Hi, Please watch the short video that explains what I mean. Thanks
Double click (or right click and edit component) on the component to open it for editing first before drawing your arc. Groups and components, are like wrappers for geometry, once you put a set of geometry into one the only way to affect that geometry directly is to open the wrapper for editing. This is correct workflow, making components of everything as you model it, it keeps all geometry separate and easy to work with.
As EF wrote, you should open the component for editing before drawing the curve so the curve will modify the geometry inside the component.
At the stage of your model in the video, you could select the curve, use Edit>Cut to cut it to the clipboard, open the component for editing and use Edit>Paste in Place to paste the curve inside with the rest of the geometry. Then use Push/Pull to push away the waste.
Done with keyboard shortcuts.
Thanks EF and Dave great tag team on my problem! This is the best forum I have ever been a part of.