Several times mention has been made of dancing machines, and the importance of firm anchoring, level-ness, etc.
Suppose a machine -- cnc or manual -- was placed on a very light but very rigid base, which was then suspended in the air by 4 or so guy wires from overhead beams. So the machine would be level and un-twisted because of this light rigid base, but it would be free to swing.
How would that affect its performance, accuracy, etc.?
I would hazard to say that there would be no ill effects at all, cuz of well, Newton'n'shit..... Heh, if there were springs in the guy wires, sure would help with building vibration, wouldnit?
But, I was wrong once before, and it could happen again.....
Would probably help with small vibrations, but without some sort of damping there might be a chance of an escalating harmonic in the system. Still probably wouldn't affect operation any more than being bolted down...until the springs or cables finally failed.
It would be a train wreck! I'm with John: YouTube for sure!
On a side note, I've seen big machines placed on thick plates of steel to somehow make up (they think it displaces the weight...) for not building a proper cement floor. Fails miserably as well.
Air-supported tables like these are the standard in optics labs, and we put our wafer probes and microscopes on them at Unitrode:
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The large leg in each corner contains a rolling diaphram air cylinder controlled by a position regulator that does a pretty good job of keeping the table level, much better than springs would. They work very well until you need to torque larger fasteners, then they bob and rock like a diving float.
Einstein was good at "thought experiments". You're not. And Newton is gonna kick yer ass in five or six different dimensions if you ever try this for real.
First, there's that stuff about how a body at rest tends to remain at rest unless acted upon by drugs or alchohol or something. That does NOT refer to the whole body, if said body is made of parts that can move with respect to each other. What it refers to is the center of the body's inertial mass. So when you hit the rapid button and make the machine's table move to the left, everything else about the machine is going to move a bit to the right, so the center of mass stays where it was. And if you move the saddle away from you, the rest of the machine will come toward you. Etc. And that's gonna be a problem.
If the guy wires were all taught and balanced and everything before you moved the table, they're not going to be that way after you move it. The platform and machine body are now in a different place, having moved to compensate for the change in mass distribution when you jogged X. So the tension on the wires is now unequal. And, gravity being what it is, the whole machine and base will start another bit of movement, swinging pendulum style toward a new location where the tension in the wires is equalized again. And that swing won't go away till friction eats it, which can take a very long time. And every new axis motion will add to the swing. And add to it. And ADD to it. And so on.
If you attempt to move Z the wires will either loosen a bit (Z- motion) or tighten a bit (Z+), for the same Newtonian reasons. And, since wires (or anything else, if you stress it enough) are elastic, they'll just naturally want to "rebound" a bit when the motion stops. So in addition to swinging back and forth from XY motions, your poor dizzy Fadal will now be bouncing up and down, platform and all. And don't even THINK about adding springs!
Worse, none of the moving parts of your Fadal is likely to have its own center of mass on a line that goes through the center of mass of the whole machine. That means the forces described above will form "couples", which will cause rotation. When the table moves left or right, the machine won't just pendulum swing in the other direction. It'll also try to twist itself around it's center of mass. Viewed from the operator's position, it'll twist clockwise if you move left, and ccw if you move right, or vise versa, depending on whether the X mass is located above or below the main mass center. Same problem when you move Y or Z (top of machine twists toward you or away from you). This causes even more distress for the poor guy wires, which will now be out of balance in god knows how many ways, and will need to hang on while the whole machine does some MORE pendulum/gravity swinging to equalize everything all over again.
And before you know it, you and your poor machine will be swinging back and forth like a 10,000 pound tether ball, twisting in at least three axes simultaneously, and screaming like a kid on one of those insane multi-dimensional amusement park rides that's specifically engineered to make people lose their lunches from both ends at once.
And then, since you asked, there's the small problem of vibration. And here's the relevant question: If you were a machine tool, and if you had a whole bunch of nasty little compression waves running all over the inside of your body, and if you really REALLY wanted those waves to go away, becaue they hurt, and itch, and make your eyes water, and more, what would you most like to grab hold of for stability?
A. Some long, skinny, flexible, stretchy pieces of cable?
B. A big-ass chunk of concrete, conveniently attached to the entire earth, with all its zillion trillion gazzillion tons of rock and iron and stuff?
If you guessed A, you'd better own a good helmet and a generous supply of barf bags.
Did you read what HalfNutz said, in one of my recent rants? That I could out-verbositize *you*!! I think we are at least neck and neck, eh? :)
Well, I didn't doubt for a minute that pendulum-like motion and rotation would accrue, but would it affect the machining *results* of a part?
And, regarding the motion, suppose the machine were constrained at the coners by horizontal springs, and in the vertical by springs. Couldn't this actually *relieve* internal stresses in a machine, with no sacrifice of part accuracy?
I wonder how this would affect the tuning of the servo system?
The more important question, is how much coolant are you going to dumping out of the coolant tank due to wave action.
Wes
-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller
I think the real "utility" to this Q, beyond any impact on accuracy (which I don't think there will be) is that a floating design might be more "structure/building friendly" say, for machines mounted on a second floor. Or, in garage-type ditties, more neighbor friendly ito of transmitted vibration to the next house, etc.
Not a biggie, just never saw the big payoff to bolting shit down like an earthquake was coming. A proper floating design would have no tendency to walk, either.
Okay. Put your safety glasses on. Stand on the platform in front of your machine. Adjust wires, springs, reductape. Go to MDI. Type in 10000 M3. Press cycle start. Type in M5. Press cycle start. HANG OOOOOOONNNNN!!!!!!!
That's a sad topic. The answer is yes, and no, and I'm pissed about it, to say the least.
I have the verbositizer shut down for the night; but I once upon a time promised an update to the group, so I'll do that later, under another topic heading.
Thank you for asking, though. Truly. This was something I was genuinely excited about, and it went down in flames.
FWIW - I once worked at a dealership under the main air compressor which was hung from the (metal) roof trusses by steel rods and compression spring brackets. We called it "Skylab", and just ignored it after the first few days of working there. It was there for 10 years before I started with no problems and might still be there now, 30 years later.
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