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ISARA 400 CMM - soft materials at CERN

In the Cern project “CLIC”, a large number of discs (‘cells’) are used in the linear collider. These cells are discs of Ø74 mm, made of oxygen-free copper (Cu-OFE). Each cell features a specific internal profile, which needs to be measured with respect to several geometric references: the center line, as defined by the outside cylindrical surface, and the top surface of the disc.

For this application, the most challenging aspect is not the required measurement accuracy; the profile accuracy is specified at 5 µm. The difficulty is measuring the work piece without any damage to the surface of the rather soft Cu-OFE material.

Given below is a plot of the Hertzian contact stress as a function of the normal force between the probe tip and the work piece surface, for a conventional CMM probe and the Triskelion C-500 probe used on the ISARA 400. The allowable contact stress, at which plastic surface deformation of the Cu-OFE begins to occur, is indicated by the red line.

  

It is clear from this graph that the conventional CMM probe will damage the surface of this soft material, whereas the contact stress caused by Triskelion C-500 remains well below the allowable limit.

The result of the profile measurement, with respect to the defined geometrical references, is shown in the whisker plot above. The profile error is shown as blue whiskers on the surface; the green lines indicate a tolerance band of +/- 2.5 µm.

More info:  Isara 400 CMM ; aspere measurement; traceable measurement uncertainty