Shallow cut perpendicular to curved surface

Create an offset surface of the curved face - offset of zero, or of 15 thou into the part (your call) Use your sketch to trim that surface (sketch is trim tool), keeping the part of it that you want to recess Insert>Cut>Thicken, selecting the surface and making sure that the cut projects past the outside of the model.
All of the edges of the cut/thicken will be perpendicular to the edges of the surface.
Ed

Spehro,
Cut-extrude-offset from surface.
Mark
Embedded software/hardware/analog Info for designers:

Thanks, Ed! That works perfectly.
Best regards, Spehro Pefhany

Normally I would go for that too, but I was simplistic in my response because he said he needed the edges to be perp to surface. I don't know why he wants that, but hey.... Personally, I avoid cut-thickens in most situations (except when thickening a completely enclosed surface body into a solid). I wonder how he will deal with draft, or if it is even necessary? Unless its a compound surface (and it is compound in the RIGHT direction), you are not guaranteed correct draft.
I am with you on your extrude-offset from surface and apply parting line draft to the root edges of the cut, because usually you want some draft, not true-perp to surface. Thanks Mark - thats usually the best way to go on cast/forged/molded parts Ed