Advice needed on filling my mill with EG


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    Default Advice needed on filling my mill with EG

    I'm in the process of designing a DIY mill and I wanted to throw out my current thoughts so that I don't screw this up too badly. I am planning on building a framework for the base and Z column out of 3/4" steel and leaving it hollow. Once all my axis are aligned and squared I am planning on pouring epoxy granite inside my framework to stiffen everything up. Will this be worth my time / effort / $$? Is there a better solution? Will the curing epoxy tweak my alignments? Picture of the mill below:



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    Default Re: Advice needed on filling my mill with EG

    First, let me point out some design ideas that would improve your machine:

    You now have (from bottom to top) rail, cart, table, rail, cart, table for XY movement and (back to front) rail, cart spindle.

    You'd get a better machine with (bottom to top): rail, cart, table, cart, rail, table and (back to front) cart, rail, spindle.

    That setup would give more consistent stiffness.

    As for filling, you do mean filling the whole structure?
    I'm not sure if that can be done with a frame like this, I think doing that would be easiest and best when the machine is upright.
    If I were to make a machine that way, I'd probably use tooling plate and screw that together.

    I would not trust welding thin steel to stay straight, but I am a very, very poor welder

    As for the filling compound, I would choose a very dry mix, like 3% for sand, or even less for small gravel, but be sure to test mixes!
    A very dry mix produces the least amount of heat, reducing the chances of distortion.

    Sven http://www.cnczone.com/forums/diy-cnc-router-table-machines/320812-aluminium-1250x1250x250-router.html


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    Default Re: Advice needed on filling my mill with EG

    Quote Originally Posted by CaptainVee View Post
    First, let me point out some design ideas that would improve your machine:

    You now have (from bottom to top) rail, cart, table, rail, cart, table for XY movement and (back to front) rail, cart spindle.

    You'd get a better machine with (bottom to top): rail, cart, table, cart, rail, table and (back to front) cart, rail, spindle.

    That setup would give more consistent stiffness.

    As for filling, you do mean filling the whole structure?
    I'm not sure if that can be done with a frame like this, I think doing that would be easiest and best when the machine is upright.
    If I were to make a machine that way, I'd probably use tooling plate and screw that together.

    I would not trust welding thin steel to stay straight, but I am a very, very poor welder

    As for the filling compound, I would choose a very dry mix, like 3% for sand, or even less for small gravel, but be sure to test mixes!
    A very dry mix produces the least amount of heat, reducing the chances of distortion.
    Thanks for the reply, I haven't looked into tooling plate before but that could be a possibility. The main reason for building it out of 3/4" steel is because I can get it cheap. Also, I'm not planning on welding much of anything, I want to avoid warping so it will almost entirely be screwed together. The column and base is designed so that it is one hollow chamber, I don't see any problems with filling it, I'm going to have holes drilled in the top of the base to allow air to escape and I plan to vibrate the EG thoroughly to get it throughout the machine. My main concern was that the shinking EG would pull everything in and distort by alignments.



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    Default Re: Advice needed on filling my mill with EG

    The dampening coefficient that you get will be a result of the sum of the components, according to their share of the total stiffness, and energy stored in said stiffness.

    For example: an 8 inch diameter pipe, 3/4th inch thick side wall has 226 inches^4
    a solid 8 inch rod is 402inches^4

    The stiffness of epoxy granite being on the order of 1/8th (maximum) that of steel, means that if you fill the .75" thick sidewall, 8 inch outside diameter steel pipe, you've only added 1/8th of the difference (402-226)/8 is 22in^4 equivalent. this means the epoxy granite will take 22/(226+22) parts of the strain, or about 9%

    Assuming the steel has a dampening coefficient of .001, and the epoxy granite a dampening coefficient of .05, this means you now have a final result of (.91*.001)+(.09*.05) which is .00541.

    .0054 is a lot better than .001, (and this is enough to make people think its no longer resonant) but the weight was nearly doubled by adding the epoxy granite, thus the resonance frequency dropped by the square root of 2 if i recall correctly, and the final dampening coefficient still worse than cast iron.


    often times very high dampening coefficient, but low stiffness materials such as sand, rubber, silicone, polyurethane (density can vary by two orders of magnitude for polyurethane) are added between tuning forks made of very stiff materials. they call this "constrained layer dampening" and the purpose of it is to cancel out specific frequencies.

    for general machine tool purposes, the best material remains cast iron because you can machine it cheaply and it is stable after heat treatment.

    if you want the highest dampening coefficient for the highest stiffness, solid granite may be the best option, basically epoxy slabs of cheaply available granite counter top together (ideally, get all the pieces cut from the same sheet) and bolt or epoxy on certified flat granite straight edges (or cut up a surface plate) with inserts drilled and tapped for the linear rails.


    there are very few other materials besides granite and cast iron that have any use in machine tool bases, you may be able to get higher dampening coefficients from epoxy granite as opposed to solid granite, but such results depend entirely on how well you fitted the mass fraction of all the separate ingredients of the epoxy granite. (too much epoxy will drastically reduce the stiffness)


    Edit: so it appears my math is wrong, i seem to recall for certain that modulous of elasticity numbers for epoxy granite have been as high as 40GPa, which is one fifth that of steel.

    but this paper here http://www.ijmerr.com/uploadfile/201...9113014336.pdf they had measured absolutely abysmal numbers on the order of 6GPa.

    so your mileage my vary... and to my recollection only one person on this forum ever cast samples of his own epoxy granite and measured the values, i can't recall what they were exactly, but 30-40GPa seems to remain in my mind.. perhaps from too many other documents.

    edit 2: another paper suggesting maximum stiffness around 3-5 GPa value http://www.ijacskros.com/artcles/IJACS-2S-26.pdf

    Edit 3: yet here is another suggesting 30GPa stiffness http://iasir.net/IJETCASpapers/IJETCAS14-802.pdf

    Last edited by Eldon_Joh; 01-16-2017 at 05:43 AM.


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    Default Re: Advice needed on filling my mill with EG

    ANY heavy filling material will greatly reduce resonance, and if the structures are completely enclosed, will make it quite a lot stiffer.
    However, to get any real benefits, plan to use hundreds of kg of material.
    This may or may not be economically worthwhile.
    3/4" steel is an excellent idea, as is bolting subs assys together.

    Practical examples:
    On my tiny 7x lathe, 12 years ago, I made a good concrete bed, and a rigid mount.
    Lathe 40 kg.
    Bed 120 mm or 5" tall mild steel, preloaded box, filled with concrete, side walls 12 mm or 0.5" thick.
    Mount 150 kg.

    Result were excellent, but not economically worthwhile.
    Vastly more rigid, noise dropped 50%, no more vibration.

    Cost about 250€ in stuff, and 200 hours work.
    Would have been better off spending the extra € for a 105 kg 8x lathe.

    My current vertical columns will be filled...
    But my machine is an industrial-type double column bed mill, and most structural members are 20-30-40 mm thick tool steel.
    And the vertical columns are 200x400 mm section sizes, 1400 mm tall, and could carry 50 metric tons.
    Top, bottom, and the 2 middle plates are 200x400x20mm thick tool steel.
    Ie 20 mm == 3/4 inch thick steel.
    Assy is bolted with about 80 6 mm shck on both sides of the box, and 8 mm bolts on the stiffeners and ends.
    Vertical flat sides 400x1400 are 10 mm thick steel.
    Ends are 20 mm thick, 200x1400x20 mm.

    Base plates, that are bolted to the floor, are 300x800x30 mm tool steel.
    The filling will not make the machine appreciably more rigid.
    Both columns are about 250 kg in mass, pre-fill, and fill will be approx 200 kg each column.

    It will make it vibrate less, less noisy, and the settling period will become lower, making final settling at tooltip faster.
    Just my observations, experiences, now doing version 5 of the machine ..
    Now doing a temporary rigid mount, to verify linear guides for binding before final assy.

    Machine quality is mostly measured by rigidity.
    It is the university textbook answer, and good pdfs exist online.
    A mill wants about 30 N/um, iirc.
    So, when standing on the spindle mount, it deflects about 0.02-0.05 mm with 80 kg mass (I expect).
    800 N / 0.03 mm = 800 N / 30 um => 26 N/um.
    Latest versions v4 deflection was about 0.08 mm iirc.

    I have measured the previous 4 versions of the rigidity with a 1 micron electronic dti, and will do the same with this the same way.
    First version was 0.27 mm flex with 80 kg mass.
    The 0.02-0.05 mm is my estimate of what I will get.

    The current "strength" of the new structure is about 20x higher than the old one, and mass is about double-triple on the new components.
    The new spindle box/ram/mount is about 100x stiffer/stronger.
    Rigidity tracks pretty well with strength.

    You don´t need strength, You need rigidity, but building it "strong and heavy" will also get You rigidity.
    (Vice versa is not always true.)
    Typical loads on machine tools are 1-2% of max strength.

    Thus, my machine has a theoretical capacity of 50 metric tons, z axis.
    12 trucks, 35 mm hiwin, on 6 rails, vertical, 1000 mm long, for 450 mm z axis movement.
    12 trucks at 4000 kgf each = 48 tons theoretical rated working load.

    I don´t expect to ever load it at even 1000 kgf (I wish), even 300 kgf would be more than I hope for.
    At 1000 kgf I would need to run a 25 kW spindle !

    A typical HAAS VF3-4 is similar sized, uses similar 35 mm truck and 32 mm ballscrews (Like an ST20 lathe).
    Bigger VF machines used 45 mm trucks.
    My screws are 32 mm, tiny ac servos, 400W.



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    Default Re: Advice needed on filling my mill with EG

    Sounds like there are definitely some pros and cons for filling with epoxy but I think I'm going to go ahead and be the guinea pig on this idea. I'll be sure to report on how everything works out but it's going to be a while before I get all my steel cut/milled. Has anyone tried using uscomposites epoxy #635 for their projects? It seems like the route I am going to go unless this has been tried before and resulted in failure. The epoxy is thin, slow curing, and doesn't contain solvents. Seems like it meets the criteria. Epoxy :Â*Epoxy Resins and Hardeners



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Advice needed on filling my mill with EG

Advice needed on filling my mill with EG