I agree with Martin. I'm using 1/2 inch Acme threaded rod on my JoeCNC 4 x 4 Hybrid mill that is 60+ inches long. I ran into whipping problems at speeds over 240 ipm. I modified the end supports to use 2 bearings to create a fixed end condition and now I can traverse at the maximum speed my stepper motors will drive the machine (over 300 ipm).
This graph shows the speed at which whipping starts (1st critical speed) for a 1/2" solid steel shaft of various lengths. This is similar to the ACME screws on Joe's design. The rpm's you can run at drastically reduce with shaft length. The black line on the chart is for a shaft that is simply supported like Joe's. The blue line is for a shaft that has fixed supports on the end. When you add fixed support the rpm's required to reach the 1st critical speed are greatly increased. At a length of 56 inches, the critical speed for the fixed support shaft is more than double that for the simply supported shaft.
I modified the screw supports on my Joe's by adding an additional flange bearing on one end and a Dumpster coupling between the motor shaft and the ACME screw on the other end. This modification makes the ACME screws behave like the blue line in the graph. Here is a movie showing the screw at high speed before and after modifying the end support:
Here is a picture of the dual flange bearing support at the end of my y axis.
