Movable table vs gantry

[aggie_67]

It seems like most of the routers are movable gantry's. Do they have advantages over movable tables. Gut feel seems like a movable table would give better tolerances as the gantry could then be anchored very securely!! Understand on a big unit the movable gantry could have less mass to move, but on a small unit (24"x18") it seems a larger stepper would compensate for the table mass?

[One of Many]

Originally Posted by aggie_67 It seems like most of the routers are movable gantry's. Do they have advantages over movable tables. Gut feel seems like a movable table would give better tolerances as the gantry could then be anchored very securely!! Understand on a big unit the movable gantry could have less mass to move, but on a small unit (24"x18") it seems a larger stepper would compensate for the table mass?

The power and rigidity expected at the spindle might be one deciding factor. The other would be foot print. A sliding gantry does not require as much area.

A sliding table should impose less flex problems than moving a gantry. The structure and sliding components for a moving gantry seem like they should need to be larger too. We have 3 industrial routers at work. 2 are very stout stationary gantry of 5'x10' and 5'x12' and 1 is a moving gantry of 4'x8'. The moving gantry is far out classed by the other 2, it doesn't even compete.

I have some of the same decisions to make on my up coming project. I am leaning toward a stationary gantry, moving table with about the same 24"x18" envelope. Then again I could retrofit one of my lathes to CNC.

Now that I have the motors, drivers and slide assemblies, I'm just not sure what I should use it for.

DC

[ger21]

Originally Posted by One of Many We have 3 industrial routers at work. 2 are very stout stationary gantry of 5'x10' and 5'x12' and 1 is a moving gantry of 4'x8'. The moving gantry is far out classed by the other 2, it doesn't even compete.

I bet the 2 big ones were a LOT more money, too.

[One of Many]

Oh so true, but amazingly enough, the smaller one was sold to us under the same specification orders as far as accuracy, materials, thickness and cutter capacity.

Moral of the story......never trust a salesman or an OEM that won't back up what they sell once it hits the floor!

Lesson learned: Buying the cheapest thing to fit the spec hardly ever pays off.

DC

[wizard]

For any amount of money I would belive that the sliding table gantry machine would be a better performer tolerance wise. It is very easy to think of a slideing table machine as a planner mill of very light duty. I'm not sure what a moving ganty machine would compare to.

A couple of things come to mind with the moving table machines that should be worth noting. #1 in my mind is that once square the machine should stay that way for a very long time. Unless a moving ganty machine is driven on both sides of the gantry that will never happen. The resultant moving table machine should end up stiffer than the the movign gantry machine.

The biggest disadvantage with the moving table machines are the size as has already been pointed out and then the variable amount of mass that one axis has to move. Niether of these is extremely serious and a moving table machine actually has na advantage in that it is possible to clear the gantry for load and unload of the machine.

For the most part a moving table machine will win on almost all accounts except for size. There are ways to deal with that too, such as moving the table across the short axis.

Thanks
Dave

[snooper]

So, how about someone lists the advantages and disadvantages of both moving table and moving gantry designs, and possibly how to combat their respective drawbacks (maybe this could be added to the FAQ section)? I'm in the planning stages of a machine that will see overhead mounting of x/y/z above the table, and would like to know how to make it work properly (my space is at a premium...). Also, is it a bad idea to have both the table and gantry moving? Just thought I'd ask...

[jestah]

Hi

What about having say the z and the y (longest) axis as a moving gantry and then move the table along the x. Surely this is a good way to be able to FIRMLY anchor the gantry and have it run along FIXED, Square and true slides with a counter weighted Z carriage and then slide the table under this.

This would result in a footprint only twice the size of the shortest horizontal axis and not much larger than its y axis?

Just an idea from a newbie...
Jestah

[lpnmb]

Hi guys,

This is my first post to this awesome site. This thread has got me very interested as I have been thinking long and hard for a while now about a system as described above - z,y on the gantry and a x axis table.....seems to me the best solution for a number of reasons.

Just to give you an overview of my what mine is shaping up to be, design wise:

- I want a mill that is able to mill up to a absolute minimum, mild steel.
- needs 800x600x200 (xyz resp.) travel
- must be very adjustable that it does not require too much precision machining of parts.
- have a adjustable toothed belt/spindle system that included some sort of angle sensing speed control for tapping of threads. not going to worry about this for the moment.
- Also aiming for .0005" accuracy/repeatability

High expectations but with a solid enuff construction should be attainable or bettered

The stand and table base and gantry sides will all be made out of steel - bolted and welded. Where weld distortion might cause misalignment I will endeavour to design in some adjustability in those planes through bolting.

The top of the gantry will be aluminium section for ease of milling and to aid in weight reduction, of course the y and z carriages will be high grade Al and precision milled.

I will try to keep the overall weight down but it is not a deciding factor in the design.

The table is still under consideration design-wise as it is going to be difficult and costly to keep the weight down AND achieve rigidity and accuracy of such a large area. will probably go with Al extrusion and a big precisely machined slab of cast iron (going to be too heavy maybe but if I have to I will) or tooling plate grade Al....cost does come into it little here – we’ll see how it goes…..I’ll have to do some basic calculations for steppers etc. I'll use recycled materials as much as I can.

I will start soon the process of designing it all up within ACAD.

needless to say it is a big project with some big costs in the ballscrew and linear guide departments. however the end product will be extremely versatile and hopefully give me some income earning potential

As far as this thread goes for the pros and cons of my design:

Pros: rigidity and ease of attaining it (design wise), use of cheaper materials for more of the machine (steel) and therefore better for things like high load milling, tapping etc.

the other thing about a moving gantry is that with all that very large centre of mass so far above the rails it will generate more flex and wear at the guides. However, I did think of maybe having the rails about 100mm above the table but then you have the problem of where to put the ball screw (I would like it between the two rail and not on one side for obvious reasons) and to have two makes things unnecessarily costly and complex.

Cons: for my requirements I can only think of a couple at the moment, maybe if I need to mill something really heavy like an engine block then maybe the inertia will be a problem. but I figure that maybe the speed sacrifice at least in that axis is worth it. however I don’t plan on milling anything more than a Motorcycle head at this stage but it would be nice to know that I could.

far out that was a bit longer than I would have hoped. Anyway very nice to be hear and I hope we can help each other out – but I think I will learn a hell of a lot more than I will give.

cheers!

[lpnmb]

Oh yeah and I think the idea (Jestah) of maybe moving the table over the shortest axis might prove better.....maybe move the table on the Z axis ;-)

[snooper]

lpnmb,

Sounds like the size I'm planning, only mine will be a steel construction... Afraid I've no drawings yet, still have to master a CAD program!

[lpnmb]

me too.....but in the middle of a course....

[LeeWay]

I think typically a moving table design only moves in one direction. At least this would be the simplest way to make it. When designed right, these machines could actually hold and mill much longer sheets than what's actually on the machine. Using some infeed outfeed ball rollers could accomplish this. You would need some registration cuts, but not too hard to figure out.
Long signs could be done on a router that is say 4' by 4'. Granted with a moving table, the foot print for a machine this size would still be 4' by 8' at least, but the table and material would be the only weight the motors would have. Could be considerable if you are milling lots of MDF.
My gantry is pretty heavy for as small as it is. The Z and Y axis can go very fast. All the mass of the Gantry has to be moved by the X and this is what really slows my machine down.
I am using one centered ball screw now, but I'm considering using dual rack and pinion to increase both speed and accuracy. Since its a single ball screw, I really have to load the side bearings on the gantry to keep it running square. That much load keeps the gantry speed down to about 35 IPM's. Certainly no speed demon.

I am building a mill rather than convert an X2. It of course will have a table that moves in both directions. I was really considering a fixed type gantry or bridge over this table. My router mills aluminum well, because of it's design. The gantry robably weighs about 150 pounds at least. No flexing at all of the Z or the gantry.
A bridge would reduce the size of material I could cut, but the design would certainly be more stable than just a one arm supported mill head.
My parts are rather small. Still on the design stage of this one.