When purchasing a CNC vertical lathe, choosing linear rails (also known as ball guides) or rigid rails (also known as linear guides) depends on your specific application needs and budget. Each type has its advantages and limitations, here are some of their characteristics and comparisons:
1. Linear rail (ball guide rail):
High precision: CNC vertical lathe linear rails generally provide higher positioning accuracy and repeatability, and are suitable for applications that require high-precision machining, such as mold manufacturing and precision part processing.
Low friction: Ball guide rails have low friction, which helps reduce power loss and improve processing efficiency.
Smoothness: The movement of linear rails is generally smoother, which reduces vibration and helps improve surface quality.
Higher price: CNC vertical lathe ball guides are usually more expensive than hard rails, which may increase the purchase cost of CNC vertical lathes.
Maintenance requirements: CNC vertical lathes require regular maintenance, including cleaning and relubrication, to ensure ball guide rail performance.

2. Hard rail (linear guide):
Lower cost: CNC vertical lathe hard rails are generally more economical than ball guide rails and are suitable for situations where budget is limited.
Durability: Box rails are generally more durable and can be used for extended periods of time under high loads and intensive machining conditions.
Stability: The hard rail has high stability and is suitable for processing large workpieces.
Relatively low accuracy: Box rails may not offer as high accuracy as linear rails and therefore may not be suitable for very demanding precision machining.

All things considered, if your application requires high precision, smoothness, and high surface quality, and you have the budget, ball guides may be a better choice. But if your budget is limited, or you need to handle large workpieces or don't have particularly high accuracy requirements, then hard rails may be a more affordable option. When selecting, it is recommended to consult with the machine tool manufacturer or supplier to ensure that the selection meets your specific needs.