Why do my contours dip at the edge?

I’m using a 6mm ball nose mill on aluminium, and trying to cut out a gentle ZY axis arc using G2 command. It all looks good except at the edges, where the cut is slightly deeper. It’s not part movement, it’s quite secure. The arc is smooth, the lines shown are the regular spacing as I’m repeating increments across the x axis. Why the dip and what can I do to prevent it?

Example cut

Any suggestions gratefully received!

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large cutter and large stepover = faster carve, small cutter and reduced stepover = longer carve times

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I think your Z axis is flexing upwards when it engages the material, then falls as it goes off the edge.
Are you trying to surface with a ball nose bit?

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Yes, I am trying to contour a surface. I think maybe I should have asked first, I imagined it would work! What would be the best way to do it?

Can you show us an example of what you are trying to do?