In this post, you will see how to refine circles or any circular curve in 2D Sketch and in the 3D Model in CATIA. 

First you have to go to Me > Preferences > Common Preferences > 2D 3D View Display > Performance

You can refer to the below image for the same

3D Accuracy:

 On the right-side 3D Accuracy box will appear where you have to change the value of "Proportional" to 0.01, which is the lowest. Then click apply. It will apply the setting to your model. The lower the proportional value, the more refined will be the model.

 Here, in the following image, two test cases are shown where you can see the difference in the model after changing the value.

Fixed: It is the fixed value (from 0.01 to 10) for calculating tessellation on all objects.

Proportional: In this case the tessellation is calculated according to object size. (More preferable)

2D Accuracy:

Below the 3D Accuracy box, you can see the 2D Accuracy box where you have to change the value of "Proportional" to 0.01, which is the lowest. Then click apply. It will apply the setting to your sketch. The lower the proportional value, the more refined will be the sketch.

 Here, in the following image, two test cases are shown where you can see the difference in the sketch after changing the value.

Fixed and Proportional meaning is the same as 3D Accuracy.

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