Final answer:
Ensure that both sheets start melting at the same time when welding a lap joint, caution must be exercised by heating the sheets to different temperatures. One sheet should be heated to a higher temperature while the other is kept at a lower temperature. When brought into contact, the heat will transfer from the hotter sheet to the colder sheet, causing them to start melting at the same time.
Step-by-step explanation:
When heating the two clean sheets in the process of welding a lap joint, caution must be exercised to ensure that both sheets start melting at the same time. This is because if one sheet starts melting before the other, it can lead to an uneven joint and compromised structural integrity.
To achieve this, the two sheets should be at different temperatures and in thermal contact with each other. This means that one sheet should be heated to a higher temperature while the other is kept at a lower temperature. When they are brought into contact, the heat will transfer from the hotter sheet to the colder sheet, causing them to start melting at the same time.
For example, in the process of welding a lap joint, if one sheet is heated to 500 degrees Celsius and the other is kept at 200 degrees Celsius, when they are brought into contact, heat will transfer from the hotter sheet to the colder sheet. This will ensure that both sheets start melting simultaneously, resulting in a strong and uniform joint.