Table for Xzero machine - your thoughts & ideas

[igor]

All points are valid. One thing to remember is that if the z axis is flexing or vibrating then there is a issue with that axis, either loose rails and bearings or more likely in this case, the cutting loads are too high for the rigidity of the machine. The Xzero machines are very good sturdy machines, but like all machines they have structural limitations, push them too hard and something will give, resulting in flex and vibration that translated into the workpiece. Z axis flex will not be negatively affected affected by a rigid table, the entire system benifits

[BanduraMaker]

I think your mixing systems....there is Rigidity and Vibration.

What you're missing is that you have to look at the whole system from tool tip to earth. Everything is related.

Let me use another analogy. Take a steel bar that's 18" long and grab it firmly in your hand 6" from the bottom. Using a mallet, tap the bar to make it ring - it will vibrate for a very short time because the vibrational impedance of your hand is much smaller than that of the bar so the vibrations are transferred to your hand which has high internal damping.

Now embed those 6" into a big block of concrete and give the bar a tap. You'll see that not only has the resonant frequency changed, it will ring for a very long time. This is because the impedance of the concrete block is much higher than that of the bar so the vibrations will tend to "stay" in the bar and dissipate only due to the damping of the bar itself. This in spite of the fact that concrete has a very high coefficient of internal damping.

The rigidity of the machine is fixed unless you want to start replacing structure with e.g. steel. The key is to match the table to the rest of the machine.

If you look at something like a bridgeport or Haas or whatever, everything on the thing is more massive and more rigid. What works in one case won't necessarily be best for the other.

If you rigidly fix a machine like the Xzero to a very rigid and very massive table, any vibrations in the machine will tend to not transfer to the table due to the impedance mismatch and will dissipate entirely in the machine. A table with similar or less mass and rigidity will tend to absorb and dissipate vibrations from the machine. I personally like the idea of George's table with legs filled with sand. Sand has very high internal damping.

Make sense?

[theremin]

I tend to agree with Andy on this one. I'm primarily a banjo maker, and there are similarities between the physics of the banjo and a machine. OK, physics and banjo aren't usually used in the same sentence but bear with me.

The banjo head is excited vertically by strings moving the bridge up and down on top of the head. On old banjos, where only a thin wooden rim was used, much of the head's energy is lost to the rim since it's mass isn't all that much more than the mass of the head/bridge. As a result, the volume is lower and the sound decays quickly. On more modern banjos, the rim is frequently capped with a heavy piece of metal known as a tone ring. On these banjos, the mass of the tone ring is high enough that very little energy is absorbed by the ring, and most of it is reflected back to the head resulting in a louder sound and a longer sustain.

The bridge and head is relatively light weight, as is an aluminum CNC machine as compared to a big cast iron mill. The vertical motion of the bridge is about the same as Z axis travel. When a very stiff, very heavy base (the tone ring) is used, the vibrations of the head/bridge are reflected back onto the head resulting while a more flexible, lighter base (the old thin wood rim) absorbs much of the vibration and the sound dies off quicker. It makes sense to me that Z axis motion and vibration would act in about the same way--heavier/stiffer base--longer lasting vibration; more flexible base--shorter lasting vibration.

Like Andy says, it's matching these impedances that's important. A heavier machine would be better served by a heavier base while a lighter machine might do better with a lighter one.

My base weighs pretty close to what the XZero weighs--a couple of hundred pounds. It's a combination of wood and steel and seems to work out well. I have no doubt that the XZero base would work out well too, and there is probably a sweet spot where the exact right amount of sand or whatever has been added and where vibration on the machine is at a minimum.

Or I could be completely wrong--but I think in banjo terms, so what can you expect?!!

Dave

[BanduraMaker]

OK, physics and banjo aren't usually used in the same sentence but bear with me.\

Oh, here we go.....

[theremin]

Oh, here we go.....

Yeah usually banjo physics discussions start with "if a banjo and a cannonball are thrown off a building at the same time, which one...."

[gio666]

Legs for stand

[gio666]

Stand corners

[gio666]

Middle extension

[gio666]

Stand corners

[azam1959]

Igor,
Thanks. Need some additional information for proper setup.

I have left over granite tiles occupying a garage corner for 15+ years. May be this is good use of these tiles. I can use them to increase mass at bottom. Do you know how much weight structure members can carry? Will 50 lb/running ft be too much on bottom corner beams? I think column should be fine. How about screws connecting the beams to columns, can they hold to this much weight? I think each column and bean joint in the bottom will have additional 200+ lbs. Do screw hold this much additional weight?

I will have 55 inch span on table. I guess I will need to use hardwood instead of plywood, right?

I thought I have enough weight capacity on caster, now I will have to double check this.

Thanks

I am the one who designed and build the tables for George. The goal was to design a modular table that could be shipped easily, assemble easily and be rigid. They key to machine performance is rigidity, and like any structure, the foundation is key...and in the case of the router, the stand is the foundation.
Vibration is dampened by mass, and this stand does not have a lot of mass....mass is expensive to ship. To address this, I add would add a plywood shelf on the bottom, and place sand bags on it. This will dampen a lot of vibration and sound. Thicker wall tubing for the legs and table sections will reduce high frequency vibrations as well as increase the overall stiffness of the stand. If one wants to make the stand absolutely dead as far as vibration one can fill the legs with concrete. My idea was not to place a table top onto the stand but bolt the router directly to the stand via the slotted inside corner brackets. This aids in the structural integrity of the assembly, and also allows access to the underside of the router for service.
We welcome your feedback, and if there are ideas that would improve the products please feel free to mention them and we will see if we can address them.
Thanks

[igor]

sure the tiles will work but you may want to glue them together or do something to prevent them from rattling. The legs and bolts will take very high loads. You are right in keeping the weight close to the corners. I like plywoos for shelves like this. I use self tapping screws and secure it to the frame. I also add a bead of silicone seal or foam weather strip between the frame and plywood to quiet it all down.....loose planks will vibrate and drive you nuts, even with weight on it. I like sand bags for weight as they are easy to add/remove and do not resonate....just my thoughts.

[azam1959]

Thanks Igor. Your message is very clear that I do not have to worry about frame strength.

I did not know that self tapping screws do exist. I can use them at lot of places. I will do some research on these screws. Learned something new today.

Thanks again.

sure the tiles will work but you may want to glue them together or do something to prevent them from rattling. The legs and bolts will take very high loads. You are right in keeping the weight close to the corners. I like plywoos for shelves like this. I use self tapping screws and secure it to the frame. I also add a bead of silicone seal or foam weather strip between the frame and plywood to quiet it all down.....loose planks will vibrate and drive you nuts, even with weight on it. I like sand bags for weight as they are easy to add/remove and do not resonate....just my thoughts.