What tool is ideal for filing inside holes?

The ideal tool for filing inside holes is a round file. This type of file is specifically designed with a cylindrical shape, allowing it to access and smooth out the interior surfaces of holes and cylindrical openings effectively. The rounded edges enable it to refine and deburr these spaces, which might be difficult to reach with flat files or other tools that do not conform to the shape of the hole.

In contrast, a flat file is better suited for working on flat or smooth surfaces and can’t easily reach into curves or holes. A half-round file may work on slightly curved surfaces, but its flat side limits its effectiveness in deeply shaped holes. A mill file is primarily used for sharpening and smoothing flat surfaces and isn't suited for filing in curved or cylindrical areas at all. Therefore, when needing to work inside holes, the round file is the most appropriate choice due to its unique design and functionality.