How can a mechanic determine if a countersink for a flush rivet should be dimpled or drilled?

The thickness of the top sheet is a critical factor in determining whether to dimple or drill a countersink for a flush rivet. When working with sheet metal, the thickness directly influences the type of preparation needed for rivet installation to ensure a proper fit and a strong joint.

If the top sheet is relatively thin, dimpling is often preferred because it creates a slight recess that allows the rivet head to sit flush with the surface, while not compromising the metal's structural integrity. Dimpling is especially effective for thinner materials because it leads to a better mechanical connection and no additional material is removed beyond what is necessary for the rivet head.

On thicker materials, drilling a countersink may be more effective. This process involves creating a conical depression for the countersunk rivet to sit in, which allows the rivet to sit flush with the surface. This method is usually needed when the material is too thick to be effectively dimpled without risking damage or undesired deformation.

Understanding the thickness of the top sheet enables the mechanic to choose the most suitable method, ensuring proper rivet installation and maintaining the integrity of the aircraft structure.