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IndustryArena Forum > Mechanical Engineering > Mechanical Calculations/Engineering Design > Moving table (fixed gantry) with two ballscrew
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  1. #25
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Ivan - The epoxy ratio is dependent on the density of the constituents and the volume ratio achieved in the pack. If you have done any concreting you will know that when you vibrate your mud the fines come to the top so you get a segregated mix. Plus concrete uses large particles with large gaps whereas small particles have small gaps and pack better. You do not need strength in a mix (its strong enough, no matter what you do. Epoxy is strong enough by itself) you need stiffness and this is a function of how much solids you can get in the mix. I published somewhere else that the concrete industry has debunked and thrown out the multi size theory as it does not work well. Sure people have tested stuff and published stuff but it doesn't matter if you look into the details. You need to do the dry/wet ratio test yourself and prove what ratio you actually need vs reading about it. Sure you can add 10% even though it needs 20% and all you are doing is having areas within the structure being porous and disconnected. Which for a machine base is probably fine but not ideal if one of those areas is in a critical place. But the right amount of resin to fill in all the gaps is the correct thing to do.

    Many concrete documents test and specify compression tests and these rely on particles being very close to get good numbers and in fact does not need much resin to achieve high compressive strength as particles are touching so the load path is not thru the resin, its basically a dry stone wall. In a machine part its under complex stress conditions and needs all the particles glued together to give good tension, compression and shear performance.

    I fill my moulds under vacuum. This removes the air so the epoxy can flow around things nicely, it removes the water that inhibits the epoxy sticking to things and the vapour barrier around things that inhibits flow. Its surprising how much resin some things take to fill. Like wood, it takes the same amount of resin by weight to fill some woods...The vacuum also degasses the resin as it fills so some say that's about 5% delivered to you in the bottle.

    There's no right or wrong in what you are doing, but you are only going to do this once so make a good go of it. The dry/wet test is the only simple way to tell with the materials you actually have.

    With carbon fibre parts and glass fibre parts we run at about 28% resin by weight using vacuum processes. This gives FG 50% fibre fraction by volume and CF about 60% fibre fraction by volume. I can't see adding 10% by weight to most things and have them correctly glued together...

    by the way the surface tension of clean water is about 70dynes/cm, epoxy is about <50dynes/cm so if the mixture wets with water it will wet with epoxy. cheers Peter

  2. #26
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    Re: Moving table (fixed gantry) with two ballscrew

    Ok Peter.
    I wiil do the water test.
    But this "... and the sand will be 70*0.5*0.5= about 17.5GPa... " ..... I don't like this value.
    I probably need to go back to the steel structure again. I'm tired of drawing projects in vain
    I wanted to avoid welded structures with their deformations and vibrations.

  3. #27
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Ivan - Yes avoiding the welding etc is worth doing. The 18GPa is not a big issue as long as the members are big to compensate. You are taking advantage of shape in a moulded part so maximise the geometry and the shape and it will work for you. You will do no worse than anyone else that has done EG with sand or granite bits and they have worked out fine. Perhaps look at using glass fibre fabric to up your E value a bit? UD cloth along mould as mentioned earlier will get you improved bending stiffness. Hand rolled out UD is 29GPa (tested at 42% resin by weight and 31% by volume eg 70x0.3= 22GPa so test was better then ROM) and if this is used on the mould surface then its doing most of the work? Then backfill with epoxy sand, then place a UD on top of that when cured so its UD all round? Peter

    edit - I looked up Zanite as they publish their properties and they use quartz. They quote E=31GPa and density as 2200kg/m3. Quartz is 2650kg/m3 so we can calculate the quartz fraction as 68% ie resin weight fraction is 14%. Now I think that's a bit dry (so there is air in there as well) but onward, If they are getting 68% Vf then 70*0.68*0.65= 31GPa.

    So I expect you will get the same. 70GPa is quartz stiffness, 0.68 is the quartz volume fraction and 0.65 is the strain transfer efficiency. Your wet/dry test will give you Vf so then you can see how you're compared to Zanite.

    Many companies quote the dry bulk density of sand as 1700kg/m3 which means its 1700/2650= 0.64 volume fraction so the 0.5 in my calc is now 0.64 which is close to the Zanite calc at 0.68 Looked up wet sand and its about 2100kg/m3 so you will find out how good your mix is!!

    The attached figures make sand and water at 2100kg/m3 which is a sand volume ratio of 67% which is in agreement with prior. So I expect your stiffness is much better then 18GPa. Plus this works out at 15% addition by weight of epoxy... Keep to the course... cheers Peter

  4. #28
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    Re: Moving table (fixed gantry) with two ballscrew

    Other variant for higher stiffness:
    sandwich from EG (thickness 50mm, stiffness about 18GPa) and 3 plates solid granite (thickness 3 x 20mm, stiffness about 50GPa).
    All plates bonded together with epoxy and bolted.

    Plate of polished granite with cutting and trimming of edges and dimensions (1520 x 1000 mm, 20mm thickness) costs in Bulgaria 100EUR


    P.S. ! Sorry Peter. While I described you also posted a message. I'll read it now.
    Attached Thumbnails Attached Thumbnails granite-plates.jpg  

  5. #29
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    Re: Moving table (fixed gantry) with two ballscrew

    Ok. I understand about hand rolled UD. Thank you.

    One more thing. Despite the advice from members that there is no need for steel reinforcement (rebar), I think that incorporating a rebar into my structure, which is not massive, will add strength.

  6. #30
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    Re: Moving table (fixed gantry) with two ballscrew

    Ivan - I don't think you need strength you are chasing stiffness. Rebar is added to concrete to improve tensile strength and to create a loadpath thru the concrete when it cracks which it does and epoxy does not. Keep it simple. The least joins materials and assemblies the betterer!! Peter

    Hi Ivan - we are both searching for the same solution. A stiff and cheap additive for epoxy so we can cast stiff parts. The only stuff that I have come across that may help is long steel wool. Available at hardware stores and cleaner supplies. Its long fibres are 100% efficient, its steel so its 200GPa stiffness and its cheap. At 50% fibre fraction it will be 100GPa stiffness so very worthwhile. In my case using a vac bag and infusion it will be compressed and stay in the mould. In an open moulded part you will need to figure out how to stack it and wet it out with the sand around it for instance. Even if you only get 25% fibre fraction its still 50GPa much better then sand/epoxy... and ahead of fibreglass. steel is same stiffness as carbon fibre.

  7. #31
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Peter. In my open molded part - will be very difficult and slow with steel wool. I won't have that much processing time to work resin.
    What do you thing about metal tube frame in Epoxy? It should increase stiffness and decrease weight. ?he welds will only be dots, so there will be no deflections.
    There will be a lot of work with holes, but it doesn't matter.

  8. #32
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    Re: Moving table (fixed gantry) with two ballscrew

    Now I see your edit post #27 about Zanite. Yes, all values are correct. Thanks for this sharing !
    Volume fraction depend from particle size. Perhaps with a particle size of 0.16-0.25 mm a higher coefficient of 0.68 will be obtained.
    I'll do the water tests. I currently have 0.315 and 0.63mm quartz sand. I'll buy smaller fractions and check. I'll share the results here.
    .

  9. #33
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Ivan - I would stay in cast epoxy world or steel world. The tube inner will vibrate and being stiffer then the casting may be the dominate structure then you may as well have fabricated it. If the pins are to couple the frame to the cast this is not needed. Just roughen up the frame or sand blast it before casting. Epoxy will stick it together, but I wouldn't do a frame. If you use the 0.63mm you need smaller then 0.1mm to fit in the gaps. Keep us informed.

    "pins" are the thread inserts yes quite a few. Peter

  10. #34
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Ivan - Here's the rub with packing. If we look at a 2D representation using spherical particles we get a possible 2D packing factor of 91% and in spheres 3D it's a bit better. But clearly no one gets this in practice Zanite gets 68% and its a graded approach so there are various things working against you in getting very high packing factors. The attached image is the 0.63mm sand particle (if its round) The "unit" solid is 0.172mm2 and the void is 0.016mm2 which gives a void of 9% this is called hexagonal close pack. So the particles are not round this is the only reason I can think of that prevents getting higher ratios. The dry/wet test is the crunch. Cheers Peter S

  11. #35
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    Re: Moving table (fixed gantry) with two ballscrew

    Ok Peter.
    Maybe tomorrow I will do the tests.

  12. #36
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    Re: Moving table (fixed gantry) with two ballscrew

    Maybe this is for another topic. The title of this topic does not correspond to these tests. Whatever.


    Today I was able to buy some others fractions quartz sand.
    So, I have 5 fractions x 25kg (prices are 0.16-0.17EUR/kg) for tests:
    0.10-0.25mm, 0.25-0.40mm, 0.40-0.80mm, 1-2mm, 2-4mm.


    Excuse me for my bad English, I'll try to explain what I did. I help myself and with Google translator and probably some expressions come out as big nonsense. However, I hope the basic is understood. So I write less and post more pictures.




    The first 2 tests - disappointing!
    I was expecting a higher packing ratio.
    Peter was right. It takes much more than 10% resin to bond all the particles.


    I tried mixing with a electric screwdriver. Not very effective.
    Then I started pouring from one vessel to another, shaking hands, shaking, vibrating, etc. Best obtained by vibration and shaking + mixing.


    I mix and vibrated for a long time. Then I poured very very slowly water into the volume of 1000ml. First 200ml. Then some times x 50 ml. The residual water of the last 50 ml is subtracted from the calculation.
    ?he water is poured until the level rises and reaches the line of 1000ml.


    Test 1
    Volume 1000ml
    particle size: 0.10-0.25mm
    the amount of absorbed water: 390ml
    quartz volume fraction: 61%


    Test 2
    Volume 1000ml
    particle sizes: 50% 0.10-0.25mm + 50% 1-2mm
    the amount of absorbed water: 295ml
    quartz volume fraction: 70.5%


    Test 2 details:
    I first measured 500ml of the first fraction, then 500 from second fraction. Then mix two fractions.
    Despite continuous and combined mixing and vibration, no uniform mixture is obtained. I think the amount of 0.1-0.25mm fraction is large. Maybe the ratio should be 70% (1-2mm) and 30% (0.10-0.25). (edit: % of volume)
    Alter mixing and vibrating the total mix volume (from 500ml 0.1-0.25mm + 500ml 1-2mm) became about 890ml. In other words 110 ml of the small fraction was inserted between the holes of the large one. ?hen I kept pouring equal parts of the 2 fractions until the total volume became 1000ml.


    Tomorrow I continue with other different tests.
    If anyone wants a specific test with my available sand sizes, please write here. I will do tests as desired.
    I'm thinking of buying and 3-5mm fraction and trying a combination with a small one.
    One more thing:
    0.1-0.25mm are very very fine particles. I very much doubt whether these small particles will stick on all sides with the resin !


    Peter, in test 2 - quartz volume fraction: 70%.
    Does this mean I need 30% resin? (edit: 30% of volume)
    What is happening with Air Bubble inside?

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