There can be many reasons for a CNC lathe to bump, the following are some common reasons that may cause bumps:

1. Programming error: Wrong machining program or tool path setting may cause the tool to collide with the workpiece or fixture. Machining paths should be carefully checked and verified when programming to ensure that the tool can move safely.
2. Fixture installation error: Unsteady fixture installation, position offset or improper fixture design may cause the tool to collide with the fixture. When installing the fixture, make sure the fixture is properly seated and does not interfere with the toolpath.
3. The workpiece size is out of range: If the workpiece size exceeds the machinable range of the machine tool or tool, the tool may collide with the workpiece. Make sure that the workpiece size fits within the machine and tool limitations before machining.
4. Improper tool length: Using a tool that is too long may cause the tool to collide with other components or the machine tool structure. Choose a tool of the proper length to avoid collisions.
5. Worn or damaged tools: Worn or damaged tools can cause the tool to collide with the workpiece or machine tool structure. Check and replace knives regularly to keep them in good condition.
6. Machine tool errors: Errors, deviations or instability of machine tools may cause collisions between the tool and the workpiece. Regularly inspect and maintain machine tools to ensure their correct operation and high-precision processing.
7. Operator errors: Operator's operating errors, such as wrong operation input, wrong operation sequence or carelessness, may cause the tool to collide with the workpiece or machine tool. Operators should receive adequate training and follow correct operating procedures.

The above are some common causes of CNC lathe collisions. To avoid collisions, operators should carefully check programming procedures, fixture installation and workpiece dimensions, and maintain good tool condition. In addition, regular maintenance and calibration of machine tools are also important measures to prevent collisions.