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IndustryArena Forum > Mechanical Engineering > Epoxy Granite > Epoxy-Granite machine bases (was Polymer concrete frame?)
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  1. #1601
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    Apr 2007
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    777
    I'll think about the foam core material tomorrow. Minimizing confusion, it's not actually modulus that you affect by putting the foam in the middle. Modulus for a given material is a constant. It's a property of a material, not the shape of a piece of it. Specifically, modulus is the ratio of stress (in psi) divided by the elongation or contraction of the material under that stress.

    What you get when you take two E/G plates in a beam that were sitting on top of one other and make them farther apart by adding foam in the middle is a change in something called the moment of inertia. The moment of inertia is a measure of how close the material is to the center of the object.

    The moment of inertia is one of the most important terms in the beam bending equation. Larry is right however that the farther away from the center of a beam a piece of material is, the more effect it has in helping to resist bending.

    This is similar to the calculation I did for brunog a while back in one of the pdf's I posted although I suspect there could be typo and I don't know the post number off the top of my head.

    Designing the proper shape for what you want to build is a function of what it is you want to build so without more details, it's tricky to be helpful. For a gantry that you want to be light, a hollow or foam core beam is great. For a machine base, the heavier solid might be more advantageous due to the extra weight to keep vibration down.

    Yours Sleepily,

    Cameron

  2. #1602
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    Dec 2006
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    6
    I use to run a Machine / Balance Shop for Rotors used in Intregated Circuits. The Balance Machine Base was the size of a coffin and made of the material you are inquiring about. Sorry, but I do not remember the name of the machine. Verteq in Santa Ana sold it to AMF Welding in Stanton.

  3. #1603
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    Quote Originally Posted by ckelloug View Post
    Wow Walter,

    With 327 pounds of weight on it, That's withstanding 566 psi of tension on the bottom! Is there anything special in that one?


    Also, that foam core construction idea is excellent for hollow structures. The foam itself doesn't likely add any strength but it is an easy way to make the part hollow.
    --Cameron
    Cameron,Walter,
    This was idea was brought back on post 447
    http://www.cnczone.com/forums/showpo...&postcount=447

    Larry,
    Have you accumulated enough beer cans yet?

    Best regards

    Bruno

  4. #1604
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    Quote Originally Posted by ckelloug View Post
    This is similar to the calculation I did for brunog a while back in one of the pdf's I posted although I suspect there could be typo and I don't know the post number off the top of my head.

    Designing the proper shape for what you want to build is a function of what it is you want to build so without more details, it's tricky to be helpful. For a gantry that you want to be light, a hollow or foam core beam is great. For a machine base, the heavier solid might be more advantageous due to the extra weight to keep vibration down.

    Yours Sleepily,

    Cameron
    Cameron,
    This is a sample design of a Gantry beam, would an L shaped design like this one be better than a hollow bottom rectagular shape?

    Best regards

    Bruno
    Attached Files Attached Files

  5. #1605
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    Bruno,

    I've looked at your proposed beam design though I did no calculations. This design would likely be respectable as far as vertical deflection but because the cross section in the middle vertical part of the L is so low, I suspect that it will be subject to deflection by twisting when a heavy cutting force is applied. Without knowing the shear modulus of the material and your definition for acceptable deflection, there isn't a way to calculate or determine whether it is bad or not.

    It looks interesting though and from the thought that want into the shape, I'd assume it has some wonderful benefits in terms of the cutting head traveling on it.

    --Cameron
    Attached Thumbnails Attached Thumbnails Gantry beam design test 1_dwg.jpg  

  6. #1606
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    Cameron,
    maybe I was a little shy in the information I gave out.

    This fixed gantry would be made out of cast E/G, 2 flat bars are embedded in E/G and anchored, originally i was thinking more of an I beam shape, however by adding width on the bottom part of the I is meant to increase strength to prevent horizontal deflection of the gantry when milling.

    The gantry would be bolted via 3 anchors 1 on the upper, 2 on the lower portion of the L. The circles on the sketch are actually holes to lighten up the structure itself.

    Maximum deflection with 100 lbs of z beam, motor and spindle would be .0002" or less.

    If the structure needs to be trimmed because of weight, I am sure I can change the bottom portion to reduced tota mass, however if you think that a major change in shape is needed, I am open for ideas.

    Thanks, best regards

    Bruno

  7. #1607
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    Jul 2006
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    1256
    Bruno,this may be to high tec for the posters and possibly should be a personal E-mail.I am embarrested,being 1/2 FrenchCanadian,I have only been able to accumalate 99cans.Americans think we do twofours every day,wear toques and flannel shirts and say Eh! a lot.This is entirelly true!Sorry I cannot live up to the Canadian tradition.Across the pond,Martin has "checked into the case".Perhaps he can,thats can fill us in the British progress, or should that read empties.Canadians have great knowledge of vacuum de-gassing or sucking back a few beers.
    Seriously,It seems strange for all the research to eliminate air entrapment in E/G and add foam which is entrapped air.Hollow E/G beams may and will have resonance which may dupilicate a steel tube.
    Larry
    L GALILEO THE EPOXY SURFACE PLATE IS FLAT

  8. #1608
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    777

    I shaped Gantry Beam

    Examining bruno's gantry, from his last post, It seems like an I or L shaped gantry beam made from E/G will be prone to twist when cutting force is applied since there is just a single skinny piece of material to resist twisting. AFAIK, I-beams are normally used in cases where there is minimal side loading which isn't the case in the gantry beam of a router.

    I've diagrammed the torque I am worried about which is caused when the router moves to the left against something it is cutting.

    Hope this isn't too distracting to the remainder of the discussion but I needed to attach the drawing which I couldn't do in a PM. Besides, I suppose even I should post something involving a router now and again.

    Regards to all,

    Cameron
    Attached Thumbnails Attached Thumbnails Gantry beam design test 1_dwg.jpg  

  9. #1609
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    May 2005
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    674
    Bruno, that's a cool looking structure but it does pose the twisting problem as Cameron pointed out, and IMO, it's impractical in two important ways.

    1.) The "extruded" shape would make sense in a mass production environment. Build a mold and then pour hundreds of em. But for one offs, you're wasting a whole lot of time for no noticeable benefits.

    2.) The 1" x 2" steel flat bars will need to be ground. This defeats the purpose of making an all-E/G structure. The whole point was to avoid having to grind, and rather rely on the flatness of the mold. If you're going to resort to surface grinding, you might as well use steel tube filled with E/G (which happens to have been my plan from day 1).

  10. #1610
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    In regards to foam core....

    I think it has a lot of drawbacks. First, foam adds zero strength and stiffness. If you're going to go through the trouble, why not use something that adds stiffness like some steel pipe?

    Second, when you hollow out a section of E/G, is the reduction in stiffness completely offset by the reduction in weight? It's a given that dynamic deflection will increase... lower moment of inertia = less resistance to dynamic forces. The question is will there be a noticeable reduction in static deflection due to gravity. I leaning towards "no". I'll do some calculations later using even load distribution beam deflection formulas.

    If you want to get rid of gravitational deflection, it's as easy as making the structure taller. Rather than a 6x6 square cross section, use an 8x4.5 or something similar. There is a "sweet spot" that can be calculated, dependent on your maximum accelerations and cutting forces.

    So in regards to shape and structure, I think the simpler the better. Make it rectangular and make it solid.

  11. #1611
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    May 2005
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    BTW, has anyone here seen E/G structures in person? What did they feel like? They sure look like solid granite.

    I'm thinking that the aggregate composition that Walter and others have been using is quite good. The problem may in fact be the epoxy we're using? I've got both West Systems and US Composites epoxies in my shop right now, and both seem to be a bit on the soft side, not rock-hard. They feel a bit plasticky.

  12. #1612
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    Hi Zumba,

    I'm interested to see what you come up with calculation wise. I plotted the Z deflection for a wide range of 4x10 through 4x16 rectangular geometries 36 inches long assuming 2000psi flexural modulus with a point load at the center and my general result was that deflection is minimized (static sweet spot) when the center is hollow and the sides are about 1-1.5 inches thick accounting for material weight. These geometries were chosen to come close to holding a deflection around .0001 inches which requires a 4x13 cross section although 4x12 is close and 4x10 is a touch better than .0002. Because the stress and modulus in beams of E/G of this cross section is so low, material that is away from the edges does almost no good in resisting bending moment.

    Making a solid structure taller might not be a good solution to deflection as casting a 4x12x36 beam solid is a cubic foot of material which is about all that one of us is likely to be able to pour in a sitting. From the plots I described above, deflection goes up as the rectangular tube approaches a solid bar.

    I agree with you that a square or rectangular beam is a better approach than an L or I shaped one due to the moments involved though bruno's beam is a sight to behold. Somebody (PlasticWorker perhaps?) also fairly recently pointed out that most homebrew machines fail to account for the rigidity of the vertical supports for the gantry and the ability to resist cutting forces. So I think paying attention to these moments and how the vertical supports and connections will do at resisting them is rather important.

    As for foam, it's a complex subject. It can either be used in a completely non load bearing way to create hollow sections or it can be used in a load bearing way to transfer shear between the stronger bearing surfaces on the outside of the sandwich.

    I wouldn't be too happy using it in a load bearing way in a machine because it probably invites deflection or deformation that is unwanted. It will also have lousy thermal expansion performance. It might be acceptable paired with E/G in a portable wood router but I'd rather not use up my error budget in a machine for metals unless the light portable nature was essential.

    As for the bars in bruno's design, they do need to be ground but it isn't particularly defeating. Precision ground bars are available from the Starret catalog and likely others such as McMaster Carr etc. If the bars are set in place parallel and at the right height with the proper metrology gear before molding, it may be possible that none of your molding for the beam has to be precision molded. It might be possible to make a steel jig to hold the bars in alignment for molding that could be removed after the epoxy has set.

    As for the plasticky nature of the us composites epoxy, most reichhold formulations with the stuff recommend high temperature post curing. I am just about to order some of this epoxy so I can't comment reliably until I've seen the stuff. I suspect there is probably a better epoxy somewhere but what will it cost? I guess I could go bug Huntsman Advanced Materials since they made most of the epoxy in the research papers we keep finding.

    Cheers all.

    --Cameron

  13. #1613
    Join Date
    May 2003
    Posts
    792
    I've seen some different types of resins, polyester for example (google epoxy system 680 or 682). Some have better properties (1.5-1.8 ksi tensile) but may shrink, require additives, could be extremely flammable, toxic, etc.
    Another words not worth the trouble.


    The E/G core I got from AkvaCNC feels like a real stone. It polishes like granite and has nothing plasticky about it.
    It's not "extremely" hard, but solid and very dense, with hard surface. The matrix is very powdery.. And I've seen some air bubbles (thermosetting?) and they're glossy inside.


    About Foam Cores..

    The article says that E/G bases are modeled after cast iron bases, with cavities and everything. It has nothing to do with E/G. It's how the machine bases are build.

    It says foam core base has better stiffness/weight ratio. Not because of foam but because of weight (foam is just a filler). E/G gains weight faster than strenght.
    -

  14. #1614
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    Quote Originally Posted by Zumba View Post
    Bruno, that's a cool looking structure but it does pose the twisting problem as Cameron pointed out, and IMO, it's impractical in two important ways.

    1.) The "extruded" shape would make sense in a mass production environment. Build a mold and then pour hundreds of em. But for one offs, you're wasting a whole lot of time for no noticeable benefits.

    2.) The 1" x 2" steel flat bars will need to be ground. This defeats the purpose of making an all-E/G structure. The whole point was to avoid having to grind, and rather rely on the flatness of the mold. If you're going to resort to surface grinding, you might as well use steel tube filled with E/G (which happens to have been my plan from day 1).
    Zumba,
    1- The design purpose is to put the right amount og E/G where it is necessary. For now finding the right structure shape is a priority, making the mould isn't.

    2-The flat bars are used as reinforment and also to bolt on the Y axis rails, and it won't be A36 fb so no grinding will be necessary. if leveling is necessary I can use epoxy grouting or epoxy paste.


    Best regards

    Bruno

  15. #1615
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    Dec 2006
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    Quote Originally Posted by walter View Post

    The E/G core I got from AkvaCNC feels like a real stone. It polishes like granite and has nothing plasticky about it.
    It's not "extremely" hard, but solid and very dense, with hard surface. The matrix is very powdery.. And I've seen some air bubbles (thermosetting?) and they're glossy inside.


    -
    Walter,
    That's where Shore D hardeness gets involved in the process, however the cured epoxy must be hard enough but not brittle.

    Best regards

    Bruno

  16. #1616
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    Dec 2006
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    East systems epoxy 9482 is formulated for marble repair and aggregate bonding.

    Best regards

    bruno

  17. #1617
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    Apr 2007
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    brunog,

    I see you've gotten ahold of some cad software. You might see if it can compute the moment of inertia around the x-axis and y-axis. It would also be helpful to see if it can compute the polar moment of inertia around the c/g too. I think most can. Dimensions are necessary to really see what happens along with the proposed mountings to the vertical supports.

    For a symmetrical shape like a rectangle or a square, it's likely possible to handwave the arguments about torsion to some extent and get an idea of the deflections anyway. Once it gets to complicated shapes however, it's probably going to require finite element analysis to figure out what's going to happen. I've never done FEA and I don't have any FEA software. I've always wanted to write some simple FEA software but it may be years before I get to it. I remember you said you were enrolled in a mechanical engineering degree so I'd suggest talking to your professors and seeing if your school has access to the necessary 3D Cad and FEA software.

    Speaking qualitatively only, I think a triangular cross section might work better than an L but I like shapes that are symmetric about the z-axis like rectangles because the deflection won't change depending on the direction of the cut. "I" beams are symmetric too but have the same problems as "L" beams with having only a very small cross section available to resist torsion (polar moment of inertia is low).

    Walter,

    As for air bubbles in the sample from akvacnc, a few air bubbles are not a big problem if they are not larger than the critical flaw size for the stress level of the material. Such flaws are primarily a problem where the material is in tension so they are much more critical near the bottom of the beam than they are on the top. The big problem is ensuring that the epoxy is stuck to the aggregate: a good sample should not have what I described earlier and called type 2 voids where there is a layer of air surrounding each aggregate particle turning it from a reinforcement to a built in flaw. From what I've seen on epoxy vendor best practices pages, there is a technique to slowly pouring the epoxy into the mold starting in one corner and gradually pouring across the mold that helps keep the voids down.

    Regards All,

    Cameron

  18. #1618
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    Dec 2006
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    Cameron,
    I am presently learning Autocad basics right now, the school has Catia V5 which I will start learning next August.

    I know Autocad can calculate moment of inertia if I draw a solid form, I'll try to work on it. That'll keep me busy for a couple hours LOL.

    Thanks

    Bruno

  19. #1619
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    Bruno,

    I think you should be able to get moment of inertias from just a drawing of the cross section if this works the way I think it does. I'd imagine that given dimensions, it might find the values for the beam you just posted.

    If I were you, I'd cozy up to the Sysadmin, get an account and start teaching yourself Catia now. You should be able to figure out how to do a kick-butt FEM of E/G beams by the time you get to the class

    My "CAD" system is an old rapidograph portable drawing board with pencils HB-6H. The only thing computer aided about it is that I can use my calculator and it at the same time so you've got an advantage. Catia especially and autocad don't come cheap.

    Good Luck,

    Cameron

  20. #1620
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    Thank you to 3M for supporting us CNC-Zoners!!!!!

    Hi all,

    I just wanted to write a post now and publicly thank 3M for samples of Novec Fluoro-Surfactants 3340 and 3342. I was figuring I'd call them today and see why they hadn't written me back but a UPS truck dropped by this morning with a standard sample package! They sent the samples to me even knowing what we're doing here. All I need to do now is order some tiny disposable pipets (calibrated eye droppers) to measure them out as these are the type of chemicals that are added at a rate of less than a drop per test specimen. The tiny amount needed is true of most of the proposed additives for E/G.

    :cheers:
    Take care all,
    Cameron

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