Run-out is an inaccuracy of a rotating
mechanical system; specifically when the tool does not rotate in line
with the main axis of the spindle. For example, when drilling, the
run-out will result in a larger hole than the drill's diameter due to
the drill being rotated eccentrically.
The first law of machining: the run-out is dynamic and cannot be compensated for.
The second law: the run-out is complex with the run-out error being
compound. It is a result of a number of factors such as imperfect
bearings in the spindle, worn bearings, imperfect chuck, collets or an
imperfect tool itself.
And here is the final postulate of machining: "Absolute alignment is impossible, a degree of error will always be present."
This is a scary thought, you invest in a machinery tool holding the best
tools money can buy, and you know upfront that no matter what, there'll
always be some run-out.
Of course, if you're to drill a 6mm hole in
the wall with a $99 cordless hand drill using a $5 drill bit from
bunnings, then a run-out is not going to be important at all. But if
you're trying to drill a 50 micron hole then even a 1 micron run-out of
your entire system is way too much. This is the kind of challenge that
we face in watchmaking. In particular, the weakest point in our system
is not the spindle of the German CNC mill nor the 'Swiss drill bit', it
is the chuck (the clamping system) that connects the two. To machine a
watch main plate alone, it takes 12 tools which are rapidly exchanged,
each one held in its own chuck. Most of those chucks have a run-out
under 1 micron. But recently, we have acquired Japanese high precision
tool holders by BIG Daishowa. These are sub micron run-out tool holders
and the difference in price between the standard and ultra precision
model is over $1,000 AUD per holder.