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Thread: Epoxy-Granite machine bases (was Polymer concrete frame?)

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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Quote Originally Posted by peteeng View Post
    Hi All- To summarise my research and practicum so far on EG:

    The optimal grading theory using 3 or 4 or 5 sizes of particles has been thrown out. This is a hangover from concrete that uses large rocks that have large gaps. You may get small pockets of good interspersion but generally its all compromised by viscosity and improper settling.

    Use a single small diameter grit and you will get a better pack and a better result. Use a very thin epoxy. Traditional hand layup epoxies are too thick to penetrate, too thick to flow, too thick to degas, too thick to mix & did I say too thick?just too thick! for many reasons...

    Peter
    Hi Peteeng,

    I am curious about your above statements. I am planning an epoxy machine build and no doubt using a single diameter grit will make things much easier. However, commercial machine builders still seem to be using multiple grit sizes so I was wondering if you have any references to back this assertion up.

    Thanks,

    Dave.



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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hi Dave - Yes go back through the thread. There is a recent document on concrete mixtures somewhere (I did a search and attached it here) . Plus experience with another poster who did many experiments. Get a very thin epoxy designed for infusion so it fills easily. Do tests with your mixes with water to determine the actual free volume in the mix and keep it simple. Peter

    summary - most epoxies people use are for laminating and have thixotropes in them designed to stop the resin from flowing. So when you vibrate it it actually stops flowing. If you use grades and vibrate it seperates into zones. It does not tend to become homogeneous. Use a single small grain and use an epoxy thats intended for infusion very thin and no thixs. In fact if you want to do it the easiest way look up infusion and do this. Dry pack the mould and use the vacuum to draw the resin through the job. That's what I do. Peter

    Dave - concrete grading is done for reasons of strength and transport and pumping etc. In a machines case you are trying to produce a stiff material in the shape that you want. If we call it EG as a general term. EG is hard to get past 30GPa stiffness. Companies do publish their figures and 35 is probably the top figure. Some do quote higher but there numbers conflict so I expect numbers like 45 are the compressive values not the tensile values. One reason EG is damp is its internal hysteresis due to its different tensile and comp modulus. For the home builder they need a simple stiff result. Using a very thin epoxy solves many problems. Using a single grit solves many problems. Read this thread backwards it has many answers in it for you. Peter

    Attached Files Attached Files
    Last edited by peteeng; 06-07-2020 at 05:56 PM.


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hi Peteeng,

    You advice deviates strongly from the body of experience in this technology collected within the german machine tool industry with respect to single grain size verses graded aggregate . It also conflicts with presentation from Koehler you posted as advocating a single grain size.

    As it states on page 16 of Koehlers presentation:
    1/ Select your maximum aggregate size (note: in the german reference below, that should be ~1/5 of the smallest feature wall thickness in your mould)
    2/ Select combined aggregates.
    3/ ...

    There is nothing in Koehlers paper to support the use of a single grain size. The summary on page 21 clearly refers to particle size distribution.

    That Köhler presentation is very much in line with the common body of knowledge. For example page 18, the benefit to packing density of rounded alluvial particles verses poorly shaped crushed particles

    In Post #94 I summarised a book (ISBN 3-478-93273-4 "Mineralguss für den Maschinenbau") published which collects that experience of the german machine tool industry.
    https://www.cnczone.com/forums/epoxy...tml#post245588

    Properly graded aggregate with the correct (low) percentage of epoxy is a pretty horrible material to get to fill a mould. You need a lot of tuned vibrational energy to get it to slump and flow.

    While it is attreactive to make things easier by using only fines and lots of epoxy to make moulding easy, and may end up with a machine base acceptable for a specific purpose, this is not the path to optimised E/G.
    Mark

    Regards,
    Mark


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hi Mark - There are many roads up the mountain and your welcome to take whatever road you need to. Home builders need a simple solution. I don't advocate vibration mixing as I use vacuum infusion to fill dry stacked moulds. Many laminating resins are non Newtonian (as they have thixotropes added) so get thicker with agitation. That's one reason people have trouble with this approach. I have been infusing many fibres, particles and materials over decades including testing these for strength and stiffness. I'm only interested in stiff easy to do solutions. Everyone will come to their own conclusions and their own solutions, in the end they generally work for them. Just because a whole industry does something a particular way for a long time does not make it the best way. That's how a change can disrupt a whole industry (film cameras vs digital, subtractive machining vs additive, valves vs transistors, steel bicycles vs aluminium vs carbon fibre, cast iron machine parts vs composite parts, disposable rockets vs landing them back on the launch pad etc etc) I'm totally open to any suggestions or commentary so your points are taken, thanks for sharing. If this is not the way, what way is?? Peter

    Mark if you dry stack a mould with whatever particle mix you like you should be able to pour a thin epoxy into the mould and fill it. If you use fines it does not need "lots" of epoxy. If you dry fill a container with the candidate mix on a scale so you know the volume and weight of the mix then fill that mix with water you will be able to calculate the volume void ratio of the system. Hence you will know the correct weight ratio to add epoxy to the mix or compare mixes. Someone here has done that and they have made all sorts of graded mixes and got similar volume ratios with many types of mixes so why make it complex? I'm more interested in the material stiffness's and the resulting bulk stiffness then the packing ratios Peter

    Last edited by peteeng; 06-09-2020 at 07:39 AM.


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hi Mark - I looked up your summary in the whole I agree. I do not agree that an EG part will be 3.5x stiffer then a CI part of same dimension. But moving along from that the EG debate seems to concentrate on the grading of the material. The grading is a second order issue. The main issue is to get a high stiffness result. We use EG for its casting ability ie to achieve a shape we want , its dampness eg many times damper than a metal, and the potential to save $$$. Which may or may not work out. The particle stiffness is very important and over rides the packing. If you use silicon sand eg beach sand then its stiffness is 70GPa. If you use Aluminium oxide sand its stiffness is about 300GPa so is significantly stiffer. The least packing factor you will get is 60% and the best maybe 90% (which I think is not achievable in practice) is only a 30% gain in stiffness. If you use steel shot then its stiffness is 200GPa so is much better then sand. Using 60% packing factor sand will get to 21GPa, using 90% its 31GPa. Both of these figures are in the realm of published EG figures. So use steel shot and you get to 60GPa which is well above published figures and if you use alumina you get 90GPa well well above usual EG and well above aluminium.

    So how do you figure out the modulus of your mix? You make a long thin test piece and buckle it then back calculate the modulus. I did this with a piece of aluminium to prove it can be done. The test predicted the Al to be 70GPa which is correct. See attached.

    Epument publish E=40-45GPa in compression. A compression test will produce a higher stiffness then a tensile test as the matrix elements can push up against each other. A tension test is better as the load has to transfer thru the matrix.

    The resin ratio added to a mix depends on the packing factor and matrix density. If using sand at 60% volume you need 27% resin by weight. If using steel shot at 60% volume you need 8% by weight, if you want a zero void result. But its fine to add less and have some air in the mix, this saves $$$.

    Composites have lots of potential for the home builder. I have been involved in composites for some decades and its the way I want to go in the next couple of machines. To point something out. If you use infusion and make fiberglass cloth parts you will make parts just as stiff as most EG, about 30GPa. This takes away much of the issues of bulk mixing and transferring a mix to a mould. If you use carbon fibre you will get over 60GPa stiffness and if your clever 80GPa. You don't need to make monolithic parts as the load is born by the part skin. Thats why we use hollow parts. Then back fill for mass and extra damping. Hope this info helps. Peter

    ps roundish particles have a 50% efficiency of strain transfer in a matrix, long fibers and rods are 100% in the long direction, roundish figures

    Attached Thumbnails Attached Thumbnails Epoxy-Granite machine bases (was Polymer concrete frame?)-modulus-jpg   Epoxy-Granite machine bases (was Polymer concrete frame?)-compression-jpg   Epoxy-Granite machine bases (was Polymer concrete frame?)-resin-ratio-stiffness-jpg   Epoxy-Granite machine bases (was Polymer concrete frame?)-grit-1-jpg  

    Epoxy-Granite machine bases (was Polymer concrete frame?)-grit-2-jpg  
    Attached Files Attached Files


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hi Pete,

    The test peices I did with 8%, 10% and 12% epoxy using Thomas's Fuller curve spread sheet, on four grades of granite aggregate from a local building suppliers + very fine powdered aluminium oxide, and a laminating resin peaked at just under 50GPa from memory (I posted it in this thread somewhere). I thought a big advantage of using 1/5 min feature largest granite aggregate is that this fills a large part of the mould with solids with zero void, and therefore zero epoxy. Also rounded granite aluvial pebbles are really dirt cheap. I posted a cost break sown on such as mix, and aggregate cost basically nothing, except the powdered aluminium oxide and epoxy driving the final cost.

    Drawing in a very thin epoxy under vaccuum sounds like a great plan, but outside my capablities.

    I see guys sort of taking your advice, but then simplifying it to sand plus 20% laminating epoxy, and getting easily cast parts, which look great, but have a stiffness down closer to the ~2.5GPa of a block of epoxy rather than the 40GPa acheivable with a little extra effort. Then again, on many small machines like this tool grinder, that is probably fine.
    https://www.model-engineer.co.uk/for...p?th=51617&p=3

    Mark

    Regards,
    Mark


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hi Mark - How did you determine the 50GPa stiffness? Peter



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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hi All - I've read back through a lot of this huge thread and infusion was discussed in 2007. There is a lot of discussion on creep and curing of epoxy. Any questions on these please ask. There are two types of time dependant change of shape in plastics. Creep and Relaxation. In machine parts if the epoxy is correctly post cured I'd expect neither of these mechanisms to be of concern. I did a creep/relaxation test over a two year period on a bolted joint and could not detect a change. The stress/strain in that joint was quite big. Strain in machine parts is quite small. Gravity loads would be the concern for creep but as the machine is designed for stiffness with large service loads the gravity strain would be very very small. Peter



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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hi Pete,

    Made test bars, fixed one end, loaded them, measured deflection and calculated the youngs modulus. The quality of my test results would be pretty low due to errors in homegenity (insufficient vibration), dimensional errors (wooden mould), deflection in the fix end mounting (clamped test bars to the surface plate), measuring errors (used a DTI to measue deflection). I think they ranged between low 30 - high 40's.

    Somewhere buried in this thread there are photos.
    Okay, found them...
    Turns out I only did two, I though it was three...
    https://www.cnczone.com/forums/epoxy...ml#post1663212

    https://www.cnczone.com/forums/epoxy...ml#post1652212

    https://www.cnczone.com/forums/epoxy...ml#post1653506

    Regards,
    Mark


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hi Mark - Your links don't work, they go to recent threads. Any clues for search words? Peter



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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Quote Originally Posted by RotarySMP View Post
    Hi Peteeng,
    You advice deviates strongly from the body of experience in this technology collected within the german machine tool industry with respect to single grain size verses graded aggregate . It also conflicts with presentation from Koehler you posted as advocating a single grain size.
    I see after several years of periodically visiting this forum peteng still doesn't let the facts get in the way of gushing out a new load of bs......... and then he keeps flogging his dead horse until people just give up arguing. Then his bs sits on the forum where people will think it is correct information (it's not). I commend you for taking the time to dispute it in this instance.

    Single grain size, especially small grains, is a stupidity on so many levels. Not in the least the enormous increase in required resin content with % increase of small grains.

    But, I wager that peteng will not let it go............

    Jonathon Clarke
    www.solpont.com


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Ah the Axeman is back - Good to know your OK over their in NZ. Jono if the packing factor (solids ratio, volume ratio etc) is the same then the void is the same, ie the resin content is the same irrespective of the size of the particle or fibre. BS or not up to you, do the math. Peter



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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Those links work for me. They are thread posts #4857, #4855 and #4865 on pages 243 and 244.
    Mark

    Regards,
    Mark


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    Default Re: Epoxy-Granite machine bases (was Polymer concrete frame?)

    Hey you plywood machine builders out there. Here's a German manufacturer that uses plywood and polymer concrete.

    https://www.cnc-holzfraese.de/

    Peter



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