Sliding VS Rolling linear slides for machinery

[gkan]

Greetings everyone,

I have a general question regarding machinery design. I have noticed that conventional machines only have dovetail slides while CNC machines usually have linear ball bearings for their axes. Even really expensive and precise conventional machines like Hardinge have sliding ways while precision CNC machinery (the ones I have seen anyway) have linear ball bearings. So I was curious, why is that? In general, isn't it true that sliding ways are generally a lot more rigid and durable than ball bearings?

Thanks in advance,

George

 

[BillPSU]

My experience matches yours, gkan, however but I was an NC/CNC programmer for twelve years. The main reason for the use of liner slides is because of the travel speed and rate of acceleration and deceleration and cost. The original NC machines utilized Turcite on scraped ways and could withstand large cutting forces. Travel speeds were moderately fast but no where near the current equipment of today. Over the years the new designs moved to even higher rates of movement and linear bearings became the norm.

I have experienced one of the linear bearing failings when an operator of a large horizontal machining center crashed the machine and broke a 1 1/2" twist drill. He replaced the drill and continued using the machine. The machine ran fine until the a precision gear box case was setup on the machine. When the machine was crashed the linear bearing pack was smashed and caused the spindle carrier to hang at an angle of about .010" per 10". When the boring bar length was changed the effective bore location changed. The initial crash was unreported and finding the cause of the problem difficult. Turcite slides would not have experienced this problem.

Bill

 

[eromlignod]

Hard ways are cheaper and handle a larger load.

Ball bearings have much less friction.

Don
Kansas City