[sundewzer]

That link is for calculating the force produced by an ACME screw. The OP has ballscrews which are much more efficient. Here is a calculator for ballscrews. For efficiency, a ballscrew is about 90% efficient.

My G0704 has the 570 oz-in steppers on all axes and has proven sufficient in X and Y but I would like a bit more on Z due to drilling forces. The ballscrew calculator says a 570 oz-in motor on a 0.196" (5mm) lead screw at 90% efficiency will give 1027 pounds of linear force. Using your 180 pound cutting force requirement there is a healthy margin. However, for a stepper design, the desired rapid speed may be the driver for motor sizing. This is because stepper torque drops off with increasing RPM and there will be a speed where the stepper will stall. For the 570 oz-in motors I can rapid reliably at about 150 IPM with double spring preload ball nuts. With single ball nuts that speed is closer to 200 IPM

You are correct on the calculator, I should have spend the extra couple minutes to find for specific for ball screws. I just changed the coefficient of friction 0.01.

Going from a single ball nut to a double you have to decrease your rapid speed by 25%? Is that typical?

Here are the ball screws I just received

Getting exciting now! I would get some steel shim stock to put between the nuts but I guess the aluminum foil would work in a pinch. I have also read about people adding spring washers between them but you have to make sure your preload is greater than your cutting force.

Have you decided on the type of motors yet? I know I will never buy anything with just steppers again. I have had lots of fun with my shapeoko, including cutting the base of the 30MV https://photos.app.goo.gl/RBULexURfKht1DuTA, but I lost to many parts due to a missed step. If you can afford it I would go with some sort of closed loop system.