Thoughts on adding vibration dampening materials inside a CNC gantry?

[Goemon]

How would you guys approach the challenge of reducing vibration in a benchtop CNC machine where you don't have the option of using 10 tons of iron? Vibration dampening materials are clearly available but they don't seem to be widely discussed here. Do any of you use them?

My CNC gantry is approx 8" thick x 8" wide x 48" long and while building up the various layers in my mold, I chose to leave a 3/4" thick x 6" x 36" section as a void in the center with the intention of filling it with a vibration absorbing / dampening material. I am trying to figure out if this a good idea or if I should just fill the void with nanotube resin and move on.

I have tried to research this topic and it seems like the industrial passive vibration dampening materials are typically some kind of elastomer. This seems intuitive as I would expect a high surface hardness to increase vibration. High hardness materials like steel vibrate more than softer cast irons etc. I have plenty of softer materials on my shelf to choose from...

I am currently leaning towards filling the void I left in my gantry with 1/2 urethane rubber and the other half an epoxy resin mixed with an additive which turns is into a flexible elastomer. It is not too late for me to change this if you guys have a better idea though?

My gantry is made from mix of carbon fiber, carbon nanotubes, fiberglass, epoxy granite and steel plates as well as various steel and carbon fiber re enforcement rods so it would already be better than most gantry structures (of comparable size and weight) for vibration control. It's just that, if I have the opportunity to make it better with a vibration dampening layer behind the steel plates (which hold the linear rails), now is the time....

I thought this topic would be more widely discussed here given that people seem to favor steel as the material of choice for diy builds while industrial designs specifically choose iron over steel for vibration control.

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[Paul-JB]

Does the gantry structure have pockets of space where you could fill it with sand kind of like a dead blow hammer??? Not sure if that would be enough to shift the nodes and natural frequency of the structure outside the operating range to be effective. Btw I think the proper term is damping.
"dampening" implies wetting something with a liquid. I know. Sorry. I had to.

[ger21]

An Epoxy granite mix is what they use for mills. For simplicity, you could just use epoxy and sand. As little epoxy as possible.

[Goemon]

It already has a large amount of epoxy granite in the structure. It's what I used to partially fill the void in the carbon fiber shell and as a binder to hold my carbon fiber and steel reinforcement rods / tubes.

All that is left of the void currently is the space I left for vibration damping materials. I understand that "epoxy granite" or simple epoxy / sand mixes are considered to be low vibration materials but they are rigid when cured. The commercial vibration reducing materials are flexible elastomers or rubber-like materials. I am talking amount materials like they use for those vibration reducing rubber feet they sell for machine tables. Or this type of material:

https://www.sorbothane.com/vibration...-machines.aspx

So.... my options are to fill the void I left with the same materials as I used for the rest of the volume (I.e. More epoxy granite, cf or fiberglass etc) or to add something softer as a a further vibration reducing measure.

At this point, I don't need to fill the void with hard materials like epoxy granite for strength or stiffness. There is more than enough strength and stiffness already in the 7"+ of carbon fiber, epoxy granite and steel. It's just a question of whether there is any benefit of using a thin layer of flexible resin or hard rubber behind the steel rail mounting plates?

I am not sure if I am confusing the concept of acoustic vibration reducing materials with reducing the vibration that causes problems for machines.

[ger21]

At this point, I don't need to fill the void with hard materials like epoxy granite for strength or stiffness.

I would argue that if you are worried about vibrations, then you want more strength and stiffness.

Machines that are made of epoxy granite are solid. You want to eliminate vibrations before they occur, not dampen them after they happen.

[Jim Dawson]

To reduce vibration you need to add some kind of squishy mass. While in this case it would not be practical, filling all of the voids with lead shot would work the best. But that's the general idea. About half of the mass of my CNC lathe is some kind of special concrete.

[Goemon]

To reduce vibration you need to add some kind of squishy mass. While in this case it would not be practical, filling all of the voids with lead shot would work the best. But that's the general idea. About half of the mass of my CNC lathe is some kind of special concrete.

I never would have thought about using lead as a vibration damping material. I actually have a bunch of .357 caliber lead air gun pellets and since I don't use that caliber anymore, it would be easy enough to melt them down and pour lead into the void. Is lead really better for vibration damping than hard urethane rubber (think car tire hardness and flexibility) or a flexible plastic resin?

[Goemon]

I would argue that if you are worried about vibrations, then you want more strength and stiffness.

Machines that are made of epoxy granite are solid. You want to eliminate vibrations before they occur, not dampen them after they happen.

Is that possible on a small benchtop cnc machine? I read that this approach is taken on very large 10 ton steel machines but I thought that smaller machines had to relying on damping materials instead.

I chose carbon fiber and epoxy granite because they vibrate less than steel, iron and aluminum. What else could I do (if anything) to eliminate vibration before it occurs?

Are you saying that there is no point in adding a softer vibration damping layer and that I should fill the void with the same epoxy granite / carbon fiber mix as the rest?

[Jim Dawson]

I never would have thought about using lead as a vibration damping material. I actually have a bunch of .357 caliber lead air gun pellets and since I don't use that caliber anymore, it would be easy enough to melt them down and pour lead into the void. Is lead really better for vibration damping than hard urethane rubber (think car tire hardness and flexibility) or a flexible plastic resin?

You would be better to use the pellets as they are and just pour them in. Much like a dead blow hammer. The loose pellets can not transmit vibration, where a solid mass might be able to. Or use the lead pellets as a filler in the plastic resin to add mass.

[Goemon]

You would be better to use the pellets as they are and just pour them in. Much like a dead blow hammer. The loose pellets can not transmit vibration, where a solid mass might be able to. Or use the lead pellets as a filler in the plastic resin to add mass.

I don't know about using loose pellets as it seems like a recipe for noise as they rattle around. Setting them in liquid urethane rubber or resin would be easy enough though.

What is it about lead that makes it desirable for this purpose? Is it the low surface hardness? Are tin and pewter also good for vibration control as they have similar mechanical properties?

[Jim Dawson]

It's the high density, volume to mass ratio. Less dense materials would work also, even sand to a lesser degree. The goal is to use a material that will dampen the vibration, so it needs to have mass and be kinda ''floppy''.

Let's take the case of a bar of steel, you strike one end and the vibration is transmitted to the other end almost instantly setting up a wave in the bar. The same mass of steel shot, would not transmit the wave at all.

[footpetaljones]

I don't know about using loose pellets as it seems like a recipe for noise as they rattle around. Setting them in liquid urethane rubber or resin would be easy enough though.

What is it about lead that makes it desirable for this purpose? Is it the low surface hardness? Are tin and pewter also good for vibration control as they have similar mechanical properties?

In addition to its mass, lead has a loss coefficient something like 2-3x better than cast iron, meaning it dampens vibrations better. I believe that a few companies used to pour lead into cavities in their castings.

[Goemon]

In addition to its mass, lead has a loss coefficient something like 2-3x better than cast iron, meaning it dampens vibrations better. I believe that a few companies used to pour lead into cavities in their castings.

Did they stop because they discovered it was toxic? I could see machine builders having an issue working with something like that every day. It's probably not an issue for a one off project and a sealed cavity though.

If mass and softness are the key reasons why lead works for this then I assume that tin is a viable substitute. I use it to make lead free pellets because it has similar properties. It's easier and safer to work with too because of it's lower melting point. It can be cast in silicone molds.

Most of the vibration damping or isolation products available now seem to use some type of rubber, foam or polymer. I wonder if any of the large machine builders still use lead in their castings.

[Goemon]

It's the high density, volume to mass ratio. Less dense materials would work also, even sand to a lesser degree. The goal is to use a material that will dampen the vibration, so it needs to have mass and be kinda ''floppy''.

Let's take the case of a bar of steel, you strike one end and the vibration is transmitted to the other end almost instantly setting up a wave in the bar. The same mass of steel shot, would not transmit the wave at all.

Do you know of any viable testing methods that can be done at home to see which materials vibrate the least?

looking at the design of commercial vibration isolation screws, it seems like a thin layer of urethane rubber sandwiched between the spindle mounting plates would help. Then again, maybe it won't make a difference because carbon fiber and epoxy granite is already better at vibration damping than iron.

[Jim Dawson]

Do you know of any viable testing methods that can be done at home to see which materials vibrate the least?

.

A load cell and an oscilloscope come to mind. Maybe an accelerometer chip and some data capture software would work also. Or you could go with something from these guys. https://www.stiweb.com/

[Vinnito1]

Don't know what your budget is, but the link below is a battery powered accelerometer with a USB interface. I've used these in the past to capture impact data at ~150Gs so they seemed to pretty durable

GCDC Products

[footpetaljones]

Did they stop because they discovered it was toxic? I could see machine builders having an issue working with something like that every day. It's probably not an issue for a one off project and a sealed cavity though.

If mass and softness are the key reasons why lead works for this then I assume that tin is a viable substitute. I use it to make lead free pellets because it has similar properties. It's easier and safer to work with too because of it's lower melting point. It can be cast in silicone molds.

Most of the vibration damping or isolation products available now seem to use some type of rubber, foam or polymer. I wonder if any of the large machine builders still use lead in their castings.

I don't know, but I would guess it was simply a business decision since the guys in boardrooms usually don't ask shop guys how they feel about their instructions.

Lead + iron requires separate processes and equipment vs keeping current procedures/equipment and simply making a longer pour.

[UrbanForge]

How correlated is dampening with mass, material density, and composite microstructure?

Silicon dioxide (granite) is less than 1/2 the density of steel, yet offers 5x dampening. Additionally, 3d micro structures can reduce material requirements while maintain good strength (think 3d printing infill).

If a composite structure and micro infill is made from a Quartz based fiber composite, can the resulting macro structure/material be light, strong, and dampening?



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[Goemon]

How correlated is dampening with mass, material density, and composite microstructure?

Silicon dioxide (granite) is less than 1/2 the density of steel, yet offers 5x dampening. Additionally, 3d micro structures can reduce material requirements while maintain good strength (think 3d printing infill).

If a composite structure and micro infill is made from a Quartz based fiber composite, can the resulting macro structure/material be light, strong, and dampening?



Sent from my iPhone using Tapatalk

From what I read, mass is important in terms of needing more or less of it depending on the vibration damping properties of the specific materials. For materials with poor damping properties like steel, they need to use more of it to reduce vibrations than they would with materials with better damping properties. To control vibration there is a choice of reduce, damp or isolate.

This is why some high end machines are made of steel instead of cast iron - there is little vibration when the machine weighs 10 tons so less damping is necessary. On the other hand, Datron claims (on their website) that their machines weigh half of what other mills weigh (and take up less footprint) because they use granite tables and polymers etc for some other components.

Some people seem to think that being heavy is the end goal with machine building but it isn't. They need to be stiff, strong, stable and control vibration. It just happens that to achieve those things with cast iron, you need a lot of it and it's heavy material. I don't know much but I am certain of this point. If a machibe is strong, stiff, stable and has good vibration control, weight does not matter.

From what I could tell, there is no direct line correlation between material density / weight and vibration damping. I.e. It's more complicated that than. I started reading something about how the thermodynamic properties of materials effect machine vibration but I glazed over. I saw enough though to realize that the subject to too complicated for the simple explanation I expected from my google search.

[bostosh]

Fill with Cerrobend

https://en.wikipedia.org/wiki/Wood%27s_metal