That is not true.
As an example, when the machine turn, it first do the rough cut, then automatically changes the tool and do the finish cut.
That shows a bilateral normal distribution on the final measure.
In the worst case, if the operator lacks to control the process as he has to do, it shows a smallest/largest extreme value distribution, but that is good enough.
It never shows a uniform distribution.
For precision machining, the most significant variation you should have is tool wear - which is all you should see in precision machining over time, then as the tool wears for an OD (for example) the resulting parts get larger. As they reach the upper control limit, they should be adjusted to the lower control limit to allow the tool to continue to wear. The resulting curve is a sawtooth curve, which is the uniform distribution. Grinding is a perfect example of that. HOWEVER, with multiple pass tooling (as you described), there can be tool pressure variation as the rougher and finisher wear at different rates. If the tool pressure influences the dimension, then you may not have a perfect sawtooth curve. Then yes, the operator has to chase around the dimension, and the multiple variations will appear to be a normal distribution. But, that's because there is no real control except the operator running to the mean, which ends up being overcontrol. I have even watched roughing/finishing OD dimensions get
smaller because of the effect of unbalanced tool pressure between the tools. Remember, to be a
true normal distribution process, you should be able to set it at mean and -
without operator intervention - the process will stay at mean. Just because you variation appears to be normal, does not mean it should be! As you know, with tool wear that would be impossible. OD's have to grow, IDs have to shrink. Otherwise you have interfering special causes, such as tool pressure variation, roundness measurement error, chip wash, etc. If you have a normal curve in precision machining, you are either lacking control or charting incorrectly.