How big is big enough?

[MunchyMonster]

Well I got all the parts for a 2'x4' workable area machine cut and fit. I ordered and installed my screws to do the linear motion. Now I've got to figure out how big do I need to go on the motors to actually make this beast move. (BTW 2'x4' cutting area is a much bigger machine than I had pictured in my head)

The long axis is my primary concern. Since my machine is a mobile gantry design, the long axis will have to move the gantry. I figure the gantry will weigh in around 60lbs with the router and motors mounted. My long axis screw is an 8 tpi 8 start ACME screw with a matching dumpster AB nut on it. I can turn it by hand, but it feels stiff to me (adding the load of the gantry doesn't seem to affect how hard it is to turn). The only point of comparison that I have is the z axis which is a 10 tpi 1 start ACME screw with a matched dumpster AB nut on it. It seems to be much easier to turn.

What should I be looking for in terms of motors? I'm looking at Probotix right now as I really don't want to mess around with the electronics. I'm going to buy a 3 axis kit and install it. I figure this is the path of least resistance here, but I don't want to buy a kit with underpowered motors.

I don't need it to scream along at 10000ipm, but I don't want it to stall the motors either. I was hoping to direct drive the drive screws from the steppers. The rational for getting the 8 TPI 8 Start ACME was that I could run at lower RPMs and take advantage of the high end of the torque curve. With 200 steps per inch without even turning on micro stepping I'm easily in the ballpark of woodworking precision.

So how big should my motors be? 166oz/in? 260oz/in? 280oz/in? 400oz/in?

[jsheerin]

The 380oz*in size would work well. Their torque curve is better than the larger 425oz*in size.