Trepanning is a machining process that cuts a hole through the centre of a solid bar and removes the core as a smaller solid bar, rather than reducing the whole hole to swarf. It can be much quicker, and the retained core can be used to make further components, so it is more valuable than swarf.
Certification of the removed core
One caution concerns the mill test certificate for the smaller bar removed during trepanning. Its diameter is much smaller than the original, and its position within the original bar means the reported mechanical properties are not representative. The way around this is to re-solution anneal and re-test the removed bar to confirm consistency.
The nuclear industry frequently relies on trepanned core materials because they can be tested against critical performance and safety specifications, and archived for later evaluation.