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How to draw this top arc?


Super Moderator
The top arc that you've highlighted is actually two circular arcs. Each is R100mm in size. One is concave, and the other is convex. Both are tangent to each other in the middle.

There are many ways you could sketch these arcs with CATIA. I would draw a Three Point Arc and then constrain it's size and position. Then repeat for the other arc.

The "trick" to drawing these arcs, are the centerpoints. The reference drawings shows convex arc centerpoint at 250mm. Both centerpoints are coincident to the vertical lines 85mm (170mm/2) apart.
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Super Moderator
Again, these reference drawings are not very good. Since there was no other dimension for that arc, we have to assume all the arcs in that view are the same size.