Say if you have a 2" bar with a cutting tool projecting .25" from the side it should cut a 2.5" diameter. If the runout near the tool is .010 then the tool will cut .010 larger than if the bar is perfectly straight. The tool is cutting from the theoretical centerline of the spindle bearings. The hole will be round to the capabilities of the setup. Just imagine a Criterion style boring head, it simply offsets from the centerline.
Ed.
Just had another thought after rereading this. That is not necessarily true. The tool will
not always cut the same amount larger as the bar is running out. If the tip of the tool is not inline with the max runout point it will cut a completely different size, depending on where it is in relation to that runout.
We'll use your 2" bar that's running out .010" with .25" tool tip projection as an example. If the max TIR point on the bar is at 0° in this case, and our tool tip is at 180°, the tool will cut a theoretical 2.490" bore. If it's at 0°, a theoretical 2.510" bore. Anywhere in between will produce a size within that range. At 90° or 270° it should hit 2.500" right on the money.
Anyway, after all that, I guess my point is: don't ever rely on tool stickout distance from a boring bar to set your boring tool unless it's for a roughing cut. Make your spot cut and set from there.
And yes, atex, I know you were also just using a theoretical example to explain
your point, I was not saying that you don't already know this.
