Several major causes of geometric errors in precision CNC lathes include:

1. Insufficient rigidity of machine tools: Insufficient rigidity of CNC lathes is one of the main causes of geometric errors. When the structural rigidity of the machine tool is not strong enough, vibration and deformation will occur during the cutting process, resulting in a decrease in the machining accuracy of the workpiece. This may be caused by improper machine tool structural design, inappropriate material selection, or wear and tear caused by long-term use.

2. Thermal deformation during processing: During high-speed cutting and long-term processing of CNC lathes, various components of the machine tool may undergo thermal deformation due to the influence of heat. Especially when CNC lathes are performing high-speed machining or large-volume cutting, due to the accumulation of cutting heat, various components of the machine tool may undergo uneven thermal expansion, resulting in a decrease in machining accuracy.

3. Motion system error: The motion system of CNC lathes includes servo motors, sliding guides, etc. Errors in these systems will directly affect the processing accuracy of the workpiece. For example, the position error of the servo motor, the gap in the transmission system, and the wear of the guide rail may lead to a reduction in machining accuracy.

4. Tool wear and deviation: The wear and deviation of CNC lathe tools will also affect the machining accuracy of the workpiece. When the tool wear of a CNC lathe is severe or the deviation is large, the quality and dimensional accuracy of the machined surface will be affected.

5. Poor workpiece clamping: Unstable workpiece clamping on CNC lathes will cause the workpiece to shift or vibrate during processing, thus affecting processing accuracy. For CNC lathes, especially for long, thin or thin-walled workpieces, workpiece clamping is more critical. Insufficient clamping force or improper clamping methods will aggravate geometric errors.

6. Improper program design and parameter settings: Incorrect processing program design or parameter settings of CNC lathes may also lead to geometric errors. For example, improper selection of parameters such as cutting speed, feed speed, and cutting depth will affect machining accuracy.

To sum up, the causes of geometric errors caused by precision CNC lathes involve the structure of the machine tool itself, various factors in the processing process, and the operator's operation and program settings. In order to ensure processing accuracy, these factors need to be considered comprehensively and corresponding measures should be taken.