Before beginning any work, what is the most important safety device that must be used when working with the on-car brake lathe?

The most important safety device to use when working with an on-car brake lathe is safety glasses. Here are the steps:

1. Before starting any work, ensure that you have a pair of safety glasses. These are designed to protect your eyes from any debris or particles that may fly off during the process.

2. Put on the safety glasses. Make sure they fit well and cover your eyes completely.

3. Check the condition of the glasses. If they are scratched or damaged, replace them before starting the work.

4. Now you can proceed with your work on the on-car brake lathe. Remember to keep your safety glasses on at all times during the work.

5. After finishing your work, clean your safety glasses and store them in a safe place for future use.

Remember, safety should always be your first priority when working with any kind of machinery.