried out. As a result, a procedure for optimal control of axial depth of cut was defined which led to a reduction of the depth of cut error from 36.5 microns to 2.5 microns.