The flattest table ever?

[Shayne Jenkins]

Looking for recommendations.

I have a very large shopbot with a vacuum setup that I need to be as close to flat as is possible. The cutting surface is about 110" x 300". On the frame is a layer of plywood biscuitted together. On top of that is 1" plastic(HDPE I think) with grids cut into it for the vacuum. Next is the 3/4" Trupan spoilboard. Pretty standard as far as I know. Ever since day one I have had issues with flatness. Usually starts off pretty flat after surfacing and gradually degrades. Sometimes slower and sometimes faster. The nature of our material(Plasdeck.com look us up) requires it to be cut at a consistent depth(within 0.005" would be nice) for fabrication purposes. It has come time to replace the Trupan and we are looking to rehab our setup. We will be checking for gaps between the layers and debris from screws in the plastic. Just about anything we can think of. Any thoughts on things to look for here would be appreciated.

Now, all this being said, I am open to rethinking this design even if it seems cost prohibitive. Are there any resources to look into or any professional I could hire to accomplish this?

[knight_toolworks]

mdf does move no matter what. but I find it moves less if you glue it to the vac table. it tends to stay flatter. keeping it thicker would help it would mean replacing it more often but it tends not to move as much if it is thicker.

[coryatjohn]

I seriously doubt there's much you can do to change the basic nature of the machine. The large area and the thin base would work together to essentially make a random surface. Fire up the vacuum and the forces imparted to the frame and base are uneven. It is impossible to surface the spoilboard under load conditions since you need it to be open and the vacuum off to surface it. A good test would be to surface the spoilboard, then place a sheet of plastic over it, turn on the vacuum and then check the tolerance of the table. I'd bet you'll be surprised at how much variation you'll find.

Then there's the temperature issue. How much variation is the temperature of the machine from the start of the day to steady production? Unless you have quite good climate control and can keep everything at the same temperature all day, you're going to see significant fluctuations in the tolerances of everything. The spoilboard is probably irrelevant or close to it. Trupan is pretty weak and flexible.

[bobmoore]

I don't think .005" is possible but the plywood and hdpe are not doing you any favors. I would get rid of them and convert to 1 or 1.5 inch aluminum or at least something more stable and stiffer.

[Burkhardt]

You could try building a torsion box table, that means a sandwich of 2 rigid sheets (like quality plywood) spaced apart a few inches with e.g. a grid of slats. That would be much more rigid than a single sheet, even one made from aluminum. IIRC, the rigidity of a sandwich construction increases with the square of the sheet distance. Obviously that assumes you have some reserve in the z-axis because the table would be much thicker.
You might be able to use the hollow table as vacuum plenum if the slat grid is non-contiguous and the top sheet is drilled in many locations.
One problem with this concept is that the large surface would need several 8x4' panels that must overlap somehow to avoid weak joint lines (maybe glueing 1 sheet from 2 thin plywood layers).
I agree that the plywood and hdpe combination is a bad thing for warping. Thermal expansion of the plastic is about 10 times that of plywood and with the large surface you will easily get a bow.

But I am not sure if you can get such a large table to be long-term stable at all unless it has a distributed rigid connection to the concrete floor.

[Shayne Jenkins]

Has anyone tried an extruded aluminum base? I have seen them used on other CNC systems.
This (http://www.techno-isel.com/LMC/Extrusions/)seems like something that could work well. No idea the cost tho.

[EricSchimel]

Check this thread:

http://www.talkshopbot.com/forum/showthread.php?24215-Let-s-Talk-About-Table-Flatness