Form errors estimation in free-form 2D and 3D geometries.

The aim of this work is to present a method for the verification of form tolerances in free-form geometries. Method: New free-form error parameters, in accordance with ISO GPS standards are defined. Geometrical errors in 2D e 3D geometries are calculated using a new fitting method consisting in an association operation where nominal profile/surface is resized (with an offset), located and oriented on the non-ideal geometry. Form tolerance is estimated using peak-to-valley deviation after the best fit. Result: Automatic algorithms are implemented to analyse 2D free-form profiles of moulds for ophthalmic lenses obtained with grinding and polishing operations and to analyse 3D free-form surfaces of an injection moulded lamp lens for automotive industry. Discussion & Conclusion: The comparison between results obtained using this approach and using traditional fitting methods shows differences on the fitting and errors parameters estimation. In particular in the traditional fitting there is a repartition of the size error contribution in the translation and form errors. The differences increase proportionally to the offset value estimated. The calculation of offset parameter allows the separation of size from geometric errors in free-form profiles/surfaces and the “true” form error estimation.