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IndustryArena Forum > Mechanical Engineering > Mechanical Calculations/Engineering Design > Moving table (fixed gantry) with two ballscrew
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  1. #21
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Peter.
    I liked those legs: Anti-Vibration Levelling Foot - Medium Duty (WDS 693) --> M16, 1200kg max load
    Anti-Vibration Levelling Foot - Medium Duty (WDS 693), Anti-Vibration Feet | WDS

    filler for EG: quartz sand- 30% 0.16mm, 30% 0.63mm, 30% 2mm (Al2O3 is expensive for me)
    Epoxy: 10% biresin CR83 (if I succeed with that small percentage).
    Will glass fibers help here or not ?

    For vibration - I have ENAR AVMU 2.3kW/3HP with vibrating eccentric head 48mm, 4m shaft (frequency V.P.M. 11500)

    About Carbon Fibre blocks - for what?. What do you mean?

  2. #22
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Ivan - CF instead of aluminium for some parts. Remember to do a dry fill and weigh then add water to figure out your epoxy ratio. 10% sounds too little for quartz. More like 28% by weight at 50% solid ratio. To achieve 10% by weight epoxy you need to get to 78% solid by volume this is unlikely. 50% by volume sand is probably tops. But do a sand/water test and you will know exactly...Share the result please...Peter

    The epoxy you have chosen has a mixed density of 1090kg/m3 so is close to fresh water.

    re glass fibres
    Depends on what you mean by a glass fibre? do you mean long fibres in stitched cloth form? or milled fibres in short form? The more components you add the less likely you are to get a consistent pack. eg Using 3 sand sizes means you will not get a consistent pack. Better to use 2 or one. If you use one size the pack will be consistent. If you use 2 then the smaller particle needs to be smaller then 1/10 the larger so use the 2mm and 0.16mm if you want to use 2. A particle is about 50% efficient in transferring strain through a matrix. A long fibre is 100% efficient. So if you used unidirectional fibreglass at the top and bottom of your members it would improve the bending stiffness, but this is complicated. fibreglass is melted sand so material stiffness is the same.

    Having a think about it you could improve stiffness by wet laying UD fibreglass into the mould and rolling it out and letting the epoxy get to gel. Say 4-5mm thick. Then fill with Epoxy/sand. This way you get a stuff outer skin. Hand laid UD will be about E=25GPa and the sand will be 70*0.5*0.5= about 17.5GPa... Peter

  3. #23
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Ivan - I'm about to make some epoxy aluminium slabs. See data sheet for powder. Quartz density is 2650 and aluminium is 2700kg/m3 say effectively the same. This powder has a bulk density of 1250kg/m3 so 1250/2700= 0.46 which is the solid fraction or volume fraction. So to get past 0.5 fraction is hard. So this converts to 29% by weight for the resin and 71% for the aluminium. 10% will be way too dry in your case. The powder arrives in the next couple of days then I'll do a dry/wet test and come back here with result. Peter

  4. #24
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Peter.
    About particle selection I read from this link:
    http://mk78.yolasite.com/base.php

    ".....The aggregate should be made of various sizes of sand and it should content also silica flour with an aim to fill the micro voids. According to the filling effect of concrete structure, coarse aggregates built the skeleton and fine aggregates fill into the voids between coarse aggregates. The bigger particles selected in higher proportion gives strength to the structure. A low percentage of granulation of coarse aggregates of basalt and resin gives the lower values of compressive strength. On the other side, when its percentage increases beyond certain limit the values of compressive strength again starts decreasing. Experiment shown that, higher than 50% coarse content in the mix leads to reduction in compressive strength of epoxy basalt because resin and fine aggregates are unable to fill voids between coarse aggregates completely...."

    ".........The strength and stiffness characteristics of E/G are governed primarily by the aggregate to epoxy ratio. For the very strongest material, this must be as high as possible while still maintaining enough epoxy to keep the the material together. The strength of the aggregate, the strength of the aggregate epoxy bond are also critical along with getting the air out etc that don't get accounted for in the theory.In general, the optimal mix can be made by mixing various sizes of sand to follow a simple equal percentage logarithmic grading curve. As wide a range of sizes as possible helps keep the packing density high. Small (micron sized) aggregate make the wetting of the mixture with epoxy more difficult. ......"


    "...
    30% basaltsplitt 2-4 mm

    30% fugensand, 0-2 mm


    30% quartzsand 0.1-0.4 mm


    8% Epoxyharz L and harter L from R&G (Germany)




    https://www.schneeberger.com/de/prod...petencesHeatly Care:Workers who fabricate and install quartz surfacing are at risk for overexposure to silica released during sizing, cutting, grinding and polishing. Prolonged inhalation of dust from silica-containing materials can lead to silicosis (scarring of the lungs). In addition to silicosis, scientific evidence indicates that occupational exposure to crystalline silica puts workers at increased risk for other serious health conditions: chronic obstructive lung disease, lung cancer, kidney and connective tissue disease, and tuberculosis. The focus of this blog is on silicosis, which has occurred in multiple workers in this industry.Prepare the mold.Vibrator is good. 8% epoxy is way stiffer than concrete and needs to be hit with the right vibration to get it to sag. 70 Hz is supposedly good frequency range (is that true and why?). I've seen a youtube where the guy took simple hammer and small piece of wood.Aggregates should be of different sizes. The recommendation is:30%..."

    "....
    Dens-Graded Aggregate
    Dense-graded aggregate mixes are also called well-graded or continuously-graded. These types of mixes are characterized by an even distribution of particle sizes, such that finer grains can fill the voids between larger ones.

    Wide range of sizes
    Grain-to-grain contact
    Low void content
    High stability
    Difficult to compact

    Particle Size
    Particle shape is important in controlling the ability of the aggregate to compact and affects the adhesion of the binder to the aggregate surface. Shapes are described as rounded, irregular, angular, flaky, or elongate. The shape is assessed by measuring the longest, shortest, and intermediate axial diameters of the fragments. In the ideal equidimensional fragment, the three diameters are the same. Particles with ratios of the shortest to the intermediate and the intermediate to the longest diameters of above about 0.6 are normally regarded as equidimensional. For many purposes, it is important that the aggregate particles have equant shape: their maximum and minimum dimensions must be very similar. Spherical and equant particles of a given uniform size placed together have the lowest space between the particles. Highly angular particles and flaky particles with high aspect ratios of the same grading can have much more space between the particles. The shape of the particles can significantly affect the properties and composition of a mixture.The overall space is also determined by the grading curve. Sometimes highly flaky particles such as slate can be used in a mixture if they are accompanied by suitably graded and highly spherical particles...."


    Over 15-18% resin - not good. At least resin use is obtained with micron particles. But "....Small (micron sized) aggregate make the wetting of the mixture with epoxy more difficult....".

    So, maybe 2 sizes: 0.16mm and 0.8-1mm. But in what ratio ?


    P.S. Thanks for the good tips:
    "Remember to do a dry fill and weigh then add water to figure out your epoxy ratio.""Having a think about it you could improve stiffness by wet laying UD fibreglass into the mould and rolling it out and letting the epoxy get to gel. Say 4-5mm thick. Then fill with Epoxy/sand. "

  5. #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

  6. #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.

  7. #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

  8. #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  

  9. #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.

  10. #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.

  11. #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.

  12. #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.
    .

  13. #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

  14. #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

  15. #35
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    Re: Moving table (fixed gantry) with two ballscrew

    Ok Peter.
    Maybe tomorrow I will do the tests.

  16. #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?

  17. #37
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    Re: Moving table (fixed gantry) with two ballscrew

    Morning Ivan - Your doing fine with your english and the tests. When you quote a ratio or % always state whether its by weight or by volume so others know what you mean. Also as a note you will need to dry your test sand before you use it!! In an oven is best. Fit it in with the sunday roast... If your achieving 70% volume fractions I think that's as good as you will get. Well done. I think this is the conclusion of the concrete companies as well. They used the high pack theories for years thinking they where getting 90%+ but in the end this is not achievable. 70% Vf which stands for Volume fraction vs Wr which stands for Weight ratio is a good number.

    Infusion epoxy is very thin and yes it will stick even the smallest particles. It will take 1hr for the resin to gel (if you get 1hr plus hardener) and in that time capillary action will permeate everything as long as the sand is dry. I have glass tissue 0.2mm thick and epoxy will flow through this quite well under vacuum. So I expect if the sand is dry and you are not living somewhere where temp is below 15C then its fine.

    Your resins density I estimate as 1095kg/m3, from the net quartz is 2650kg/m3 if you achieve 70%Vf then it goes like this if we work in litres:

    700 litres quartz weighs 700*2.65= 1855kg
    300 litres resin weighs 300*1.095= 328.5kg total = 2183.5 kg

    sand is 1855/2185.5 = 0.85 by weight
    resin is 328.5/2185.5= 0.15 by weight or 15% resin by weight. This is a weight fraction not a weight addition!!

    The weight addition is 328.5/1855= 18% weight addition based on sand weight. Weight additions are usually used for pigments and modifiers, not the functional parts these are usually expressed as weight ratios. Check the maths and check your ratios on the day!

    Not sure what you mean by air bubble inside? Can you see large air voids or lots of small bubbles? This sort of open process will always have small air bubbles unless you use a vacuum process. Air is 1.2kg/m3 so it takes a lot of air to change the numbers. As more info you can add a small drop of detergent to the water to improve its wetting of the sand and its bubble release. There are surface tension improvers for epoxy as well and I have played with these. They are expensive and I have not found them to be useful. If the air is in the epoxy resin (the hardener is very thin and it seems to self release) I have found heating it to 40-50 degs releases a lot of air as it thins out. But then allow it to return to ambient temp before use!! If you use a fast mixer its inevitable that air will be sucked in, that's how you whip cream and make pavlova.

    When you use epoxy 1) make sure sand and mould is dry (water is your enemy, especially the stuff you can't see) 2) you could warm it slightly if air temp 20degsC or less but if 25C up then alls good. 3) when mixing do it slow with a paddle or spade vs fast with a mixer 4) use a long gel time hardener. This takes the pressure off getting it done, it reduces the exotherm peak and stresses the mould less.

    If you have a scale you can weigh the test as well and this can then cross check your figures.

    Onward to making some hard stuff.. Peter

  18. #38
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Ivan - some updates on the quartz stiffness now some groundwork has been done. I have always thought quartz was E=70GPa but seems being a crystal it is anisotropic (always a fly in the ointment). So it varies 80-100GPa. Attached doc confirms density at 2650kg/m3 which is great. So reading attached, it seems using 90-100GPa is fair as this is the tested quasi isotropic figure. So your sand mix 90GPa x 0.7Vf x 0.5 strain efficiency = 31.5Gpa and zanites quoted figure of 31GPa is spot on. So I expect you get better then 30GPa so back to the FE work and finalise that design!!! Using 100GPa E=35GPa so range E= 31-35GPa not bad? Half that of aluminium

    Oh yes E of epoxy =3.5GPa so full value using rule of mixtures (ROM)

    (0.7x90x0.5) +(0.3x3.5)= 32.55 GPa estimated modulus or using 100GPa, E mixture=36GPa are you the optimist or the pessimist? 33 or 36 close enough for FE work.... Peter

    I've just had some aluminium powder arrive so I'll be making a block of that in the next couple of days 0.4mm and 0.2mm

    Looking at your Biresin, its thinner then the one I use which is great, I'd go with the CH83-6 hardener so the peak exotherm was down and it gives you over 2 hrs gel time. Doesn't quote the mixed density so

    100g + 30g = 136g vol = weight/density kg/(kg/m3)
    0.1/1.14=0.0877 m3
    0.030/0.94=0.0319m3 total =0.1196m3

    density = weight/volume = .136/0.1196= 1137 kg/m3 so the prior figures need to be corrected as I used 1095kg/m3 (1137/1095 = 104% missed by 4%)

    cheers Peter

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    Re: Moving table (fixed gantry) with two ballscrew

    added the resin data sheets - Peter

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    Re: Moving table (fixed gantry) with two ballscrew

    Morning Peter. Thanks for the comprehensible calculations and explanations
    Everywhere I mean % of volume or ratio of volumes. I corrected 2 places where I did not mention this.
    Today I'm going to dry in a oven the fractions for the tests. Yes, I have a digital scale, but I'm not sure of its accuracy (absolute values). But as ratio (relative values), it can be used.
    Other tests - afternoon.

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