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 to ensure safety:

1. Before starting any work, ensure you are wearing safety glasses. These will protect your eyes from any debris or particles that may fly off during the process.

2. Check the condition of the brake lathe. Make sure it is in good working order and there are no obvious signs of damage or wear.

3. Ensure the area around the brake lathe is clear of any obstacles or hazards. This includes making sure the floor is clean and free of any oil or other slippery substances.

4. Before starting the lathe, make sure all guards and safety devices are in place and functioning properly.

5. Always use the correct tools and equipment for the job. Never attempt to use a tool or piece of equipment in a way it was not intended.

6. Always follow the manufacturer's instructions when using the brake lathe. If you are unsure about any aspect of its operation, seek advice from a qualified professional.

7. Never leave the brake lathe running unattended.

8. Always turn off and unplug the brake lathe when it is not in use.

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