[merl]

Hey djshop,
I hear ya on the squeeling pig analogy. I tried putting way oil on my uhmw bushings and letting some coolant flow through the tube too with the same resault. I made some bushings from delrin wich is harder than the uhmw hoping that they wouldn't gall up inside the tube but, I had one go into a harmonic vibration at 6000 rpm and explode so now I just stick with the 2' sticks like you do.
I used a Fagor controle on a large engine lathe at another shop. It was OK except it didn't use all the same Fanuc standard G & M codes and that was confusing. Doubley so because it wouldn't talk to Master Cam either and I had to programe it at the controle.

[merl]

Geof ,
I just read your post about the frequency issue.
Consider also the varying densities of the aggrigate and the epoxy.
The highly visquious nature of the epoxy may hold the aggrigate in place until you set up the vibration at wich piont the density issue may take over.
As I recall when settling out lapping grits in a linseed oil vehical, it wouldn't have made any differance to vibrate the jar.

[jsage]

Agreed.


We probably (likely) have gone beyond our practical abilities. We have reached the end of the internet, please return now.

As an aside my house is set into the side of hill and deer walk on my flat roof, soon to be replaced with a pitched one. One has been walking on it for the past 30 minutes. Great view, yes but getting annoying. ; )

Merl, I was thinking specific gravity as well. At this point, whatever process works best.

[ahlbebuck]

Hi all

This is my first post on this very interesting subject.

I started reading the topic from the start, then skipped many pages then again read the last few pages and the overall impression I get is the followin g:

As DIYérs we are trying to build machine frames the Pro's can only dream of!

I really believe that we are pulling the pond from under the duck on this one.

Right at the beginning PMINMO asked a very important question - why not ordinary Portland Cement based concrete?

This is the way I will go. I design concrete structures with 1 or 2 mm of deflection over 6 to 10 m spans. We are looking at concrete sections where the slenderness ratios are rediculous - there is no "slender"ness! Forget about worrying about deflection.

I will not worry about concrete creep, as proper curing will minimise this to negligable quantities and even so a good design will provide adjustable fixtures.

In machine tools you want mass. Vibration damping is a product of mass. Ordinary concrete of around 50 MPa will do the job nicely as long as you reinforce it properly ie exactly the right amount in the right places.

Transferring vibration to the frame can be done by appropriately spaced cast iron inserts cast into the concrete. These can be machined in situ afterwards to accommodate any fittings to be attached ie linear rails etc. AND they make it possible to provide for adjustment when required.

Keep it simple. I am starting to build a combination CNC mill/lathe out of concrete cast into permanent mild steel shuttering (in places) and properly cured under water for a few weeks. By proper design of support components I really believe that concrete is the way to go.

And if it doesn't work - break it up, re-use fixtures and try again - small cost compared to exotic materials!

Just my 2c worth.

regards

[jsage]

Ahlebuck,

I agree. I think within your reply is the reasoning. With the expenditure of effort and money, you would like something that appeared and to some degree performed more like a precision, professional grade machine.

Besides this 1986 country house has a lot of ugly white ceramic tile countertops which beg a granite like composite ; )

Concrete is mechanically similar and has a lot of advantages.

Underwater huh, is that to prevent microcracking. I think also to compensate for drying variations pulling apart more complex angles.

Jsage.

[djshop]

That is also my intent. My goals are to have a platform that will be rock solid, high precision, low maintenance and very dependable. It will also allow for increasing capability as my software and programming knowledge increases. I am more familiar with epoxy and composites than with concrete so I think that is the way I will go. I like the idea of a molded base like the CMS lathe I have. I will have to experiment with some molded sections. Is the current concensus that quartz filler is the best for smaller cross sections? Why are larger fillers used for larger sections? I am assuming that different aggregate sizes have different dampening frequencies? I'm not sure that would mean much in a smaller machine. My lathe will be about 3 foot long and 1-1/2 feet square.
One additional thought is the addition of very short, chopped fibers as fillers. The fibers do a lot to increase the shear strength of the resin and being short would mix in well with the aggregate.
Dave

[Zumba]

Right at the beginning PMINMO asked a very important question - why not ordinary Portland Cement based concrete?

Time.... somewhere in the last 667 posts, it was concluded that ordinary concrete would take weeks to cure, cast iron would take months to stabilize, and epoxy/granite would take days.

[jsage]

If you really want to go all out. I found some pricing on system threes phase two. I didn't look at the spec sheets but to my recollection, it has better thermal stability and lower modulus of elasticity, better overall structural strength. At 140 degrees farenheit it is doable for smaller thinner applications. It also allows you break down the process of compaction and curing into two steps.


Possibly

1) Mix and compact

2) Vacuum as necessary

3) Post Cure

The price from this link is not unreasonable considering the relatively small epoxy component.

I think this also might reduce the surface problems as epoxy entraps air as mixed.

Would I use it. I don't know. I'll have to finalize the design first.

http://www.jamestowndistributors.com...pid=4956&PTECH

[ahlbebuck]

Zumba

Somewhere between post 1 and post 667 I could have cured a few concrete structures.

My point is that everyone is in uncharted waters - therefore I would stick to a cheap material with very similar eventual results to the very exotic and expensive polymer.

Using super plastisisers and other appropriate admixtures, you can continue working on a concrete framework within days of casting - the curing goes on.

My advantage is that I'm a structural civil engineer and therefore I feel very confident with concrete and I know what I'll get. Time is not really an issue - there is plenty to carry on with while I wait for my concrete to cure!

JSAGE

Yes (under water curing) - here in South Africa we deal in extreme temperatures therefore the extreme curing measures. I don't know about colder climates - there I will simply keep the temperature and moisture content constant. Evaporation of water is the enemy - it causes shrinkage cracks and separation from the aggregate.

Using properly graded (6mm to 25mm non acidic stone with clean river sand) and shaped (as rounded as possible) aggregate will lessen this effect

regards

[jsage]

Albebuck,

Thanks for the response. Doing a bit with concrete myself but still in the learning stage.

I don't think anyone is really disagreeing. There has been an underlying grumble about why not just concrete instead. I was a bit cynical myself. However, I see some benefits and applications above and beyond the basics. No doubt were reaching in the end I think it is a personal choice and it not necessarily for everybody.

It is kind of fun to try to reverse engineer some of these fancy composites.

I'm also working on thinner faux rock concrete veneers, so overcoming pure thickness and the need for reinforcing is one of my goals.

Concrete is still king. You can't get away from it and it continues to evolve.

Best.

Jsage

[ger21]

Zumba

Somewhere between post 1 and post 667 I could have cured a few concrete structures.

From what I've read here in previous threads on this topic, concrete continues to cure (and shrink and move) for years. Just what I've read, though.

[ahlbebuck]

Ger21

You're right - concrete does continue to move, but its all relative. After a few months the movement is reduced to microns over meter intervals - far less than temperature effects on metal based machine frames. This is precisely why I'm bolting linear rails and other precision attachments to cast iron inserts - to provide for adjustability. Surely all machines need to be adjusted regularly to maintain accuracy?

I must just mention here that I also look at polymers in awe, but it is just unaffordable / unavailable here where I live.

I will use epoxies to seal my concrete structure when its ready though - nothing beats epoxies for wear 'n tear.

Also I can see that many posters get their kicks from building the "ultimate machine" - I however am building the machine as a tool for more interesting endeavours. Just like the plasma cutter I'm finishing off to cut parts for the concrete moulds (amongst other things of course)

Doesn't this forum just rock? So many solutions to so many common problems - wonderful!

regards

[ger21]

Also I can see that many posters get their kicks from building the "ultimate machine" - I however am building the machine as a tool for more interesting endeavours.

That's what they all say....at first.

[ahlbebuck]

Gerry

Do I sense a "been there, done that" moment?

regards

Neil

[walter]

You're right - concrete does continue to move, but its all relative. After a few months the movement is reduced...

You lost me right there.

[ahlbebuck]

Walter

Concrete finds its near final shape through creep during the first 7 days after casting. Therafter the creep slows down to a point after 28 days where it is accepted that all significant movement is over.

Like glass that flows over decades to thicken at the bottom, concrete will always creep - the amount of creep (contraction) is negligable after a relatively short time. Only the guys with thick lensed specs see it as a factor. In the real world we ignore it.

Outside, in harsh conditions, bridge beams, floor / roof slabs and walls will show hairline cracking due to creep (aided by loading) years after casting, but in a controlled environment like a workshop this will not happen - hopefully!

Also - concrete structures in civil engineering are usually very optimally designed for economic reasons, therefore factors like creep and cracking due to expansion, contraction and deflection will be more pronounced. In our application of machine design we over design by a large factor thereby removing most of the issues related to movement and cracking.

Hope that helps

Neil

[walter]

Ahlbebuck,

I must say I really appreciate you joining the discussion. There is a vast amount of knowledge here already, all we needed was an concrete engineer. Thanks for being here.

Here are some of the problems:

-Regular portland cement uses the water in the mix as the curing agent so voids are left within the cured mass.

-Big manufacturers are using concrete inside tubular structures, inside rubber bags, with additives to prevent shrinkage.

- We probably will have to use it on the "outside", with steel tubing or I-beam core frame.

- Water+steel=IO2=rust

How do you solve those problems?

[lgalla]

Concrete has a very low coefficient of thermal expansion. However if no provision is made for expansion very large forces can be created, causing cracks in parts of the structure not capable of withstanding the force or the repeated cycles of expansion and contraction.

"As concrete matures it continues to shrink, due to the ongoing reaction taking place in the material, although the rate of shrinkage falls relatively quickly and keeps reducing over time (for all practical purposes concrete is usually considered to not shrink any further after 30 years). The relative shrinkage and expansion of concrete and brickwork require careful accommodation when the two forms of construction interface."
I am 55 years old,cannot wait for my slab to cure.The Hoover dam is not fully cured.
Did I hear someone mention creep?Martin to the rescue.
I wish concrete was the answer but the risk is not worth the cost of E/Q.
What do you do with a 2000lb cracked slab?How do you remove concrete from weldments?Companys doing 5000 lb pours of E/G or E/Q would use concrete if it gave acceptable results.Epoxy and whatever works,concrete is iffy.I am a gambler,but reading all trillion posts,I place my bet on E/Whatever
Larry

[jsage]

Walter,

I'm sure ahlebebuck has some better answers. I have been looking at some acrylic binders. May not prevent voids completely but eliminates the porosity problem and if water freezes and thaws it does have an effect (expansion contraction) . It's part of why most natural rocks degrade over time.

There are other polymers besides epoxy in polymeric concrete. What I described above is really a fortifier. I ran across a binder at lowe's yesterday. It was related to a specific bonding process, however on the back it said it could also be used in place of water in concrete. I'll check that section again.

Last night I came across an epoxy used for coating rebar. If i remember it was powdered perhaps electrostatic. However, that was production oriented, just sand blast and epoxy coat.

On a big enough scale or a reasonable time line these probably aren't that big of deal. When they design steel boats rusting is assumed and designed in.

Just a quick shot to tide you over ; )

[walter]

Well, I gave up on concrete long time ago- this Hoover Dam situation is scaring the hell out of me lol..

But I guess I still like to hear the other side of the argument, some people might actually use that in their projects.