Almost all current-model CNC turning center that are equipped with three-jaw chucks have a way to select clamping direction. An inward (toward chuck center) clamping direction is, of course, required for external (o.d.) clamping while an outward (away from chuck center) clamping direction is required for internal (i.d.) clamping.
Most turning centers are pretty well interfaced in this regard - at least from a safety standpoint. If, however, clamping direction can be changed while the spindle is running, the results could be disastrous. Consider inadvertently changing clamping direction from o.d. to i.d. while the machine is in cycle. The chuck jaws would open - releasing the workpiece being held by the chuck. The faster the chuck is rotating at the time, the worse the results could be.
Again, many turning centers are set up in such a way that chuck clamping direction cannot be reversed unless the spindle is stopped - which eliminates the disastrous results just mentioned. But we must point out that not all machines are so well interfaced.
We know of at least one turning center manufacturer that uses a mechanical valve to reverse chuck clamping direction. That is, the setup person simply moves a lever to change clamping direction. The builder we're thinking of, knowing a dangerous situation could exist, provides a long bolt that is must be removed before clamping direction can be changed (this bolt also eliminates the possibility that the valve lever can be accidentally bumped into its other position).
But if this bolt is not replaced each time the clamping direction is changed, nothing will prevent the valve lever from being moved while the spindle is running - meaning a workpiece could be released at very high rpm - and again - the results will be disastrous.
If you run or work with a CNC tuning center of any kind, find out how chuck clamping direction is changed. Determine whether or not the clamping direction could inadvertently be changed while the machine is in cycle. If this is possible, be sure that all safety protocols that the machine tool builder recommends are in place.