Table for Xzero machine - your thoughts & ideas

**amasters** · 01-18-2012, 12:05 AM

I have recently received most of the parts from George for a Raptor32. The actual assembled dimensions (excluding motors) is 33 x 48 inches. So far it is a fantastic looking and rigid beast, but I have yet to actually put it under motor control.
I plan to cut woods, plastics and light metals. There will be a 2.7HP Teknomotor spindle in use as well.

I have been thinking about a table for this machine. I think a table that will match the machine's rigidity and also absorb the movement and vibration from the machine would be the way to go.

I have read most of the threads here in the Xzero section as well as many threads for similar machines. I have seen welded tables, wood tables out of 4x4's, and many others. I think heavy welded tables seem to "win", but I'm not a welder.

I am leaning towards purchasing a welded table and bolt the Raptor down to it.
Here is a link to a heavy duty table I found. 1/4" table top, 3/16" legs.
Extra Heavy Duty Work Bench | 20,000 lb Capacity | 36 X 48 Extra Heavy Duty Workbench | 579313 - GlobalIndustrial.com

I would like to hear your thoughts, opinions, or past experiences down this road.

Is the above table overkill?
Is there a better alternative you have seen or used?

Similar Threads:

**xsnrg** · 01-18-2012, 08:32 AM

George sells tables. I bought one of his tables for my Raptor 32. It should be here this week.

**barnaclebeau** · 01-18-2012, 05:57 PM

Made mine out of 2x4, 4x4, plywood, and bench bolts from Rockler. It's plenty solid.

**gio666** · 01-18-2012, 08:49 PM

**barnaclebeau** · 01-19-2012, 12:09 AM

Awesome to post that, George. How much for an 8-pack of corners?

**BanduraMaker** · 01-19-2012, 08:06 PM

Originally Posted by amasters

I think a table that will match the machine's rigidity and also absorb the movement and vibration from the machine would be the way to go.

I think that your statement here brings up a very interesting question: is table that's very rigid do a better or worse job of damping vibration from the machine.

My gut feeling is that the answer is no.

I built a wooden stand out of doug fir bolted together. I also used the leveling casters that I learned about on this site which have rubber feet that descend out the bottom when in use.

I have pretty decent accels on my machine (I think "45" and "55" using whatever the inch units are in Mach 3) and my bench shimmies and shakes when I'm running a program but the feet don't slide around on the floor. On some operations, you can see that the machine is vibrating by the machining marks on the wood - too fine to feel but visible on the material when you look at it at the correct angle. I have a program that machines a guitar fretboard which changes directions at 400ipm and you can see these lines perpendicular to the direction of the tool-path but as I said, they're so shallow that you can't even feel them and at some angles, you can't even see them.

Let's use a musical instrument to draw an analogy - when you pluck a string on an acoustic guitar, the string vibrates and volume decays quickly compared to how quickly the volume decays on an electric guitar. The difference is due to the vibrational impedance differences in the bodies of the guitars.

An acoustic guitar bridge and soundboard is slightly more massive than the string vibrating it so the energy is transferred at a relatively fast rate.

An electric guitar bridge and body are much much more massive than the string vibrating it so the energy stays in the string much longer. Consequently, the volume of an un-amplified guitar is much much quieter than that of an un-amplified guitar.

I believe that a bench with a little sway in it is going to result in less vibrational artifacts in the final part than a stiffer bench rigidly attached to the floor with a machine like the Xzero. In the interest of full disclosure, I used a wooden table not because I thought it would be better but because it was much cheaper than a steel table.

My opinion is based on watching the machine in operation on my table when the bolts came loose as the wood continued to try and shrink in my basement. With the bolts tightened down the table is quite rigid, though not as rigid as a steel table, and most of the CNC-shimmy-dance is in the rubber feet.

Remember that compared to something like a commercial CNC machine for machining steel, the Xzero is much much faster and ridiculously lighter than anything in that category.

If you do get a very stiff bench and run at high accelerations, I think you'll find the bench sliding around on the floor. If you bolt it to a concrete floor, I think you'll find that any reverberations are "staying" in the table/machine system rather than being "dissipated" someplace else. Whether that will affect your part I can't tell you but it would be very interesting to do a comparison. My thoughts are that you'd see more vibrational artifacts on an Xzero mounted to a very rigid bench than one that has a little give.

**amasters** · 01-20-2012, 01:22 AM

Andy,

You bring up very good points. I'm not sure about the "swaying" table absorbing the vibrations and a rigid one not. I think it depends on materials and natural frequencies of the table. Its either going to absorb or add to the machine's vibration. Of course the machine is not going to have a consistent vibration. It will depend on spindle speeds, movement, and the mass and material of the object being cut.

I know that many high end mills and routers use cast iron or granite as a base with the story being rigidity and vibration absorption. Both these materials have 2 things in common; high stiffness (bordering on brittle) and lots of mass.

I have decided to go with George's table (the 8 corners, legs, hardware, and feet) and I supply the main 2x2" tubes. I was thinking of using either 1/8" or 3/16" wall tubes.

I presently have the machine sitting on a folding plastic table, but I'm not running it at the moment. I fully expect it to start jumping around when I run the first program.

It would be interesting to compare the performance of the machine on George's table to one of the 2" thick wood top/metal leg tables I have.

Also, what would adding a mass under the table do to the performance? What about loading the low legs of the table with several hundred lbs of steel or a tank of water? What would adding vibration dampening mounts between the table top and machine do?

I'm probably over thinking this anyway...

When I get there, I will be happy to post the results.

**BanduraMaker** · 01-20-2012, 08:06 AM

Originally Posted by amasters

I know that many high end mills and routers use cast iron or granite as a

As I mentioned, those machines are in a totally different league than what we have.

I'm probably over thinking this anyway...

I'm sure that Georges table will work out perfectly. If it was available when I bought mine, it's probably what I'd have.

**igor** · 01-20-2012, 10:44 AM

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

**amasters** · 01-20-2012, 12:38 PM

Igor,

Thanks for the info.

That parallels my thinking too on the rigidity and then adding mass, but sand bags are an even better idea.

**BanduraMaker** · 01-20-2012, 02:29 PM

First off, I think the table will work out great but, I think you're missing the point.

The weak link (I think) is the rigidity of the Z-axis. The more mass in the table, the longer the Z-axis will reverberate on sudden accels.

Allowing for some movement in the table, the z-axis shouldn't vibrate as much or as long. When I say vibrate, I'm talking well under .001" in amplitude.

**miljnor** · 01-20-2012, 02:49 PM

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

Vibrations are taken care of by mass and can be absorbed by certain materials, whether or not this is going to show up in machining really depends on what your doing and how.

Rigidity on the other hand effects how the machine flexes or bend to cutting forces and this is where the table is more important. If the table can move via rubber pads through any means (ie vibration or whatnot) then the cutter becomes not true vertical causing one edge to dig the part more than the other.

Both of these effect one another to a degree but I see rigidity to be the more important factor here just simply because most of the hobby machines lack it. But then again rigidity is more effect by machine design than that of the table.

feel free to ignore my armchair musings!

**igor** · 01-20-2012, 03:24 PM

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** · 01-20-2012, 03:25 PM

Originally Posted by miljnor

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** · 01-20-2012, 03:44 PM

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