Has anyone built the machine from the book plans and made substitutions for the store-bought threaded rod? I wanted to spend a bit more money to ensure smoother/faster/more accurate travel with the le

You can use any type of threaded rod, i.e. ACME profile, hi-lead, ball screw, etc. The only change you will need to make is the way the lead nut fastens to the nut mount on the machine. Lead nuts for these non-allthread screws typically have flanges with mounting holes, so the mounting of this type of nut is far better than the standard nut used in the book build. These better screw options will also allow your machine to move faster and with less backlash. https://buildyourcnc.com/item/mechanical-leadscrews-antibacklash-nut-!5-5-starts-10-tpi https://buildyourcnc.com/item/mechanical-leadscrews-lead-screw-!5-5-starts-10-tpi