What are the three heads of a combination square?

The correct answer identifies the three heads of a combination square as the stock head, center head, and protractor head.

The stock head is the main part of the combination square and typically features a straight edge that can be used to establish a degree of precision while measuring or marking. It also serves as a reference for a right angle to ensure workpieces are accurately aligned.

The center head is specifically designed for finding the center of round or cylindrical objects. This head allows for precision in marking and measuring, which is essential when working on components that need to be symmetrical or balanced.

The protractor head, on the other hand, is used for measuring or laying out angles. It has a semicircular scale that enables users to determine the degree of an angle accurately, making it vital for tasks where angular measurements are essential.

Other options either combine terms that are not standard or include incorrect descriptions of tool components. Understanding the functions and names of each head helps in recognizing the versatility and utility of a combination square in various aircraft metal structural repair tasks.