The screen-shot is of a sectioned, non-return valve body, part of a tiny but complex assembly whose original design was more prettyness than practicality! I’m investigating an alternative way for more feasible making; but as the real thing needs extremely high-precision machining and fabricating to work at all, more as an Alibre exercise than reality.

The arcuate horizontal section is a joint-face with a horizontal, hollow cylinder to which the component is soldered.  I I modelled the cylinder but found its internal parts too difficult to model, and the original drawings (reduced for printing in a book) are almost illegible, so I omitted them.

The problem lies with that port sloping down into the left-hand cavity.

My modification replaces an awkward slot with a circular hole placed to drill without damaging the upper rim of the chamber, or so low it prevents the valve working.  Such problems are normally solved by drilling horizontally through both walls, then plugging the outer wall.

It took over an hour to plot this wretched port, and then only by cutting in from a rectangle.

If however, I had to illustrate it as a round hole, with its axis sloping down the cut plane, and at a definite height in the cavity, how is that done, please?

(Yes, the port does graze that external corner but I cannot modify the drawn outline to suit.)