[Geof]

Possibly less messy and a lower odor factor, paraffin wax. Melting that on the kitchen stove is probably going to generate fewer complaints from SWMBO.

[NC Cams]

The additition of something "solid" (parafin wax, concrete, tar based asphalt, etc) surely would affect the stiffness once placed inside a device but what you're looking to do is affect the natural frequency of the assembly WITHOUT DEFORMING THE HECK OUT OF IT when you do it.

Thus, it isn't just the fact that you add SOMETHING to the body to stiffen it but it is also how much you can stiffen it in proportion to the weight that you add in the process of adding the whatever.

Essentially, the natural frequency is (using Excel format):

Nat Freq = (K/M)^0.5

or the "square root of (the spring rate of the device divided by the mass)".

Yes, the addition of parafin might add stiffness but is parafin going to get and remain as stiff as concrete???- Definitely not, especially in a warm shop or when/if the machine should heat up appreciably. Ever see candles start to droop when temps get above 100 Deg F???

Yes, the addition of asphalt might also add stiffness but it it as stiff as concrete??? When you consider that asphalt is essentially aggregate mixed with tar (which at room temp is actually a cooled liquidus, almost thixotropic fluid), again, I'd say a resounding NO. Note also what happens to asphalt pavement stiffness on 100+ deg days.

Yes, the addition of either substance would add Mass (M) to the machine BUT would it proportionally add stiffness (K)??? I"d say no. Unless you drastically increase K, you're only adding weight which is not the prime nor full intent of the effort.

Fine aggregate concrete will "cure" to a hard, solid, dense, stiff "spring/mass/damper" which is what you're trying to achieve. It will also flow into small spaces, especially if properly vibrated.

In this case, it is the DIY'ers dream because is it cheap and available and relatively benign to handling (asside from the mess and weight). I'd contend that you'd want to pour in stages so that the heat generated in during curing can escape without causing expansion/distortion issues. Keep in mind that the polymer stuff that was mentioned earlier that is specifically formulated for stiffening machinery, has been tailored specifcaly for this task.

I'd guess that it is stable, free flows into small spaces, cures with excellent size stability and doesn't shrink/expand appreciably, which could really hurt the straightness of ground whatevers on the fabrication that one is trying to stiffen.

Finding out that pouring stuff into something only to find that it distorted the hell out of that something when it cured AFTER THE FACT is a bit too late.

Then again, and after all, it is only a DIY project and it doesn't have to be rocket science, now does it????.

[Geof]

I had not mentioned it earlier but will now; forget about adding stiffness using concrete, epoxy, tar, wax or anything else. Make the steel structure as stiff as it needs to be. This is fairly simple but a steel structure, particularly a tube, makes a wonderful resonant structure - think bells. Now use the concrete or whatever to damp the resonance. As I pointed out earlier I do not think you want a homogenous mass inside because it is going to have its own natural frequency; this gives you three fundamental frequencies to contend with, the steel structure, the filler mass and the beat frequency between them. What is needed is something that doesn't really have a natural frequency because it is not solid. Sand, gravel, lead shot, all fill the bill but you need something to bring them into good contact with the steel structure otherwise it just resonates and bounces off the filler. This is where some form of non rigid binding agent comes in because it ties a structure that will willingly vibrate at certain frequencies to a mass that has very little tendency to vibrate at any frequency.

[NC Cams]

"....forget about adding stiffness using concrete, epoxy, tar, wax or anything else....."

For whatever reason, this method (concrete-like filler material) is used and considered viable by as notable a machine tool maker as Hardinge - saw the process being done while on a bearing engineering visit.

The key however is/was that they aren't just dumping in concrete in a wanton fashion. They were concentrating it in selected areas to stiffen and 'tune' the frequency of the structure that they are trying to make.

Like a lot of discussions that have occurred on CNC Zone, acceptable technologies can get "Cliff noted" and oversimplified. Sometimes the technology can be scaled or applied directly and other times, it can get morphed into a useless caricature of what it really needs to be.

Imagine the guy who tries to build a concrete bridge WITHOUT using rebar. Sure he made it out of premium concrete but, without the rebar, it is not going to be capable of holding up its own weight let alone the traffic that it is intended to hold.

As is often the case, what you DON'T see incorporated into the application of the technology has as much to do (if not more) with the success of the the structure as in what you DO/CAN see.

Re: homogeneous mass versus beat frequencies of non-homogenous resonating structures =

I"m definitely NOT a engineer who's well versed in vibrations and the effects that structural dynamics have on machine vibrations or damping of same.

This is the only reason why I'm pointing out some of the "issues" that dumping concrete or whatever into a structure can cause. It simply is not that wise or easy to call the local ready-mix supplier and have him deliver 2-3 yards of "fix em up" to my mill column to fix an otherwise poorly designed structure.

The simplification I put forth in post #22 was an attempt to show the affect that mass and stiffness have on the natural frequency of a structure. How you apply the basics of natural frequency to machine tool structure design have even come close to being properly conveyed in this exchange of ideas.

Yes, Hardinge and other machine tool suppliers DO use aggregate fillers to add stiffness and mass damping characteristics to their machines (see post #19). However, you an bet that the process was the result of careful work, study and analysis and testing - hardly the way most DIY'ers (including myself at times) go about doing things.

[Geof]

....Imagine the guy who tries to build a concrete bridge WITHOUT using rebar. Sure he made it out of premium concrete but, without the rebar, it is not going to be capable of holding up its own weight let alone the traffic that it is intended to hold......

Wrong. If it was an arched structure concrete would be an ideal material and rebar totally redundant. Concrete, fundamentally, is no different to rock and there are numerous rock bridges and building that have successfully held themselves up, sometimes hundreds of feet up, for centuries.

And to get back to my point; which is that for a DIY it is much easier to build in the required stiffness using a standard structural material such as steel. And it is relatively easy to introduce vibration damping by using some form of (loosish) filler and a non-rigid binder. If you have the resources of Hardinge possibly to can get fancy and do it all in one.

[NC Cams]

"Wrong" is probably a bit too general of a statement.

What if it was a column and girder construction which is the way most pre-stressed concrete bridges are made today??? Without the rebar, the girders that carry the bridge trafic it would probably crack while they were trying to lift it into place.

Your point is however QUITE illustrative of the need to PROPERLY use and load the construction member(s) being used. Concrete wants to be in compressive loading otherwise it will crack. Cast iron, too, tends to prefer compressive loading. Steel will tolearate some compressive loading BUT it prefers tensile loading.

Right material for right job - perhaps the MOST important message to be conveyed from via this thread. More over and more imporantly, the point is NOT to simply dump in a yard of "ready mix fix" to address a poorly engineered structure. I agree completely with the need to properly engineer and build a structure.

As is often the case, the initial inquiry about "adding concrete to increase stiffness" evolved/morphed into a much more detailed thread/reply than the inquiring member ever imagined.

[ataxy]

As is often the case, the initial inquiry about "adding concrete to increase stiffness" evolved/morphed into a much more detailed thread/reply than the inquiring member ever imagined.

but thats great... no?

[miljnor]

"Wrong" is probably a bit too general of a statement.

Too general regarding what?

Imagine the guy who tries to build a concrete bridge WITHOUT using rebar. Sure he made it out of premium concrete but, without the rebar, it is not going to be capable of holding up its own weight let alone the traffic that it is intended to hold.

I would say in regards to this statement its completely right. But if you added "using current building procedures or techniques" then it would be "too general" But you didn't.


oh my! a first!

And yes I am older than a teenager but couldn't resist the ribbing!

Probably should have added na ni na ni na na!

All in good fun though!

:cheers:

[BobWarfield]

You can achieve significant dampening with ordinary concrete, applied correctly. Do a Google search for a document called "Principles of Rapid Machine Design". This was the PhD thesis of an MIT mechanical engineer, and it's all about building high precision machine tools. Many useful techniques and data are covered there.

His approach of using thick walled round tubing to achieve greater rigidity also seems to me has application to these kinds of projects.

Worthwhile reading.

Best,

BW

[Geof]

You can achieve significant dampening with ordinary concrete, applied correctly. Do a Google search for a document called "Principles of Rapid Machine Design". This was the PhD thesis of an MIT mechanical engineer, and it's all about building high precision machine tools. Many useful techniques and data are covered there.

His approach of using thick walled round tubing to achieve greater rigidity also seems to me has application to these kinds of projects.

Worthwhile reading.

Best,

BW

Don't give away secrets (chair) (chair) (chair) (chair). All in good fun as miljnor says; worthwhile reading it was.

[miljnor]

Here is another thought my business partner brings up when I mention papers on thesis.

Theses are written for grading and generally the engineer has no "real world" knowledge. So while the thesis might be a shining light of information and formula it might in fact have no basis in reality.

Of course the guy is a PhD so he might have gone in the field and grabbed himself a taste of the real world.

but it is good stuff regardless.

[Geof]

...Of course the guy is a PhD so he might have gone in the field and grabbed himself a taste of the real world.....

And discovered it tastes horrible.

[svenakela]

Well, I will not argue about the stiffness, dampening, all-about-the-rest concrete theories in this thread. But as it works with the machines I've seen and been using - so why not?
The tarmac lathes are made by a bunch of really respectful engineers, I will not argue about their design either and it's a fact that their machines are awesome to see in action!

[biomed_eng]

According to Wikipedia:
-"Dampening refers to the musical technique of muting an instrument while playing it. For the scientific term see Damping."
-"Damping is any effect, either deliberately engendered or inherent to a system, that tends to reduce the amplitude of oscillations of an oscillatory system.

Like an electrical system, you want to "ground" your machine mechanically such that any forces that occur at the table level will be transmitted to the ground with absolutely minimal movement and vibration i.e. ultra high stiffness. This would be analogous to an electrical system with a very low resistance ground path.

Concrete or similar would be a cost effective solution for achieving your goals.

In addition to the stiffening nature of concrete, steel and concrete have very different vibrational transmission speeds (steel ~6100m/s versus concrete ~3400m/s), this will enhance the system's natural vibrational damping due to the two materials' impedance mismatch.

[Tristar500]

Finding out that pouring stuff into something only to find that it distorted the hell out of that something when it cured AFTER THE FACT is a bit too late.
..

My plan would be to first weld, then fill with concrete mixture, then face mill for trueness. I'm thinking of using the latex additive to the Portland cement that is commonly sold for use in tile grout to make it more flexible and more resilient. Anyone?

[Mcgyver]

Well, I will not argue about the stiffness, dampening, all-about-the-rest concrete theories in this thread. But as it works with the machines I've seen and been using - so why not?
The tarmac lathes are made by a bunch of really respectful engineers, I will not argue about their design either and it's a fact that their machines are awesome to see in action!

are you sure its concrete? For example Hardinge uses something the call harcrete, as near as i can tell it is cement but made with a polymer instead of water, ie their own proprietary polymer concrete. you do NOT want to use regular concrete as it takes years to cure and will move about. this matters on a machine where you are working to thou's or less. the worlds best machines, that work to millionths of an inch, ie optical equipment, often use crushed granite and polymer mixes for bases - that is what I'd pursue as a filler. Incidentally it has 2x the dampening properties of cast iron. Do some searches here - there's lots of material on it. concrete, unless its a polymer concrete is not for machine tools imo.

[miljnor]

you do NOT want to use regular concrete as it takes years to cure

I don't know about that, Is there any documentation or something that supports this?

I've done the concrete thing in the backyard and had to rip it up a couple of weeks later do to a pipe leak and it looked plenty dry to me. But then again this was only 4" thick concrete.

Since concrete is cured via a chemical reaction no from water evaporation I would have a hard time believing the stuff doesn't cure at a Simi-uniform rate. Otherwise you would never be able to dump the stuff in a river to make a bridge.

But hey what do I know.

I would like to see the actual cure rate though, if you have any info.

[pastera]

While it does take a few weeks to cure to ultimate strength, concrete is solid shortly after mixing (unless it's very cold out).

Aaron

[Geof]

I don't know about that, Is there any documentation or something that supports this?...I would like to see the actual cure rate though, if you have any info.

Oh, ye of little faith. I have known since I was a boy at my pappy's knee that concrete took years to cure. But since some people are stubborn, disbelieving sons of @#$%^ I tried to find something definite. How frustrating, here are some excerpts:

Soon after the aggregates, water, and the cement are combined, the mixture starts to harden. All portland cements are hydraulic cements that set and harden through a chemical reaction with water. During this reaction, called hydration, a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates. ..............The rate of hardening depends upon the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions. Most of the hydration and strength gain take place within the first month of concrete's life cycle, but hydration continues at a slower rate for many years. Concrete continues to get stronger as it gets older
===================
The concrete (or specifically, the cement in it) needs moisture to hydrate and cure (harden). When concrete dries, it actually stops getting stronger. Concrete with too little water may be dry but is not fully reacted. The properties of such a concrete would be less than that of a wet concrete. The reaction of water with the cement in concrete is extremely important to its properties and reactions may continue for many years.
===================
Concrete hardens as time passes....It takes a great deal of time (even years!) for all of the bonds to form which determine concrete's strength. It is common to use a 28-day test to determine the relative strength of concrete.

I think you will have to whistle in the wind for something definite on actual cure rates. Calcium chloride is added to make concrete cure faster in cold whether but I didn't find anything about how much faster. Also didn't find anything about dimensional changes although I know concrete shrinks during curing.

[bigbam]

Here is a link that has a lot of info on the use of concrete for machine design. They use concrete mixture with powder aluminum so that the concrete will expand when it sets up. Also use a mast construction of a pipe in a pipe and the concrete poured between the two pipes. This will help reduce the overall mass but retain good stiffness.


http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf