Probing force

Especially when measuring soft materials such as plastic or aluminum with conventional (tactile-electric) styli with stylus diameters smaller than 1 mm, permanent deformation and thus damage to the workpiece may occur (Fig. 62). The special probes used for the measurement of micro parts have stylus diameters of several 10 µm. This exacerbates the problem of contact with the measuring object, as the probing force acts on a very small area. In addition, the styli are deflected in the order of magnitude of the part tolerance during tactile measurement in order to ensure reliable detection of the workpiece surface or contact retention in scanning mode at an acceptable measuring speed. At high measuring speeds and strong unevenness of the workpiece, the minimum deflection is often out-of-limit value many times over, resulting in higher probing forces.

During probing, both the stylus tip and the measuring object are deformed. The initially point-shaped contact changes into a surface-shaped contact due to the deformation corresponding to the elasticity of the materials. Permanent deformation is avoided if the permissible surface pressure (Hertzian pressure) is not out-of-limit value[1]. However, the permanent deformation that occurs in practice is always smaller than the original penetration depth of the ball. With soft materials such as plastics or aluminum, the penetration depths are considerably greater, which is why the greatest risk of permanent deformation exists here. Additional forces that increase the deformation are caused by the abrupt impact of the stylus tip during punctiform probing. A sufficiently accurate prediction of any permanent deformation is not possible due to the many unknown parameters. Experience has shown that an experimental investigation of the resulting damage is recommended for stylus diameters smaller than 1 mm.