Confused about stepper sizing

[Jeff]

Hi

My mechanical build is almost complete 20mm HiWin rails and bearings on X & Y axes with 2010 lead screws X axis is 1500mm and Y axes are 1000mm. Total weight of my gantry including the spindle is around 40Kg and I've been trying to calculate what size motors I need for my axes, but both my own calculations, and the output from several online motor calculators are giving me results that I wasn't expecting.

For example on the Y axis with a total load of 40Kg and a cutting force of 5N my total required torque @ 80 rpm (800mm/min) is coming out with a figure of around 0.18Nm, this would easily be achievable with the 175oz Nema 23 stepper on the openbuilds store which should put out around 0.7 Nm @ 48v and 80 rpm.

My gut instinct was to go for motors in the 400+ oz range, which seems to be what everybody recommends for this sort of size machine. Are my torque calculations way off or is there another reason that I need to massively overspec my motors?