How is zakat calculated for printing presses in detail?

If the printing presses are not for trade but rather for printing papers, then there is no zakat on them, because they are prepared for personal use (qinyah), and whatever is prepared for personal use has no zakat on it, due to the Prophet's (peace be upon him) saying: "There is no sadaqah (zakat) on a Muslim for his slave or his horse." However, if the printing presses are for trade, then their value is assessed at the end of every Hijri year from the time of acquiring their original assets, and then their zakat is paid, which is one-quarter of one-tenth of their value.