Not quite sure what you're asking - do you want to know how much force the motor can apply if the Z-axis pushes straight down into the material?

If so, then the answer is dependent primarily on what speed the motor is running at and clearly what motor/driver you are using. For sake of argument lets say it's a 3Nm motor on 70V, operating below the corner speed (about 880rpm = 4400mm/min), which is realistic as the Z-axis rarely goes very fast. Under those conditions the torque is a 2Nm.

Formula for driving force, F=T*2*pi*e/L where T is torque, L is pitch (m/rev) and e is efficiency (around 90% for ballscrew).

So with 2Nm on a 5mm pitch ballscrew, remembering that lead is in meters, you have; F=2*2*pi*0.9/0.005=2262N

We should factor in the weight of the Z-axis, but as you can see ballscrews/leadscrews can apply a huge amount of force so this is negligible in comparison. So in short don't stick your fingers under it!

Does that answer your question?