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  1. #1
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    Dec 2018
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    Epoxy granite but graphite?

    I've been trying to design a diy cnc machine that is able to mill steel. After a long time of looking at other designs, both from commercial and diy machines, I've seen a lot of good things about epoxy granite. Then I started thinking about the use of graphite powder instead granite pieces and wondered whether they would come out with similar or even superior qualities. The reason I say this, is because carbon fibre is made in a very similar way, well....using carbon fibres. Carbon fibre being a very rigid material, I figured it would serve as a good replacement. There's probably a million reasons why this is a stupid idea, but I haven't come across any builds that confirm this so I figured I would bring it to the smart people. Is there any specific reason that substituing graphite powder in for granite might be a bad idea? (Also don't hate pls I'm a 16 year old highschool student with absolutely zero education in physics, at least for now, as I am considering going into mechanical engineering.)
    Last edited by yoobsterlol; 02-25-2019 at 09:05 AM.

  2. #2
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    Jun 2018
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    Re: Epoxy granite but graphite?

    Welcome and great to see enthusiasm!

    Perfect opportunity to research the physical properties of granite and carbon based composite materials. You'll need to find out about density, vibration, rigidity and the ways these physical properties apply to the mechanical aspects of mechanical motion based machines. Look at the forces applied in 3 dimensions (or more) and how to addresses those. How to move mass around (and stop it).

    Other experienced builder folks here can chime in and provide some feedback.

    Great project with a practical goal/application in mind.

    Good luck!

  3. #3
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    Apr 2004
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    5728

    Re: Epoxy granite but graphite?

    I don't think that graphite powder plus epoxy will be any stronger than epoxy and any other kind of powder. The physical structure of the granite, optimally graded from coarse to fine with a minimum of glue holding it together, is what gives epoxy-granite its strength. Carbon fiber is different from graphite powder - due to its linear structure, it does add tensile strength to materials it's mixed with. But don't take my word for it - mix up some test batches of graphite and epoxy and granite and epoxy, cast them into bar shapes, and then try breaking them with compressive forces. See which one breaks easier.
    Andrew Werby
    Website

  4. #4
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    Quote Originally Posted by awerby View Post
    I don't think that graphite powder plus epoxy will be any stronger than epoxy and any other kind of powder. The physical structure of the granite, optimally graded from coarse to fine with a minimum of glue holding it together, is what gives epoxy-granite its strength. Carbon fiber is different from graphite powder - due to its linear structure, it does add tensile strength to materials it's mixed with. But don't take my word for it - mix up some test batches of graphite and epoxy and granite and epoxy, cast them into bar shapes, and then try breaking them with compressive forces. See which one breaks easier.
    See, I can totally understand the reason why carbon fibre has the properties is does(very similar to wood grain structure for example), but the exception I'm seeing with graphite and carbon powder in general, is that it is already used to make very stiff objects, in the form of carbide tooling, with this epoxy graphite mixture using epoxy as the matrix as opposed to cobalt or something like that. While of course I can't expect something held together by epoxy to come close to carbide tooling, I still feel like there must be something there to go off of. I will take your suggestion of making some test pieces, who knows, maybe I just created a super material ������

  5. #5
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    Apr 2004
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    Re: Epoxy granite but graphite?

    Yes, carbon is a component of tungsten carbide (and diamonds), but that doesn't mean that adding carbon to things always makes them harder. Making some test bars and destroying them will give you more visceral insight into all this than any amount of explanation online.
    Andrew Werby
    Website

  6. #6
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    Dec 2018
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    Quote Originally Posted by awerby View Post
    Yes, carbon is a component of tungsten carbide (and diamonds), but that doesn't mean that adding carbon to things always makes them harder. Making some test bars and destroying them will give you more visceral insight into all this than any amount of explanation online.
    I'll definetely try to do some tests and see what happens. My understanding with the use of granite is that larger pieces have less surface area than if the same area were to be filled with powder, aiding in having the lowest amount of epoxy as possible, and of course a large piece of graphite would be quite brittle so that wouldn't work so well. I hope I don't come across sort of a know-it-all, I'm just trying to absorb as much knowledge from people as possible, and find out if any information I have already is accurate.

  7. #7
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    Nov 2013
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    Re: Epoxy granite but graphite?

    Hi,
    graphite is carbon in the sp2 hybridized state. In that state it bonds to three other carbon atoms forming sheets. The only forces between sheets
    holding the graphite solid together are Van der Walls forces. Consequently graphite is weak and brittle.

    Diamond on the other hand is carbon in sp3 hybridized state and bonds with FOUR other carbon atoms in a 3D lattice and is strong.

    Carbon fiber is graphite but is aligned 'in the plane' of graphite or if you like a 'sheet cut into long narrow strips'. While graphite might
    be weak between adjacent sheets a strip pulled lengthwise is strong.

    When a carbon fiber composite is made the strength of the product is high in the direction that the graphite strips or ribbons are laid
    but weak in other directions. To make a strong product requires carbon fibers must be laid in all directions.

    One of the great challenges of making fiber reinforced products is to minimize the resin relative to the fiber, the fiber being strong but
    the resin not. A hand lay up is unlikely to achieve a fiber to resin ratio of better than 3:1. Even achieving that becomes harder and harder
    as the number of different fiber directions increases. So much so that if you chop the fibers into short lengths and mix it with resin the ratio
    of fiber to resin is likely to be 1:2, that is more resin than fiber, sometimes called 'fiber filled plastic'.

    Were you to attempt to make a machine that way the cost of the resin would soar. I realize you are proposing to use graphite rather than
    carbon fiber but the probability of you achieving a castable mixture of less than 30%-50% resin is slim. Even that would count against
    the economics of the technique.

    Where granite-epoxy comes into it own is a graded series of sizes granite down to granite sand. Then resin to granite ratios of 1:10
    have been reported.

    If you could take solid graphite and crush it into a graded mix of little chunks right down to sand sized grains you might achieve a reasonable result.
    But then how does that differ from making a graded mix of granite?

    Craig

  8. #8
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    Dec 2018
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    Quote Originally Posted by joeavaerage View Post
    Hi,
    graphite is carbon in the sp2 hybridized state. In that state it bonds to three other carbon atoms forming sheets. The only forces between sheets
    holding the graphite solid together are Van der Walls forces. Consequently graphite is weak and brittle.

    Diamond on the other hand is carbon in sp3 hybridized state and bonds with FOUR other carbon atoms in a 3D lattice and is strong.

    Carbon fiber is graphite but is aligned 'in the plane' of graphite or if you like a 'sheet cut into long narrow strips'. While graphite might
    be weak between adjacent sheets a strip pulled lengthwise is strong.

    When a carbon fiber composite is made the strength of the product is high in the direction that the graphite strips or ribbons are laid
    but weak in other directions. To make a strong product requires carbon fibers must be laid in all directions.

    One of the great challenges of making fiber reinforced products is to minimize the resin relative to the fiber, the fiber being strong but
    the resin not. A hand lay up is unlikely to achieve a fiber to resin ratio of better than 3:1. Even achieving that becomes harder and harder
    as the number of different fiber directions increases. So much so that if you chop the fibers into short lengths and mix it with resin the ratio
    of fiber to resin is likely to be 1:2, that is more resin than fiber, sometimes called 'fiber filled plastic'.

    Were you to attempt to make a machine that way the cost of the resin would soar. I realize you are proposing to use graphite rather than
    carbon fiber but the probability of you achieving a castable mixture of less than 30%-50% resin is slim. Even that would count against
    the economics of the technique.

    Where granite-epoxy comes into it own is a graded series of sizes granite down to granite sand. Then resin to granite ratios of 1:10
    have been reported.

    If you could take solid graphite and crush it into a graded mix of little chunks right down to sand sized grains you might achieve a reasonable result.
    But then how does that differ from making a graded mix of granite?

    Craig
    This is actually some really good information, thank you. I wasn't particularly set on the use of graphite or anything, mostly asked to find if anybody has done anything with it before, and it's apparent that there are some serious issues that would be present with the use of graphite. I think it would probably be best to use graded aggregate considering the information I have been given. Me being a hobby luthier has taught me the importance of having as little adhesive between the pieces as possible, and it seems that it's the same for epoxy granite. I really do appreciate the information, there's more in this one reply than what I've been able to find on my own for the past 2 weeks(although I've spent more time modeling my machine rather than researching frame materials).

  9. #9
    Join Date
    Mar 2003
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    35538

    Re: Epoxy granite but graphite?

    The strength in carbon fiber comes from the fibers. Graphite powder has no fibers, and hence, no strength at all.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

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    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  10. #10
    Join Date
    Mar 2019
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    1
    Quote Originally Posted by joeavaerage View Post
    Hi,
    graphite is carbon in the sp2 hybridized state. In that state it bonds to three other carbon atoms forming sheets. The only forces between sheets
    holding the graphite solid together are Van der Walls forces. Consequently graphite is weak and brittle.

    Diamond on the other hand is carbon in sp3 hybridized state and bonds with FOUR other carbon atoms in a 3D lattice and is strong.

    Carbon fiber is graphite but is aligned 'in the plane' of graphite or if you like a 'sheet cut into long narrow strips'. While graphite might
    be weak between adjacent sheets a strip pulled lengthwise is strong.

    When a carbon fiber composite is made the strength of the product is high in the direction that the graphite strips or ribbons are laid
    but weak in other directions. To make a strong product requires carbon fibers must be laid in all directions.

    One of the great challenges of making fiber reinforced products is to minimize the resin relative to the fiber, the fiber being strong but
    the resin not. A hand lay up is unlikely to achieve a fiber to resin ratio of better than 3:1. Even achieving that becomes harder and harder
    as the number of different fiber directions increases. So much so that if you chop the fibers into short lengths and mix it with resin the ratio
    of fiber to resin is likely to be 1:2, that is more resin than fiber, sometimes called 'fiber filled plastic'.

    Were you to attempt to make a machine that way the cost of the resin would soar. I realize you are proposing to use graphite rather than
    carbon fiber but the probability of you achieving a castable mixture of less than 30%-50% resin is slim. Even that would count against
    the economics of the technique.

    Where granite-epoxy comes into it own is a graded series of sizes granite down to granite sand. Then resin to granite ratios of 1:10
    have been reported.

    If you could take solid graphite and crush it into a graded mix of little chunks right down to sand sized grains you might achieve a reasonable result.
    But then how does that differ from making a graded mix of granite?

    Craig
    You have explained well Craig. Just learn lots of things from your post. And also learn about the best driveway sealer from this post Best Driveway Sealer Reviews: 2019 Most Popular List. Useful information about sealers.
    Last edited by zentork; 05-24-2019 at 11:48 AM.

  11. #11
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    Jul 2018
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    6254

    Re: Epoxy granite but graphite?

    Hi Craig - In your post you state that its unlikely to achieve better than 1:2 that's 1 kg fibre to 2kg of resin. Most of the people that I deal with use stitched cloth and they get 1:1 by hand or better. One guy regularly runs at 1kg fibre to 800-900g of resin. Thats with rollers and brush not vacuum bagged. If you use infusion you can get down to 28-30% resin by weight. Cheers Peter S

  12. #12
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    Nov 2013
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    4282

    Re: Epoxy granite but graphite?

    Hi,
    if you read my post you will see that that applied to chopped fibers. There is that much resin it becomes a 'fiber filled plastic'
    Hand lay ups of fiber cloths and rovings are better and 1:1 ratios are common. Using prepreg cloths and rovings you can get
    ratios of 3:1 or better, ie approx. 25% resin. Vacuum compaction is usually required.

    For OP to get 10% resin ratios will require graded filler, be it granite or graphite. Graded chunks of graphite are likely harder to
    obtain than granite.

    Craig

  13. #13
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    Nov 2017
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    591

    Re: Epoxy granite but graphite?

    One thing to add, if you cast the test blocks, don't just add weight until they fail and record the force needed for failure. In this case I don't think that data is very relevant. What you want is force vs deflection. Slowly add weight and record deflection, preferably with a dial indicator. Graph out the points and compare the graphs of the different castings. I don't think failure point is all that important for this application.

    I have also considered composites in a machine, but not powder or chopped. actual carbon fiber layup. For a base structure that doesnt need to move, I don't think there's any reason to use anything other than epoxy granite, but for special machine layouts where you need extreme strength in moving structures, I think a carbon fiber layup could be beneficial to get the best strength possible and keep weight down. Another added benefit is thermal expansion which is basically zero with carbon fiber.

  14. #14
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    Jul 2018
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    Re: Epoxy granite but graphite?

    Hi Yobster - Granite is used because its available and its often used in monolithic form for precision machines. It's bulk modulus is about 50GPa some say 10-70GPa. Now epoxy is 3.5GPa so you need to use a high volume ratio of granite or anything else for that matter to create something stiff. Something to look into is steel metal fibres. They are increasingly being used in concrete reinforcement. Since they are 200GPa stiffness you need less to get to a reasonable bulk stiffness. These should be available through your local concrete supplier or they should know where to get them. They also should be cheap as they are mass produced for concrete. I did a quick search and there's lots of info on it so should be straightfwd to track down. eg

    https://www.bekaert.com/en/products/...-reinforcement

    There are plastic fibres as well but these are same stiffness as epoxy so not good for what you want. Plastic fibres are used for crack control, steel fibres are used to make structural slabs without steel mesh.

    If you pack the fibres into your mould very well and get say a 40% fibre volume ratio (you can figure this out with a small test mould) you will get 0.4x200=80Ppa which is stiffer then aluminium at 70Gpa. If you pack really well I'd expect to get to 50%Vf. Steel is the same stiffness as carbon fibre yet its much less cost. Moulds can be made from formply and use PVA as a release PVA is polyvinyl alcohol not PVA glue which is polyvinyl acetate and should be noted as PVAc. In fact if steel wool is cheap that's just as good!! So an epoxy steel composite will be quite damp and very stiff. Good Luck. Peter

    Oh yes - epoxy. Do not get a laminating epoxy they have thixotropics added. Which means they are thickened so the resin does not sag when you laminate on a vertical surface. You need infusion or pouring resins for benchtops that are clear. These will not need vibration as they will pour nearly like water. If you warm the mould surface and the steel wool through with a heat gun prior to pouring it will release bubbles easily. If you have access to vacuum pumps then you can put a vac bag over your job and it will fill perfectly as you can remove all the air and moisture from the mould prior to the pour. Use the vacuum to suck the resin in. Look up infusion it's a great process.

    If there is a machine shop nearby you could use the steel swarf. But it will be sharp and have oil on it so you would have to wash it first. Don't use the family washing machine for this!! Peter

  15. #15
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    Re: Epoxy granite but graphite?

    Hi Yobster - Granite has a modulus of around 50GPa. (Some say 10-70GPa) Epoxy is 3.5GPa so you need to have a high volume ratio of granite or any other stiff material to get a high bulk modulus material. Granite is used because you can get it in size grades and there is a ratio of grades to create a close pack arrangement. But I suggest you look at steel fibres used in concrete. Steel has a modulus of 200GPa so if you achieved a 40% Vf you would have 0.4*200=80GPa material stiffer then aluminium at 70GPa. You could also look at using steel wool. Easy to use friendly fibres and if you can get it at a good price then your away.

    https://www.bekaert.com/en/products/...-reinforcement

    use formply to make moulds and use PVA from the concrete guys as a release agent. Do not use PVA glue!! PVA glue is polyvinyl acetate and should be written PVAc. PVA is polyvinyl alcohol and is water soluble so it's easy to release things and is easy to wash off. Again the concreters use tonnes of this stuff so should be easy to get.

    If you have a machine shop nearby you could use the swarf or chips if its free. But it will be sharp and oily. So will need to be washed. If its from a CNC machine it will be in small strings not long strings ideal. But do not wash it in the family washing machine, guarantee that's trouble. Good luck. Peter



    sorry about the double up, first one didn't appear for some time so I thought it was lost.

    Oh yes Epoxy - Do not use a laminating epoxy as these are thickened so it does not sag on non horizontal surfaces. Use an infusion or pouring grade epoxy that has ZERO thickeners (thixotropics) . This will pour into and around your fibres and will only need a few bumps vs lots of vibration. If you have a vacuum pump you could infuse the mould. Look up infusion its a great process. if you go this way use a heat gun to warm your fibre stack and mould surfaces a little bit before the pour, this will help it flow and release bubbles.... again good luck

    On the subject of graphite it is often added to epoxy to make bearings, it is soft and slippery so I think its out for your project.

    Sorry for double up, was slow to come up and I thought it was lost....

    But I think that reconstituted steel from swarf is very worthwhile to pursue. Need a catchy name for it "newsteel" or Eswarf or Eric or rustystuff ...

  16. #16
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    Mar 2017
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    926

    Re: Epoxy granite but graphite?

    I just finished a CNC build using carbon fiber and epoxy granite. As far as composites go, it is an excellent combo for machine bases. Both have excellent properties (for milling) individually and they work even better together. The end result (if designed right), is incredibly strong, stiff and with better vibration damping properties than iron, steel or aluminum.

    While some people use the terms "graphite" and "carbon fiber" interchangeably, they are very different. For a good idea of graphite properties, think pencil lead. I.e. It's brittle and weak with none of the desirable properties for CNC that carbon fiber, has.


    Btw With carbon fiber, the type of lay-up has no relevance to the fiber to resin ratio. A hand lay-up just means you are laying carbon fiber in a mold, by hand. A hand lay-up is just a common when using prepreg.


    For applications where the ratio of resin to solids is important, relative precision is easily achieved by weighing out the resin and solids beforehand. I regularly do this when making my own pre-preg.

  17. #17
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    Feb 2005
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    2

    Re: Epoxy granite but graphite?

    Goemon, can you provide any information on the ratios of epoxy/carbon fiber/granite that you used for your CNC build ? I'm interested in a similar project.

  18. #18
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    Sep 2015
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    Re: Epoxy granite but graphite?

    Graphite is soft.

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