To move the slide 18" in 5 seconds is equivelant to moving the slide 216" (18" x (60/5)) in 1 minute.
if your air ratched operated at a speed of 100 revolutions per minute the pitch of the thread on your threaded rod would want to be 2.16" that is 216" divided by 100.
You will need to know the exact revolutions that your air ratched gun will run at, the dificulty will be to get an accurate rpm on the gun as it will be determined by how difficult it is to rotate your screw, the more work the gun has to do the slower it will be, so the greater the pitch on your screw will want to be to make up for the drop in speed, this in turn could make it harder for the gun to turn your screw
I would probably go for a ball screw as it has ball bearings in the nut that run in a grouve in the screw, this makes it easier to turn. Have a look at the following http://uk.misumi-ec.com/eu/ItemDetail/10300079310.html or http://uk.misumi-ec.com/eu/ItemDetail/10300081930.html for ball screws or lead screws that might work for you.
I didn't know if this would be a good place to pose a question of this type, but it never hurts to ask. I appreciate the response.
After doing some research on 1/2" impacts, I found clock speeds ranging from 1,000 to 10,000 RPM. I have also done some research on ball screws which is uncharted water for me. Thanks again!