Which tool is used to round the edges of the filler for easier fitting into the stringer?

The tool utilized to round the edges of the filler for improved fitting into the stringer is a belt or disc sander. This tool operates with an abrasive material that removes material continuously, allowing for precise control over the shape and smoothness of the edges.

When rounding edges, it is important to maintain a consistent profile, and a belt or disc sander provides the capability to do this effectively. The flat surface of the disc or belt can be used precisely to create a smooth, rounded edge that will fit snugly into the stringer without creating sharp points that could lead to stress concentration or difficulty in assembly.

In contrast, a router is typically used for more intricate edge profiles and cuts rather than for simply rounding corners. A jack plane is primarily a woodworking tool used for flattening surfaces or straightening edges but does not provide the same efficiency in rounding as a sander. A band saw is more suited for making straight cuts or intricate shapes but would be inefficient for achieving the rounded edge required in this situation.