How can one accurately measure the diameter of a cylindrical workpiece?

To accurately measure the diameter of a cylindrical workpiece, using calipers or a micrometer is the most effective method. Calipers can provide precise measurements by allowing the user to measure both the inside and outside diameters, as well as depths and steps. They are designed to make direct contact with the object being measured, ensuring a more accurate reading.

Micrometers are specifically designed for measuring small dimensions very precisely, making them ideal for checking diameters of round objects where a high level of accuracy is required. They can measure dimensions down to the thousandth of a millimeter, which is crucial in precision machining.

The other methods listed are less suitable for this task. A ruler or tape measure, while useful for general measurements, lacks the precision required for accurately measuring a cylindrical workpiece's diameter. A protractor is used for measuring angles, not diameters, and a square gauge is primarily utilized for checking squareness or ensuring surfaces are flat, not for measuring cylindrical dimensions. Therefore, utilizing calipers or a micrometer ensures accurate and reliable measurements for cylindrical workpieces.