Re: modal frequency, 12' 1/2-13 screw and buckling

Hi Elliot,

This is on topic as it pertains directly to building and operating a CNC machine.

At this time, the table requires less than 3 pounds of force to move. very light. But the unforseen
chip or tool bit or misplaces pencil will drive those forces much higher.

As you mentioned, this set-up cannot be used for pushing on the leadscrew.

Boy am I glad it's a prototype, and I left room for a 1 inch leadscrrew.

(and I didn't waste a 10 ft length of ballscrew).

Dave

--- In CAD_CAM_EDM_DRO@y..., "Elliot Burke" <elliot@h...> wrote:
> I forgot to mention buckling. The screw will stay straight when the nut is
> pulling it straight, but when putting the screw under compression it can
> buckle. A long skinny column is subject to Euler buckling, this is pretty
> straight forward stuff. for example see
> http://physics.uwstout.edu/StatStr/Columns/colse61a.htm
>
> with steel, 12'long .4" diameter, the buckling stress is 17 lb. If the nut
> was loaded with over 17 lb while it was compressing the screw at full
> extension, the screw would buckle. Bad, very bad. Control
> acceleration/decceleration!
>
> Further, if unsupported over 12', the .5" midspan deflection of the screw
> would reduce the buckling stress much lower. Now it doesn't act like a
> column, but rather like a very skinny piece of bent wire.
>
> I'm starting to think that it may be necessary to tension the screw to give
> it any axial strength at all.
>
> Am not yet able to calculate load stiffening (tension) effects with FEA, but
> hopefully in a couple of days problems will be cleared up.
>
> If this is off topic, please let me know.
>
> Elliot