Re: [CAD_CAM_EDM_DRO] Sizing a Ballscrew ( was Re: Price of the ballscrew??

turbulatordude wrote:

>I am looking at the size ball screw that may be needed for a 8 ft x 4
>ft table. seems that if routing sheet aluminum and oak were the
>expected materials that a pretty hefty screw is needed.
>
>I found the beam deflection file, is there a screw-sizing
>calculation ? after it gets so long it will sag under it's own
>weight. and if it is too small it will bend when pushed.
>
>

Sag is not the big problem. The first thing you run into is first
critical speed, or
"whip". It's not just a simple beam, it is rotating. General practice
is not to
use very long rotating screws. On screws much over 3 feet length, the
whipping
effect becomes so severe that you are forced to go to non-rotating
screws, and
use rotating nuts. These are a real pain, but on an 8' screw, you are
not going to have much
choice. 2" screws would most likely have to be used to get any
reasonable speed,
unless you used a VERY coarse thread, like 1 TPI. Almost everybody with
6 to 8 foot
routers uses rack and pinion or belt drive due to this problem. Note
that wood
dust is very hard on ballscrews, and there is hardly any practical screw
wiper
to keep the dust out.

Jon