DIY Mill

[Susquatch]

It's not rigidity that makes CI better at damping, its the boundary layers between dissimilar material within it- i.e. the nodular structure of cast iron. Steel actually has a slightly higher Youngs modulus than CI. There's other factors as per the bridge example, the natural frequency of the structure, but all things being equal, materials with lots of surface area (boundary layers) between dissimilar materials will damp the best (decay the wave, i.e. the amplitude is reduced more quickly)

I don't think anyone here ever said anything about "rigidity" of cast iron making it a better damping material. I certainly didn't - or at least didn't intend to. All of my references to rigidity were in the context of components of a machine and its stand and as such were an outcome or resulting consequence of the overall design.

I agree that it is the grain structure of cast iron that gives it it's high damping. But I'm not sure it's correct to describe the materials as different though. I guess that depends on how strict your definition of different materials is and the individual type of cast iron being considered. Certainly if you want to consider cementite to be a different material than pearlite and that it occurs in grain boundaries then you would be correct. But that's a little deeper into the individual microstructure of cast iron than I thought you were suggesting. Regardless, I don't believe dissimilarity is as important as the fundamental granular nature of cast iron itself.

Looking back at what you said earlier, it's easy to see how we might have needlessly disagreed. Your first comment about the boundary between two materials led me to believe you were talking about the interface between steel and cast iron or some such other dissimilarity on a very large scale when in fact you may have been talking about the molecular structure of the grains that make up the entire mass of cast iron - which I would have agreed with.

Here is a quote from my first response "FWIW, I think there are two kinds of damping being discussed here. One, as you say is at the boundary between two materials, and the other is throughout the grain structure of heavy masses."

Your later comments suggest you were referring to these two as one and the same and I just thought you were talking about the first but not the second.

In any event, this granularity in the second was my whole point. Concrete, epoxy granite, sand, and other materials exhibit a similar energy absorbing behaviour to different degrees for the same reasons. It takes energy to move the particles and grains within a body and that energy is dissipated in the form of heat.

Sometimes I hate the English language.

 

[Manfred]

As to what I expect of my mill:

Roughly the performance regarding accuracy speed and rigidity equivalent to a standard manual Sherline or Taig. Support of full three axis CNC control in an envelope of at least 150mm X 150 mm X 200mm.

In time I would like to add a fourth and potentially fifth axis under CNC control with roughly the same envelope.

I understand that I will have to increase the rigidity of the machine when I add additional axes or increase the size and power of the spindle.

 

[TorontoBuilder]

As to what I expect of my mill:

Roughly the performance regarding accuracy speed and rigidity equivalent to a standard manual Sherline or Taig. Support of full three axis CNC control in an envelope of at least 150mm X 150 mm X 200mm.

In time I would like to add a fourth and potentially fifth axis under CNC control with roughly the same envelope.

I understand that I will have to increase the rigidity of the machine when I add additional axes or increase the size and power of the spindle.

Okay well you dont need to go overboard like I have.

 

[Susquatch]

Damping is a function the boundary between two dissimilar materials (the resin and the sand) and its very stable.

FWIW, I think there are two kinds of damping being discussed here. One, as you say is at the boundary between two materials, and the other is throughout the grain structure of heavy masses.

@Mcgyver - I wish you had caught my misunderstanding right there. I think it was plainly obvious that I misunderstood when I separated your singular statement into two different kinds. Even my subsequent discussion must have had you going "huh?"!

I was misled by your choice of the word material and your example.

Now that I know what you really meant, I agree with you.

Anyway, my apologies for the massive disconnect and any aingst it caused you.

It's not rigidity that makes CI better at damping,

Can't figure out where you got the idea that someone said rigidity makes Cast Iron better at damping. I suspect it was just your turn to misinterpret what someone else (maybe even me) said.

 

[Mcgyver]

Can't figure out where you got the idea that someone said rigidity makes Cast Iron better at damping. I suspect it was just your turn to misinterpret what someone else (maybe even me) said.

@mbond post seemed to suggest it

As to why steel is poor, and other materials are better, steel is relatively flexible and more elastic.

The steel is more rigid (slightly higher Young's Modulus) than CI, however it's homogeneous - doesn't have the nodular structure creating a lot of boundary layer area that CI does. That is what makes it damp less well than CI, and why EG is so good at damping

 

[Susquatch]

so for stands or benches that our machines are mounted on, is there any remedial modifications that can be done to help dampen vibration?

I think the best return for the effort is rigidity improvements in the machine itself. Plinths on lathes, low stick out on tooling and work, better vises and work holding, solid tooling, better fastenings, more mass, etc etc. It boils down to understanding the work you want to do and it's relationship to your machine. Lots and lots of opportunity to make significant improvements this way, but I think most of us either ignore it or take it for granted until it bites us.

One can also add rigidity by filling cavities with energy absorbing materials but it doesn't always pay. It's effectiveness depends on the specific machine geometry and the individual work envelops for what you usually do.

Regarding stands, I think it's better to design and build a rigid stand from the beginning than it is to improve it later on. But, it's not a hopeless situation. Even things like adding plate or energy absorbing materials over wide areas can help.

If I were you and wanted improvements, I'd start by assessing what the root cause of any problems are and then go from there. Otherwise, you are just stabbing around in the dark.

Another suggestion is to assess the OEM stand if there is one. It's been my observation that a lot of these stands are much more effective than we realize. If it has heavy cast iron components, it's a pretty good guess that it was done intentionally and with purpose - or was copied from another OEM that did. I wouldn't hesitate to buy a stand from an OEM that was right sized for my machine. It's usually a mistake to think you can make something better that is cheaper.

Last but not least is managing expectations. You cannot magically turn a small bench top machine into a 1 ton toolroom machine. Just do the little stuff and whatever makes sense and enjoy making stuff.

 

[Susquatch]

@mbond post seemed to suggest it

I don't think that's what he meant. But fair enough.

The steel is more rigid (slightly higher Young's Modulus) than CI, however it's homogeneous - doesn't have the nodular structure creating a lot of boundary layer area that CI does. That is what makes it damp less well than CI, and why EG is so good at damping

At the risk of misunderstanding you once again........ You are mixing together two different kinds of rigidity. Yes, steel has a low strain to stress ratio (deforms less), but that's a static characteristic independent of its damping characteristics. This is also the more common definition of rigidity.

Cast iron's damping benefit is a dynamic characteristic somewhat independent of its strength. I agree with your reason for why CI and EG are so effective. I think we probably shouldn't use the word rigidity to describe this characteristic, but we all do.

The common dictionary definition of rigidity is: "Rigidity is a state or quality of being inflexible or not prone to bending, deformation, or change". This definition doesn't separate the static condition from the dynamic one. I think that's where the confusion arises.

 

[Manfred]

I have found Barrymounts to be able to work wonders: https://novibration.com/

You may have to put in some thought and be willing to experiment to find the best size and location though.

Alternatively, tennis balls might be useful. My team was using an optical platform to evaluate a stabilised camera system that could produce high quality images from a few thousand meters away. Even though it was mounted on a granite table that was about 1'X4'X8' and that was mounted on a sturdy steel structure on a massive cement block we found that trucks traveling on the highway a few km away would induce vibrations in the optics. After our experts had spent weeks doing their sums and the buyers getting quotes one of the junior engineers suggested filling the frame that supported the base with a single layer of tennis balls - it worked like a charm. Only problem was that the tennis balls had to be replaced every few months because they simply lost their bounce.

 

[Mcgyver]

I don't think that's what he meant. But fair enough.



At the risk of misunderstanding you once again........ You are mixing together two different kinds of rigidity. Yes, steel has a low strain to stress ratio (deforms less), but that's a static characteristic independent of its damping characteristics. This is also the more common definition of rigidity.

Cast iron's damping benefit is a dynamic characteristic somewhat independent of its strength. I agree with your reason for why CI and EG are so effective. I think we probably shouldn't use the word rigidity to describe this characteristic, but we all do.

The common dictionary definition of rigidity is: "Rigidity is a state or quality of being inflexible or not prone to bending, deformation, or change". This definition doesn't separate the static condition from the dynamic one. I think that's where the confusion arises.

The suggestion was made that more rigidity is a (important) factor in damping and that cast iron was more rigid which explained its better damping. Both of which I disagreed with - that it is total area of the boundary layer between dissimilar materials that gives a material its damping ability, and the better damping cast iron is less rigid (slightly) than steel based on their Young's Modulus.

 

[mbond]

If I have caused confusion, I'm sorry

ordinarily the point of this forum is to learn I think

 

[Manfred]

Okay well you dont need to go overboard like I have.

I had a look at what you did. Way over my skill, budget and competencies.

I plan on using my mill - once it's up and running - to build some tools as well as some of the supports that I would need for a solid EG base.

 

[Mcgyver]

If I have caused confusion, I'm sorry

ordinarily the point of this forum is to learn I think

100%, it's a good discussion and I don't pretend to have an exclusive on being right

Now, in addition to machine tool vibration damping, we have to figure out how to un-confuse a large hairy person. Scope creep, story of my workshop life.