CNC router Z axis travel

Andrew wrote:

>There are lots of routers around- my beef is that they don't have
>much room in the z-axis. It would be nice to see a machine that
>would be able to carve either a large panel (say 4' x 8') or, using
>a "rotisserie" 4th axis, to cut a big fully-round part (like a
>lifesized figure). The quill wouldn't have to make the adjustment if
>there were a way to either raise the bridge or drop out the table
>(the latter is probably easier.). With a setup like that, about 14"
>of Z travel would probably suffice, although more is always better.
>Sound feasible?]

Andrew,

Sure its feasible, IF the price is right...

I am glad you said bridge, rather than gantry as it is better to have
a large z on a bridge design. On a gantry, it becomes unstable (at a
given price) VERY quickly. Unfortunately, the bridge design costs
much more to build and has a MUCH larger footprint.

Assuming a 4' x 8' x 14" router:

Gantry design will be about 5'x10' footprint and have about 34' of
linear bearings. These will need to be highly rated, to deal with the
moment loads of the large Z axis.

Bridge design will be about 6'x14' footprint (minimum, I'm assuming
6' between table support bearings, which will require a STIFF table)
and will need 42-46' of linear bearings. The load rating for these
bearings will be less than required for the gantry design, but the
structure of the bridge will be larger and more expensive.

For the bridge design to work on BOTH flat and "rotisserie" work will
add even more problems. The table is a moving and necessary part of
the structure (as a support for the axis bearings), so it would need
to be built as a frame with removeable top surface. And the
ballscrew, if used (could be rack/pinion) would need to be shifted to
one side to make way for the rotating work. (this would mean the
linear bearings need to be increased to handle the uneveness of the
axis drive). If done this way the 14" travel of the Z could be used
for both flat and round work.

If the table is kept solid, then the z travel needs to be increased ,
as will the clearance between the table and the bridge.(to make room
for the round work) This will also require 'beefier' structure and
bearings(and more lineal bearing footage).

The third way is to 'raise the bridge', which is commonly done on
small engraving systems. On a 4x8 router, it is more than a trivial
task! (at least if accuracy is to be maintained)

Now back to the gantry:

The table can be removed or lowered to accomodate 'rotisserie' work,
and the structure for the rotary axis is probably in place at each
end. But remember, the moment loads of the Z AND Y axes are carried
by the X...

Now to the REAL problem...

Regardless of the bridge or gantry design, you will face two
limitations: Tooling length and structure interference! (Z axis
support and spindle structure).

Assuming a 'pointed' tool extending from a spindle collet, you will
develop a "cone of interference", based on the length of the tool and
the diameter of the spindle/supporting structure. ANYTHING you intend
to carve will HAVE to account for this, and thois will generally
present much more of a problem than the length of the Z travel!!!

Commercial 3 axis carving machines used in the wood industry do not
generally have large Z travels for precisely this reason.

Carving of aerospace components is often done by 5 axis machines
where the cutter is 'oriented' to "get the machine out of the way".
This is(was, now CAM programs can model the machine and detect and
account for the interferences) often a LARGE part of the programming
time involved.

So, to sum up, it CAN be done, I'd be glad to consult with you on
this (contact me off list). BUT..., There's a lot more involved
than 'just' increasing the Z axis travel... And, It's a lot easier to
design a good 'in the round' OR 'flat' machine than to try to do both
in one design.

Hope this helps.

Ballendo