How far should a copper tube be inserted into a cup for a brazed joint?

For a brazed joint involving copper tubing and cup fittings, the tube should be inserted all the way to the shoulder of the cup. This practice ensures that there is sufficient surface area for the brazing alloy to properly fill the joint. When the tube reaches the shoulder, it allows for an optimal alignment that results in a stronger and more reliable connection. Additionally, brazing involves melting the filler material, and having the tube inserted fully allows the molten filler to be capillary drawn around the joint, facilitating a uniform and effective bond.

While other options may suggest alternative insertion depths, they do not provide the same level of integrity and strength as inserting the tube to the shoulder. For instance, inserting only halfway or a minimum of one diameter can lead to weak joints, prone to failure under stress, while being short of the shoulder doesn't provide the necessary contact area for effective heat transfer and filler flow. Thus, inserting the tube all the way to the shoulder is the best practice for ensuring a robust and durable brazed joint in copper tubing applications.