The Research Of Precision Milling And Form Error Evaluation For Freeform Surfaces

As the development of the precision machining technology and optical design, requirements of the function and performance of optical freeform components are becoming increasingly diversified, and product structures are more and more complex. These optical devices are always of non-rotational symmetry and complex irregular structure. For spherical and aspherical optical lens, it is viable to manufacture by traditional 2-axis ultra-precision turning. Nevertheless,there are a lot of complex freeform surfaces which can not be machined using precision turning, instead,it is inevitable to turn to precision milling.So far, There is little attention drawn on freeform surface’s precision milling.So it is of great significance to study on precision milling and form error evaluation for freeform surfaces.Tool path generation in precision milling is directly related to the quality of free surface. A tool path generation algorithm based on chord errors control and the residual errors control is proposed for freeform surfaces’ precision milling.And then the ball end cutter’s actual radius value is accurately measured based on laser aligning instrument.At last,A freeform surface with the mirror effect is obtained by experiment.Combined with OMM and mirror compensation model, the machining errors caused by cutting force and tool wear are compensated by a process like "compensating—measuring—compensating—measuring". A probe calibration model is established and the probe radius compensation method is studied; A compensation method which takes compensation smoothness into consideration based on the mirror compensation model is proposed,and it is used in freeform surface precision milling.Experiments show that the method can compensate the machining error with a high precision,and ensure the freeform surface mirror effect at the same time.A form error evaluation algorithm is proposed for freeform surfaces of precision milling. The algorithm consists of initial maching position recognition for measured surface,coarse matching and fine matching. A new two-dimension geometric shape descriptor derived from Z-map model is put forward to represent the surface shape characteristics,which can be used in initial maching position recognition.After a good initial matching position is provided by coarse matching, ICP algorithm and genetic algorithm is used in fine matching. Talysurf PGI 830 surface profilometer is used for freeform surface measurement, and the form error evaluation results prove the algorithm’s validity and the precision of the freeform surface shaped by precision milling.