When using a flat file to deburr a hole, which surface should be in contact with the material?

Using the rough surface of the flat file to deburr a hole is the best practice because the rough side is designed to remove material. The file's teeth are intentionally cut in a way that they can effectively shave off any burrs or sharp edges left after the drilling or machining process. When the rough surface is applied to the material, it maximizes the cutting action, allowing for a cleaner finish and a more precise deburring. This is crucial in aerospace applications, where a smooth edge can prevent stress concentrations, enhance aerodynamics, and promote safety.

In contrast, using the smooth surface would limit the removing capability of the file. A smooth surface could potentially burnish the edges rather than effectively remove the burrs. Therefore, employing the rough side optimizes the deburring process, making it the correct choice for ensuring quality and safety in aerospace components.