Re: Machine construction.

Hi Builder,
For my use of this design I have obtained two racks that will be
driven from a common axle to the pinions. since the racks will be
high, the axle connecting the pinions will run parallel and next to
the gantry, with the drive motor mounted in the middle.
In Your example using a screw, having the screw in the middle
would further impair access to the work area, and, still allow
skewing as forces are applied when positions approach the outer
limits. But if you run the screw on one side, their will be even
more "skewing" on the opposite side of the gantry. A compromise must
be made. Increasing the strength of the screw side by widening the
rail bearing mounts and beefing up the gantry's structure would
reduce this "skewing" and allow more forces when working at the far
side. But doing it this way you lose travel distance because of the
wider glide design. This would then require longer rails, screw,
frame, etc. to achieve the same desired travel distance.

Bloy

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "builder4wd"
<builder4wd@y...> wrote:

>
> How are these 'portal' mills driven? By two screws or one? I am
> planning on a similar design, but I only have one ballscrew for
> the "wall" axis, so I'm wondering how robust this configuration

is.

> Using linear rails, will there be a problem with the axis getting
> skewed under cutting loads?
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "John Heritage"
> <john.heritage@v...> wrote:
> > > One way to maintain accuracy using a gantry might be
> > > to raise the table/frame sides high and securely (since you are
> > > talking 12" z-axis movement, and glide the basic horizontal
> gantry
> > > over these so the gantry's "arms" are as short as possible.
> >
> > This is in fact the way almost all new gantry machines are

built.

> They are
> > often referred to as 'portal' mills, as the structure ressembles
> and
> > archway, which is also termed, a portal. The machine is
> essentially sitting
> > on top of a pair of walls.
> >
> > With big machines, you can literally work around in the work

area.

> But I
> > thought that things might get a bit squashed if you want to

scale

> it down to
> > a desktop sized area, as the walls would block your access to

the

> table
> > quite a lot.
> >
> > I have seen numerous portal mills specified for a positional
> accuracy of
> > 0.001mm. The design offers excellent access to the work piece

for

> the work
> > it's self and can be built to be just as strong, if not more so,
> than a
> > standard VMC - I have seen two manufacturers offering the design
> right up to
> > the point of machining solid titanium.
> >
> > Best wishes,
> > John