Which factor should NOT be present to ensure a successful thermite weld?

The presence of bolt holes is not a necessary factor for a successful thermite weld. In thermite welding, the focus is primarily on the quality of the molten metal being produced and the proper alignment of the components being joined. Bolt holes are not a part of the thermite welding process and can actually complicate the weld if not correctly aligned or if they interfere with the flow of molten metal.

In contrast, controlled temperature is crucial, as the thermite reaction generates extremely high temperatures that need to be carefully managed to ensure proper melting and fusion of the rail materials. Stable rail alignment plays a vital role in the success of the weld since any misalignment can lead to weak joints or incomplete fusion. Additionally, the elimination of vibrations is essential throughout the welding process, as vibrations can disrupt the molten metal's ability to settle and bond properly, leading to defects in the weld.

Thus, a successful thermite weld does not rely on the presence of bolt holes, making that factor not essential to the welding process.