What should be the motion used during preheating to ensure uniform heating?

The correct choice for the motion to be used during preheating to ensure uniform heating is a slow, constant, circular sweeping motion. This method allows for an even distribution of heat across the area being brazed. By using a circular pattern, you minimize the risk of overheating certain spots while underheating others, which can lead to issues such as poor joint strength or even damage to the workpiece.

In a slow, circular motion, the heat source continuously covers the entire surface area, allowing the heat to dissipate evenly. This is particularly important in brazing, where maintaining a specific temperature range is crucial to achieving a successful joint.

Other methods, such as back and forth shaking or fast linear movement, might not provide the same level of uniformity, as they can lead to areas getting too hot while others remain inadequately heated. Random motion lacks the control needed to ensure that all areas of the joint reach the required temperature consistently.