Machine Build ball screw mount question

[chevydyl]

so I am building a mill, ive been gathering parts and starting mockup using 3/4 mdf. for X I have a 24mm bosch screw and 20mm bosch rails. the x travel is about 18.5in I will be using 25mm bosch rails for Y, I want the travel to be in the range of 12-15in. I haven't decided on the screw yet but I am almost certain it will be a 32mm. I plan to use a 10x50 mill table that may change as whats available at the time its needed. I will also probably be using the Tormach 1100 mill head, undecided at this point, maybe the 770, but it would need an upgrade in horsepower.... so on to the question, what do you guys think about using a 32mm screw with a .200" lead gives a resolution of .0001" with 2000 steps, but the screw would only be supported at the drive end? the OAL of the screw would be in the neighborhood of 23-24in. I don't really see any issue with such a large diameter screw and the fact that the rapid wont be fast. the Y screw doesn't look to be supported on the far end on some or most knee mills. Thanks

I posted here as im not quite ready to do a build log yet.

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[awerby]

Most mills have a bushing of some sort at the far end of the screw that retains it without constraining it too much, just to eliminate the possibility of whipping, which can bend the screw. The driven end is usually equipped with dual angular contact bearings that hold it firmly.

[chevydyl]

I know of the ac bearings just like my other ball screw pillows. So your saying that the threaded end would be in a bushing? I'm talking about the actual threads or ball tracks of the screw would be in the bushing. Not a machined end.....

[LeeWay]

It is usually done on a machined end. It just works better that way. I have used short screws without anything though. You take into consideration the length from the nut to the end of the screw for most cuts. That considerably shortens the length you have to plan for. I doubt you will get any whipping on short screws of a relatively small mill.

[chevydyl]

i know how its usually done, i was just asking if it would be done in that manner cause the screws i was looking at are machine specific for the y axis and do not have a machined floating end.....
so back to my build, can i get away with no bearing on the far end in your guys opinion? the nut is about 4.5in long. i really dont see whipping to even be a concern given the speed of around 90ipm maybe a little more. i my self think it will be fine in my build. im not looking for super fast rapids. and with the screw being 1.5in and the bearing journal to be 25mm i think it will work...

[LeeWay]

I don't see a problem with it. I did use them on the mill that I built because they were already there when I bought the screws.
I didn't use them on the lathe I made. I turned the screw ends myself and didn't bother with the opposite end. No problems and it was a 5/8" screw. Maybe 20" long

[chevydyl]

Funny I was just looking at your lathe thread. Pretty neat! Thanks for the info. Now I just gotta figure out the pillow block...do I turn my own or buy it? Looking for excuses to buy a harig 612 surface grinder and that block plus several other items on the list are making the purchase seem justifiable lol