Additive manufacturing (AM) allows a high degree of complexity and the creation of geometries that are not possible with conventional manufacturing methods. However, the quality and the accuracy of the parts are often not sufficient and a precise post-processing is inevitable. In the case of subsequent machining, especially of thin-walled and non-rotationally symmetrical components, residual stresses generated during additive manufacturing may result in distortions. In order to achieve the required accuracy a stress relief heat treatment may be required. If for other reason this is not possible the optimisation of the additive manufacturing procedure as well as the cutting process is necessary to reduce the distortion of the workpiece. In this paper the influence of different machining parameters is investigated for AM-manufactured cylindrical half shells made in Invar, and compared to non-additively manufactured and heat treated workpieces. A post-process stress relief annealing eliminates most distortions, whereas on the machining side a single deep cut is more beneficial for smaller distortions instead of several small cuts and higher cutting speed. By this, a decrease of 33% in distortion could be achieved. The results also show that different AM scanning strategies have no significant impact on the distortion of the workpiece.

^a RhySearch, Buchs SG, Swizerland
^b inspire AG,  St.Gallen, Switzerland
^c SwissOptic AG, Heerbrugg SG, Swizerland

*The full paper is available to CIRP members

Point cloud based tool path generation for corrective machining in ultra‑precision diamond turning

Marco Buhmann^1,2, Erich Carelli³, Christian Egger³, Klaus Frick³

Published online: April 20, 2022

The increasing demand for machining non-rotational optical surfaces requires capable and fexible cutting tool path generation methods for ultra-precision diamond turning. Furthermore, the recent interest in on-machine metrology and corrective machining requires efcient as well as accurate algorithms capable to handle point cloud based surface data. In the present work, a new computation method for the tool path generation is proposed that focuses on three-axes corrective machining. It is based on the principle of defning the surface to be machined by a point cloud of given density, since surface measurement data is usually available as point cloud. Numeric approximation techniques are used to compute the surface normal vectors and calculate the resulting positions of the cutting tool path preserving a uniform radial axis motion for face turning. Investigations are performed in order to quantify the error between the calculated tool path and the exact analytical solution. The error dependencies are analyzed regarding the local surface slope and numerical parameters. Error values below 1 nm are achieved. In addition, form deviation results prove the method’s capability for corrective diamond turn machining.

¹ RhySearch, Buchs SG, Switzerland
² Institute of Machine Tools and Manufacturing (IWF), Swiss Federal Institute of Technology, Zurich, Switzerland
³ Institute for Computational Engineering (ICE), Eastern Switzerland University of Applied Sciences, Buchs SG, Switzerland

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Implementing on‐machine measurements on ultra‐precision diamond turning machines follows the principle of measuring close to the machining process without reclamping the workpiece and sets up a closed‐loop process consisting of machining and measuring. The probe integration into the machine tool inevitably comes along with measurement errors due to alignment, probe system and kinematic imperfections as well as varying scanning conditions. In order to reduce these errors an approach is presented to align the optical axis of an interferometric one‐dimensional optical probe parallel to the machine tool spindle axis, necessary to reduce the cosine error caused by misalignment. Furthermore, the approach allows to determine the gain error of the probe system. The machine tool’s X‐axis straightness deviations in Z‐direction are measured and compensated by post‐processing of the measurement data. An experimental validation method is presented and yields to a maximum measurement error range of about 45 nm for a measurement of a non‐rotational symmetric freeform surface with a diameter of 20 mm.

¹ RhySearch, Buchs SG, Switzerland
² Institute of Machine Tools and Manufacturing (IWF), ETH Zurich, Switzerland

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The measurement of machined surfaces directly on ultra-precision turning machines yields to notable advantages, e.g. in correcting formdeviations without re-clamping. In the present work, a one-dimensional probe is moved by the machine axes to optically scan the workpiece. The integration and positioning methodology of the probe within the machine tool coordinate system as well as a measurement simulation model are presented. Simulations and experiments are conducted to identify the influence of positioning errors of the probe regarding the measurement results. The measurement error along the spindle axis, which mainly affects the measured form deviation, depends on the gradient of the geometry to be measured.

^a RhySearch, Buchs SG, Switzerland
^b Institute of Machine Tools and Manufacturing (IWF), ETH Zurich, Switzerland
^c Institue for Computational Engineering (ICE), University of Applied Science NTB Buchs, Switzerland

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The ability to verify the geometrical quality of a workpiece on the machine tool itself can be a crucial advantage in ultra‑precision diamond turning. This work presents a test procedure for single point distance measuring optical probes integrated on diamond turning machines (DTMs). To be able to specify the probe´s characteristics a strategy using the axis of the DTM itself is developed.

Figure 1 shows the experimental setup consisting of a measurement target mounted on the z‑axis and the probe mounted on the x‑axis of the DTM. The flat metallic diamond turned surface used as measurement target is positioned orthogonal to the z‑axis. While running a NC program, which commands a specific trajectory, for example a sinusoidal path, the glass scale signal of the DTM and the probe´s distance signal is recorded with a frequency of up to one kilohertz. By analyzing the glass scale signal the actual movement of the z‑axis can be extracted and used as a nominal value to evaluate the quality of the probe signal and its characteristics. An estimation of deviations caused by misalignment of the probe and measurement target is described. Besides that, effects caused by a possible time delay, temperature drifts and dynamic effects of the machine axis are taken into account and strategies to contain these effects are shown.

By adjusting the programmed trajectory the probe´s measuring range to be tested can be varied. Furthermore, by tilting the probe in regard to the measurement target´s surface (see Figure 1 (b)), the important behavior in non‑orthogonal measurements on high reflective surfaces can be investigated. In the present work an interferometric probe is employed. The obtained deviations slightly differ depending on the probe´s tilt and distance value. With a linear compensation for a measuring range of four micrometers the maximum error can be reduced by fifty percent to residual deviations of less than twenty nanometers.