Re: [CAD_CAM_EDM_DRO] Re: Machine construction.

Commercially, these machines are usually driven from both walls I think.

The problem is that there is almost no way to account for skewing. The
workpiece can be off set for skew using a touch probe, but that's only
because the workpiece is fixed at that skew by it's clamping whilst it is
machined. If the machine it's self is skewing dynamically, it's virtually
impossible to predict accurately.

I read an article about portal type designs suggesting that driving the
gantry from both walls also produces better movement performance.
Apparently, by using two smaller servos either side, as opposed to one
bigger one on just one wall, the acceloration of the gantry is improved.

It all depends on what kind of accuracy your after and what size of span the
gantry has to achieve I would expect. These machines were looking towards
0.001mm, which is a bit extreme for a home enviroment. Using linear ways on
either side, and a short gantry span, you might be able to achieve something
very usable driving it from just one wall.

You could design your machine to run from both walls, but only implement one
for now. Then, if the machine is skewing too much for your liking, add the
drive to the other wall to keep it all rigid.

If you like, I can send you a nice high resolution picture of a commercial
portal machine being put together. Looking at the picture, the gantry
appears to only be driven from one wall.

Best wishes,
John.

>
>
> 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
>
>
>
>
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