Belt drive tables, whatever their other virtues, can easily run fast or slow because of diameter error of the drive pulley and/or belt slippage. Of course in the best tables the error is minimal.
Quartz clock controlled direct drive tables, whatever their problems may be, inherently run at correct speed. Consider that your $25 Timex watch keeps time to a few seconds per year. What's that in percent?
Interestingly, there is a clock that can be incorporated in a piece of equipment that has absolutely no error at all! In a Missile Fire Control System that we designed, serious consideration was given to incorporating a Cesium Vapor Clock which, was then the NBS definition of time. Cost would have been about $100,000, a relatively small sum in context of the entire Fire Control System. Error due to the clock would have been zero, by definition. However, for much less money we used a quartz crystal clock system which is so close to perfect that it doesn't matter.
Quartz clock controlled direct drive tables, whatever their problems may be, inherently run at correct speed. Consider that your $25 Timex watch keeps time to a few seconds per year. What's that in percent?
Interestingly, there is a clock that can be incorporated in a piece of equipment that has absolutely no error at all! In a Missile Fire Control System that we designed, serious consideration was given to incorporating a Cesium Vapor Clock which, was then the NBS definition of time. Cost would have been about $100,000, a relatively small sum in context of the entire Fire Control System. Error due to the clock would have been zero, by definition. However, for much less money we used a quartz crystal clock system which is so close to perfect that it doesn't matter.