Touch probe performance on CN machining centers

The use of a CNC machining center (using a touch trigger probe) to perform dimensional control is described in the present paper; in this case, the machine tool acts as a coordinate measuring machine (CMM). In general, measurement performance of a CMM depends on both probe and machine structure. Probe behavior is mainly described by “probe lobing”, a polar plot of residuals from a least square fitting circle (when measuring circular or spherical reference specimen). Machine characterization is a more complex task, moreover, most literature references do not deal with machine tools applications for metrology. An experimental procedure was set up to assess measurement performance of a Famup MC60 machining center equipped with a Marposs E86 touch probe. A first set of tests was carried out to asses the system repeatability in 1D measurements, 2D system performance were tested as well. The effect of contact direction was evaluated by one-directional measurements of a planar surface carried out at 360 different angles of oriented spindle stop. In this way, the probe approaches the specimen at different angles and the results are comparable with the probe lobing diagrams. The effect of probe tip misalignment (with respect to the spindle axis) was eliminated by using Fourier Transform and the resulting residuals were evaluated as a function of the spindle angle. Three different stylus lengths and three approach velocities were taken into account: the magnitude of the residuals proved to be dependent on stylus length, while data scatter is heavily affected by approach velocity. The magnitude of pre-travel was evaluated by touching two opposite sides of a prismatic reference block. Pre-travel was found to be linearly dependent on probe velocity and increasing (not linearly) with stylus length. The effect of stylus length can be modeled by assuming bending and overrun effects; the results are consistent with physical input data (moment of inertia of the stylus, probe tilt to detect contact, etc). Basing on the results of this analysis, the effect of operating parameters (stylus length and approach velocity) on touch probe measurements was evaluated, yielding interesting suggestions for performance optimization.