Why might thread-like grooves appear on the braking surface of a newly turned drum?

Thread-like grooves on the braking surface of a newly turned drum typically indicate that the lathe was not adjusted properly during the turning process. When a brake drum is turned, it is supposed to create a smooth and even surface for optimal contact with the brake shoes. If the lathe is incorrectly set up, or if the cutting tool is not aligned properly, it can lead to uneven cuts and the formation of grooves, which can negatively affect the performance of the brakes.

Proper adjustment is crucial to ensure that the drum is machined evenly, allowing for the correct friction surface necessary for effective braking. This can lead to problems such as reduced braking efficiency, increased wear on brake components, or even noise during braking. Therefore, identifying that the lathe was not set up correctly addresses a crucial aspect of brake drum maintenance and machining.