Trade offs between Moving Gantry and Moving Table

[gregmary]

I am looking to get some input on the trade offs between selecting a moving gantry or a moving table for the X-direction.

If I am targeting about a 3 foot X direction, which seems to be the consensus for an open CNC design. the moving gantry would result in a smaller machine. The lenght of the machine would roughly be the length of the gantry travel plus the width of the gantry. A moving table would be roughly twice the length of the table ( 6 feet) as both ends of the table need to be positioned underneath the table.

The other considerations are the stability of the moving componenet. The moving table seems to be the best from a predicatable platform as it would be easiser to create a stable platform for several reasons: 1.) the rollers would be farther apart, say at the four corners of the moving table. This would result in a less movement at the router head given a fixed movement at a bearing on track movement. 2.) The primary force of the router would be down and be the same direction as gravity driving the table onto its track instead of lifting it off the track as in a moving gantry version.

The moving gantry version has the advantage of being smaller and thus costs less as the cost is directly proportional to the size and amount of materials used. The question is can the performance disadvantages be minimized or eliminated to the point to make this the best choice.

Any thoughts?

Greg

[CNCadmin]

Space is a issue for a movable table for me.

[Al_The_Man]

One of the down side to a moving table would be that if work of appreciable weight is carried then the motor torque may have to be increased accordingly. i.e. higher mass to move.
Al

[bradodarb]

If were going to build a moving table system(and I am), I would move the table on the Y to minimize footprint.

[microcarve]

The main thing a moving table design has against it is
the extra space it requires for the same cutting area.

Other than that, it's a stronger and easier to build
machine. As well as maintain.


Picture #1...

There are few, if any, moving gantry machines that'll be stronger
than that. There will be no twisting/skewing/racking of a moving
gantry...because it doesn't move.

The table rides on 4 widely spaced bearings, so it won't rack
either.

The nut can be supported to prevent whip at each end of the
table.

A finer pitched screw choice will give it all the mechanical
advantage it can use, though only slightly slower cutting
speeds, but still well within a range it can cut in any
practical manner anyway.

Well, that machine's a big industrial beast....


Picture #2...

Looking at that machine and asking why it's made like it is
points to a need for extreme rigidity. It must do some very
highly precise work to need that kind of strength.

It's pretty easy to copy the basic design of it for hobby
purposes and still get many of the advantages such a machine
can give.

Mine is just a big MDF version. The table is a separate element
in the design that can very easily be slightly adjusted to as
near perfect square with the gantry as possible.

The bridge can be built in such a way as to make raising or
lowering it a simple thing to do for doing something like
computer case or large electrical box work where project height
can matter. Just a few bolts will easily lower or raise the bridge.


Picture #3...

A prototype that proves it works Great. It's a lot more rigid
than my larger moving gantry machine. Easier to maintain...A
moving gantry can be a PITA to get square and adjusted to
prevent possiblity of skewing. This isn't. It's cheap & easy...


But it really all depends #1 and Foremost on what you need from
the machine. It can be useful to think of these type machines
as "special purpose" machines. There's a "right design" for any
particular type of project.

Mine will cut about 24 x 15 x whatever depth I want to adjust
the bridge to....so it's *Ideally* suited for something like
a guitar body....where things need to proceed as well as possible
on an expensive chunk of Nice wood....
(Do Not want any chance of things going wrong halfway through
a job like that! Things going wrong are minimal with a simpler
design)

Sure it takes a bit more space for the moving table/fixed bridge
design, but again, it depends highly on what the machine needs
to do. Large signs may very well need a moving gantry...just for
the large-ness of the project alone.

I don't build a machine any larger than it needs to be for any
type of work it'll do. The smaller the machine can be, the easier
it can be to build and maintain. The simpler it is, the less
maintaining needs to be done...

My $0.02


John

[bradodarb]

Well said, a static gantry machine is much more rigid pound for pound.

Nice machine too.

[Pplug]

Originally Posted by bradodarb Well said, a static gantry machine is much more rigid pound for pound. Nice machine too.

I love my moving table router! It has very little noise and resonances from it's shear mass. The base is made from 5" of granite and the gantry is made from giant aluminum castings and a dense ceramic. The new gecko 540 and ballscrews I installed have reduced noise and increased the speed. I'll eventually replace the router with a spindle but as it is, it still cuts faster and smoother in 3d than my commercial Techno lc4896 and for about 1/5 the price!

If you have the room, do it!

[PEU]

if you use rails a moving table design will need twice as long rails compared to moving gantry.


My 1st router is moving table, my second will be moving gantry, both use linear guides.

[Harpye]

The Break even point between Moving Gnatry and moving plate depends on your machine structure:

For
a specified Z axis ( upper point for max distance to x rail)
a given Cutting engagement force ( + reactio on Spindle)
a given Speed ( acceleration of mass = Force)

you need a certain stiffness which in one point belongs to the depth of the gantry support

if the required x range would be much more than the effective gantry depth then a moving gatry is the better way.
if x range and gantry depth would be equal and you can effort the space it is a cost thing to go for moving plate
if there would be shorter travel than gantry depth I would go for moving plate in case of cost and stability

but finally this is my opinion and you'll have to make your decision yourself!

[cyclestart]

The common concensus is a moving table needs wide bearing block spacing along the rails while moving gantrys get away with narrower spacing. I'm experimenting with a different design with stationary bearing blocks. From a torsional point of view it has advantages over a moving gantry. It does have a problem with the table tilting fore/aft due to the central placing of the bearing blocks. The plan is to use roller bearings to support the table. The bearings will be mounted to square tubing which will also be part of the base structure and give additional support to the y axis (bridge).

The rails are 48", the screw is 38". The square tubing will be much longer, the current pieces are what was handy.

It's an experiment. I'll play some more before wasting any metal on it. Hopefully the pictures add some clarity to my word salad

[Harpye]

Cyclestart - I wouldn't 100% agree with you :

On a compact built with fixed and loose bearing thread support you would have to calculate :

Moving plate
over all thread length is ( minimum)

_effective x movement
+nut length
+engagement length for emergency spindle end switches
+loose side spindle end length
+fixed side spindle end length
_________________________________

-> this means that the loose side support has to be lower than the plate so the plate can run over it so you save a little bit less than half of the thread length


Moving gantry
over all thread length is ( minimum)

_effective x movement
+gantry depth or nut lenght (the longer dimension of both counts)
+engagement length for emergency spindle end switches
+loose side spindle end length
+fixed side spindle end length
_________________________________

[cyclestart]

I was working from memory, never a good idea
Some tape measure numbers +/- the fractions:

The effective travel of the screw is 31", the rails are 47".
The cutting length on the x is equal to the screw travel. The 6 rail blocks could be placed further apart but the x travel would be reduced. A longer screw could be used but the 6 rail blocks would need to be placed closer together. It's a very flexible design in that way. Whether it's a good design is another question altogether. It's fun and functional. Pretty easy to slap together. If it's eventually scrapped I get to keep all the pieces.