Look for "shaft couplers". Servocity, Mcmaster Carr, etc. have them.
Keep in mind that there are often severe restrictions on the amount of weight you can hang from a motor shaft. It is in the motor specifications, the term might be radial load.
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NEMA 14 tells about the outer physics of the motor but tells nothing about torque, current etc.
Search the internet for timing belts and pulleys. There's plenty out there. If You have the courage to buy from Ebay, they have pullies.
NOWHERE in your calculation to you account for using a lead screw for movement. If it is of reasonably good quality, the TORQUE required to turn the screw will be so close to zero that ANY stepper motor can turn it.
I got it. Thanks for the documentation.
You can always trade speed versus power. Small pulleys gives more power but move more slowly. Larger pullies moves faster but has less force.
Those steppers are not very powerful running at 0.6 resp. 0.8 Amps.
What is that platform supposed to carry? You have not told the whole story yet. Is it a mini mill, a plotter or what?
Heavy objects have a larger inertia.
The steppers on my Anet A8 3D printer all have a 5mm output shaft and the 'Lead Screws' are 8mm, a gear with 5mm hole half way through and 8mm from the other end. On the numerous occaisions I have had to replace the rods, bearings & lead screws even the cheapest chinese traders have offered a choice of 5mm, 8mm or 'half n half'