Use "slot" in Sketcher (0.21.2 shortcut: G, S) and remove equal constraint between two arcs.

I would have used the arc tool to do the ends of the cam but this is a good demonstration of the trim tool.

If you know you are going to draw a bunch of construction geometry, without any geometry selected you can click the construction geometry tool to toggle between drawing regular geometry and construction geometry. This should save you a few clicks because you won't need to select the regular geometry that you want to convert to construction, you simply draw construction geometry. Just another workflow for your quiver. The tool icons will show either white for regular geometry or blue for construction geometry as an indicator for what mode you are in. (These colors are for the default theme pallet. If you change your theme colors, obviously you will need to convert from what I've said above to your custom colors.)