Making model engines requires frequent rounding over of the ends of linkages, clevises, etc..
I made this jig to simplify this operation.

Top view.  The part to be rounded pivots on a central pin which can be changed to suit the size of the hole in the part.  The two stops are absolute essentials since the cutter (For safety, I use a rotary file rather than an endmill.) has a tendency to grab the work and pull it out of one's hands.  The main bar of the stop is repositionable and can be swung into rough location.  Setscrews in the end of the main bar then allow the secondary bars to be slid in and out for fine adjustment.