Big Epoxy Granite machine, Which frame is better ?

[EZELab]

Hello !
I am in the design phase of my new CNC machine. I am aming to 120x60 cm working area, And most of my milling will be in Aluminum, Brass, And some steel!
The frame will be epoxy granite and a welded laser cut steel skeleton inside.
I prototyped 2 design to choose from, Here's my 2 prototypes
28848

I really like the U base design (#1) its easy to enclose & Chip control is great.
But, Is the design #2 stiffer?
Which one should I go to ?

Thank you all in advance (:

[Voicecoil]

I can't help but think that you would get better results with both designs if you rotate your working area by 90 degrees to minimise the gantry length.

[EZELab]

I can't help but think that you would get better results with both designs if you rotate your working area by 90 degrees to minimise the gantry length.

Thank you for your addition. I'll think about this.

[pippin88]

Why put a steel skeleton inside?

If the steel is for stiffness it is better to put it outside. Material at or close to the neutral axis does very little for stiffness.

Save the money you would spend on laser cut steel and put it into more epoxy granite for bigger sections.

[EZELab]

Why put a steel skeleton inside?

If the steel is for stiffness it is better to put it outside. Material at or close to the neutral axis does very little for stiffness.

Save the money you would spend on laser cut steel and put it into more epoxy granite for bigger sections.

Are your sure about that ?
Sure I'll put steel outside for screwing and rails, ballscrews .. etc.
Epoxy granite alone is that stiff for a large machine ?

[phill05]

Why put a steel skeleton inside?

If the steel is for stiffness it is better to put it outside. Material at or close to the neutral axis does very little for stiffness.

.

How do you arrive at that thinking? most things have the skeleton inside for strength of the main structure and appearance there would be very little point of having steel outside of Epoxy granite other than to cut down on resonance of a steel box frame you could fill the box with epoxy.

Phill

Edit: I would be tempted to go for No2 image

[EZELab]

Edit: I would be tempted to go for No2 image

Thank you for your replay.
Yes, design #2 have many Advantages, Easier to align, Easier to machine flat, uses 3 motor not 4.
But will it add any more stiffness more than the #1 design ?

[phill05]

I believe it will, a fixed gantry has got to be more stable than a moving mass gantry, the only thing is you need more room for the moving table.
I have a fixed gantry and no problem at all.

Phill

[Zorbit]

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[phill05]

Pippin is correct. That's why a piece of box section steel is stiffer than the same amount of steel in solid rod form. It's a cube law - the stiffness is proportional to the cube of the thickness.

Beware of mixing materials with such differing properties, you may end up with a structure that performs worse than either would on their own.

Did you not read the OP's post "The frame will be epoxy granite and a welded laser cut steel skeleton inside" you would not cast an epoxy granite frame then clad it outside with steel Box section Box section was not questioned totally different build.
Come on!!!

Take a look at this project: https://www.model-engineer.co.uk/forums/postings.asp?th=139042

Phill

[Zorbit]

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[mekanik]

Hi EZELab
Welcome to the forum.
I would like to point you to John McNamara's epoxy build( https://www.model-engineer.co.uk/forums/postings.asp?th=139042)
A very thorough and informative thread.
Kindest Regards
Mike

Sorry Guys didn't see the last couple of posts, was trying to find the link

[m_c]

From the various builds I looked at while researching ideas, using an internal skeleton to essentially hold the key mounting surfaces in place was a method quite commonly used.
However my concern was if the skeleton was too rigid, would there be any long term issues with the different materials and changing temperatures. It's not likely to be an issue on small builds, but over a 1m length, the internal stresses could get quite high.

The common thing I found, was although different people used different methods, I never found any sort of academic/industrial papers that gave much practical scientific answers. It was more a case of people used what they thought would work for them.

Personally I was swaying towards using embedded mounting surfaces that could be machined after casting, as it would be near impossible to keep accurately positioned surfaces in position within a large casting, however epoxying surfaces onto the main casting after curing can also be done (Chaz used this option on his big build which can be found on here).

John's build was also one worth looking at, as he did do a lot of research, but the basics of his build did still come down to what suited him.


Even looking at the commercial options, from what I gleamed from various sources, was they all seemed to have their own proprietary epoxy/granite mix, and seemed to use different methods for final surface accuracy, ranging from surfacing the cast EG, to embedding machinable rails. The techniques used all seemed to come down to what equipment they had available.

[JAZZCNC]

Personally I think a structural skeleton with the intention of adding strength is wasted money as it could have negative effects long term and interfere with resonances which is the main reason for epoxy granite. If the skeleton is to hold mounting plates etc and was a minimal wireframe affair then that's slightly different.

Where I think most go wrong with Epoxy builds is not putting enough effort into the molds, I would Save the money for laser cut steel and put it into a better quality and strong mold which would hold mounting fixtures, etc and allow for good vibrating. This is how the Big boys do it.!

Regards the Moving gantry or fixed then from the 2 you show then I'd say the Moving is the better stronger option. The fixed Gantry model has too much overhang on the table and when cutting at the outer edges you'll get vibrations.

If you support the table better along its full width then the fixed Gantry is by far the stronger choice but not how you have it drawn.!

[EZELab]

Hi EZELab
Welcome to the forum.
I would like to point you to John McNamara's epoxy build( https://www.model-engineer.co.uk/forums/postings.asp?th=139042)


Thank you (:
I checked it a couple of days ago, It's a great build, and learned a lot from him
Actually, I searched the forum for any "epoxy granite" related topic and checked what others did with their builds


Seems like the word "Skeleton" made some miss understanding here.
I am not talking about a big chunk of steel inside, It will be a waste of money.
I meant by skeleton is like any other EG build, some steel embedded inside the pour to provide some stiffness and mounting plates for stuff will be bolted on. Nothing more.
Here's a quick sketch for what in my mind.
28861

Something like this, a thick 10mm steel on front for mounting rails & other stuff, And some 4 mm steel support inside the pour.
Am I on the right track or not?

And about the overall process, It will be not easy, I Know!
I'll do my best in molding. My plan is cnc'ing the mold out of Melamine wood And lay them on an epoxy leveled flat floor.
I don't have any hopes to get the Mounting plates flat after pouring on a big size like this. The base will go to a big milling machine to get everything flat & Parallel. Which will be another challenge since the estimated weight of this base around 600~700 KG :grief:

I think I'll go with the fixed gantry design, I'll make the process easier than the other design



If you support the table better along its full width then the fixed Gantry is by far the stronger choice but not how you have it drawn.!
This was a fast drawing, And sure I'll make the table much more stable, Thank you.

If anyone has any more suggestions, tips, Please share it (:

Thank you All for helping

[jarjar]

Trying to stiffen an epoxy-granite structure with interlinked pieces of steel will only lead to issues due to the huge difference of thermal inertia. The steel will expand much faster than the EG.

Forget about the bracing. If after calculation the beam is not stiff enough to your liking, increase the cross-section.

[Zorbit]

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[EZELab]

Thank you guys, Got it.

But, In John McNamara's build he put a LOT of enforcement inside, Like this base photo.
28865
I saw a lot of EG builds, most of them puts some kind of enforcement inside.
Is there some way to do it right, Or should I just put plates on the surface for bolting ?
Could someone please explain?

[Zorbit]

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[jarjar]

Indeed many hobby people put reinforcements in their EG structure. Probably because they don't really know the material and treat it like a regular concrete.
Look at how the big guys do it. Studer/Granitan, Durcrete/Nanodur, Rampf/Epument, ... You will never see reinforcements in their castings.

Only place inserts where needed to bolt things on.

[m_c]

I can't help feeling that the thermal expansion of epoxy concrete, typically twice as much as steel, is going to tie that machine in knots. I'm happy to be educated otherwise.

This was something I failed to find any kind of definitive answer to.
The conclusion I drew, was it can't be that big a concern, given there are some very big EG machines with linear rails mounted to them, with no apparent allowance for expansion. So if it's not off any concern over a couple metres, I doubt I'd worry about it on your typical home build.

As for John's build, what I took from his internal structure, was it was more about keeping things in place during the pour/cast, rather providing much in the way of structural support. Remember in that photo, the outer steel plates are the actual mold and designed to be removed after curing, so they can be reused.

[Zorbit]

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[JAZZCNC]

I can't help feeling that the thermal expansion of epoxy concrete, typically twice as much as steel, is going to tie that machine in knots. I'm happy to be educated otherwise.

I don't think so.! Some of the best and most accurate lathes and Milling machine in the world use Epoxy granite because of it's thermal advantages and the fact they can build in extra mechanisms ie: running coolant thru the frame to increase this even more. Kern for instance and the Bigger DMG Mori's etc all use EG for there frames.

[pippin88]

Kern uses aluminium for the linear rail mounting I believe. They cool / thermally control all the mounting surfaces.

[Zorbit]

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[Voicecoil]

Being intrigued by this I went digging and found the thermal expansion coefficient. It will vary with the mix (more epoxy = more expansion it seems), but typical figures are 12...16 x 10-6/deg C. Tool steel came out as 11 x 10-6, so they're pretty close.

[Zorbit]

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[Voicecoil]

I found figures ranging from 12.5 - 28 for epoxy concrete. I'm guessing that the lowest figure is for carefully selected and graded aggregate and the best performing resin, although I can't find out which resin that is.

So, careful mixing would get pretty close as you point out. The next question is what difference would it make ? For example, steel/epoxycrete with a difference of 5um per deg C per metre, with a 10 deg temp change and a 1 metre linear guide means a length difference of 50um, not a lot. I don't know what effect that would have, perhaps someone with some engineering expertise can answer that. In my head I can see the bi-metal strips from school science class, they bent dramatically but the materials were carefully selected to do that.

One thing has become clearer to me - careful mixing is vital - the coefficient of expansion of neat epoxy can be 10 times as high as epoxy concrete, the mix needs to be very even.

The papers I read seemed to say that the major deciding factor was the % of mineral filler, and then the particle size distribution - you seem to need a fair % of small stuff to keep the minimum distance of epoxy small and fill in around any larger lumps. Some people were going as high as 90% mineral, albeit with a material which was little more than sand, hence would pack together tightly - the low % of epoxy would eliminate areas of overly-expanding undiluted epoxy I guess. Removing air bubbles is another issue - good luck with your build!