Re: [CAD_CAM_EDM_DRO] Re: Ideas for matching drives on a YY axis design?

Will the lead error in the thread of an acme screw cause much problem
when driving with two screws on the x axis ?

dakota8833 wrote:

>
> --- In CAD_CAM_EDM_DRO@y..., "Doug Harrison" <prototype@c...> wrote:
> > I have seen quite a few gantry drives with one screw down the center
> under
> > the table. This presents two problems. First, the table must be
> supported
> > at the corners which limits stiffness. Second, the moment loads
> developed
> > when the head is at either end of the gantry cause wracking of the
> gantry
> > (and deflection of the limp table caused by the first problem).
> >
> > The usual solution to these problems seems to be overbuilding the
> table and
> > gantry. Neither work very well. A more effective solution is to
> design the
> > gantry so it can be driven from one side. This works surprisingly
> well,
> > though it requires some unconventional thinking to accept. I would
> choose
> > this approach if I were doing it again.
> >
> > Another approach is to use two screws and link them together with
> belts to
> > the motor, which will then have two pulleys. The motor is placed at
> one end
> > of the table between the screws. Any loss of accuracy from the long
> belts
> > will be insignificant compared to Abbe errors, thread drunkenness
> and
> > thermal stratification in a sub$80K machine.
> >
> > Doug
>
> Funny you should mention driving the ganty from one side ,.. I tried
> this and noticed that it was more efective to drive it down the sides
> or single down the middle. What one has to keep in mind is the lever
> action. Driving down the center halves these forces as opposed to a
> single drive at one side. Looking from under the table the structure
> would look like bowtie,.. two triangles |><|,.. great stiffness.
> Additional stiffness in the X axis run would come from rail
> "overbuild" and with the table top asmy. Another problem with single
> sided drive is with the twisting of the ganty uprights. It is doubled
> for the same size mill. Any of these problems can be overcome though
> ,..by overbuilding. I like unconventional thinking and may use the
> same concept with my next mill. I would like to hear some more about
> your idea of a single sided drive.
>
> Frank ,.... CAD CAM FEA backround
> >
> >
> >
> > ----- Original Message -----
> > From: Jim Brown <jagco1998@y...>
> > >
> > > Just curious............
> > > Why cant you just space out the linear bearings on the z/y axis'
> so it
> > will have less of a chance of binding when traversing in the x axis,
> and
> > drive it from the center with the ballscrew/leadscrew?
> > > I realize that this will be a big feat to perform, being the width
> of the
> > gantry. But It can be done, cant it?
>
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