racking was Re: Dual screws for large router tables

Chris,

It all comes back to basic levers. The "spread" of the gantry vs.
the "spread" of the bearings supporting it.
Better quality bearings change the lengths and ratios, that's all.

Note that the screw itself can even be afactor since any racking will
try to twist the nut and bend the leadscrew.

Hope this helsp.

Ballendo

--- In CAD_CAM_EDM_DRO@y..., Chris L <datac@l...> wrote:
> I agree with smoke here. I get countless inquiries asking how I
keep my
> machine from "racking". If it "racks", I've never detected it. It
is only 36"
> wide with one screw down the middle.
>
> I think the real key here is to design a "box" or a "non-twistable"
upper
> spread (Y gantry), support it on substantial gantry legs, stand the
legs on a
> substantial bearing carriage and then Tie together the bottom of
the gantry
> with a rigid crossmember.
>
> My machines center lead screw "pushes" on a piece of 1/2" thick
cast aluminum
> about 4" wide. That is bolted using 4 bolts on each end, under the
bearing
> carriages. This does not create all that much mass while being
very, very
> rigid even where you can see that I "dropped" a portion of this
crossmember
> to clear the under table motor (I did not want a motor sticking out
the back
> of the machine).
>
> If you want to see how it looks, just have a look:
> http://www.data-cut.com/page5mv.html
> Rotate a few pictures down to see what I mean.
>
> The really BIG issue with lead screws is the "length" of them,
especially if
> you run a fine lead. 600ipm is about all my 1/2 diameter, .5 lead
screw can
> handle before it looks like the "wobblys" are going to rip it
apart. And, it
> is "pulled" gently with a nut and double row angular contact
bearing on each
> end! The longer you go, the more wobbly she will get unless you up
the
> diameter. Fortunately, I have plenty of room to do that in the
future if
> necessary. 600ipm is plenty fast for an old man like me.
>
> Chris L
>
> smoke2k01 wrote:
>
> > I really wonder why so many people think dual drive screws are
> > necessary for those large tables. With a properly designed system
> > only one screw would be required for any system.
> >
> > I'd put the x and Y screws right on the centers of the
machine...no
> > matter what size the table was. What I'd do to prevent twisting
> > would be to mount a set of guide bars along the sides of the frame
> > for the long axis and a set of guide bars close to the router
> > mounting for the X axis. If Z movement were required, I'd do
> > something similar for it as well.
> >
> > I'd support all the guide bars well enough so I wouldn't need to
> > worry about flex in the middle causing an unwanted change in the Z
> > depth.
> >
> > A system built that way could be built just about any size you
wanted.
> >
> > BTW The inletting/copy machine I'm building is designed that
way.
> >
> > Smoke
> >
> >
> >
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