--- 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?
