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IndustryArena Forum > MetalWorking Machines > Benchtop Machines > Concrete to stiffen mill upright?
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  1. #1
    Join Date
    Dec 2003
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    189

    Concrete to stiffen mill upright?

    I'm going to build a desktop mill and want to be able to machine aluminum and steel with a good degree of accuracy. I'm going to be using THK linear rails and bearings and ballscrews.

    Question: Any thoughts on using a section of 4 inch, thick wall (.25 or so) square stock as the mill uprights with welded and faced steel pads. My idea is to fill the square stock with concrete to stiffen and dampen any inherent resonance the steel tub might have. Any one tried this? Any thoughts?

    Larry in NC

  2. #2
    Join Date
    Jul 2005
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    78

    hmm

    concrete isn't that stiff (10x less than steel I think?)... but it is cheap which is its main advantage.

    concrete also has negligible tensile strength, so you need to pretension some rebar or threaded rod going through it, to get the best of both worlds. This will give you the same tensile strength as the steel, but greatly increased stiffness. Concrete is probably the cheapest way to get stiffness like this. But if you just pour it into steel section there will be no pretensioning and you might as well not bother.

    In terms of damping... it adds mass, but I suspect that's about it. in other words, you lower the resonant frequency, but in no way eliminate oscillations.

    Good luck, let us know how you get on.
    /Mat

  3. #3
    Join Date
    Aug 2006
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    I think that sounds like a pretty good idea. I'm sure steel is stronger than concrete...but concrete is very rigid and really cheap. I would worry about it breaking/cracking. Maybe if you somehow integrated a steel mesh (with something strong like 1/2 rebar) to keep the metal from pulling away from the concrete it would probably work pretty good.

  4. #4
    Join Date
    Aug 2005
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    90
    Concrete only does 3 things, gets hard, turns white and cracks. You might consider boxing in an "I" beam instead of the tubing. You can also weld in gussets before boxing it in...just remember welding will "draw" as it cools, so small tacks and alot of them before welding it all the way up. I've done this on a frame jig I made years ago and it worked well.
    Robert

  5. #5
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    Jul 2005
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  6. #6
    pepo Guest
    Lead would be better,or maybe even bird shot. Thermal expansion would be much improved by using something that would hold a temperature. To get a weldment like yours stiff and resist harmonics when it is made from steel is best done by putting the peices in tension and compression. Cant say for sure,but I think 4" round tubing would be better. Stress loads,moments of inertia and all that.

  7. #7
    Join Date
    Jun 2006
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    4
    Look there, a mini mill base make of polymer concrete.
    http://www.cnczone.com/forums/showth...threadid=24879

  8. #8
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    Aug 2006
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    78
    I think your approach has definit merit. I have a post somewhere here suggesting exactly the same project as yours. MY idea was to use 4 x 6 or 6 x 8 steel sections and fill them with plasticised concrete. These steel sections are plenty stiff with a 1/2" wall thickness but vibration and resonance is the problem. My idea was to use a plasticized cement mixture made with portland cement, fine and coarse sand, acrylic admixture and water based epoxy. There is a water based epoxy product called Latapoxy. It is used for setting ceramic tiles when fast traffic reopen time is essential. Such as when repairing a floor in a shopping mall. This product is fairly expensive, but does not have to be used full strength. I think a 50/50 mixture of admix cement and latapoxy will work great as a dampening medium and will definitly not crack. One important factor in preventing cracking is using the exact required amount of liquid (H2O) in the masonry. Enough to complete the calcification reaction but no excess which evaporates and causes shrinkage and subsequent cracks.

  9. #9
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    Jul 2004
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    374
    You may want to consider the use of brass in your design, as it also has very good vibration dampening characteristics. I have been highly impressed by the use of brass in toolholding. (tool shank clamped by an intermediate brass sleeve) Maybe it could be applied in your application?

    Since you already have linear bearing rails and ballscrews, make sure you go way overkill on ballscrew support bearings and definately use clamping elements on your linear bearing rails. [against a machined edge]

    Justin

  10. #10
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    Jul 2005
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    Quote Originally Posted by dcprecision View Post
    ....will definitly not crack. One important factor in preventing cracking is using the exact required amount of liquid (H2O) in the masonry. Enough to complete the calcification reaction but no excess which evaporates and causes shrinkage and subsequent cracks.
    I have seen descriptions about using epoxy binders and the same emphasis about concrete cracking and I wonder if trying to avoid cracks might be self defeating.

    My reasoning is that the vibration damping characteristics of cast iron arise from the fact that cast iron is full of micro-cracks, or so I have read years ago. The micro-cracks are the principal reason that cast iron has low tensile strength and uncommon behaviour when loaded under compression; deformation under compression becomes more difficult with increasing load because initially it is the cracks closing up not true elastic deformation. The cracks damp vibration because the entire mass of material does not vibrate as a single unit but as a lot of partially separate units that have all manner of different resonant frequencies.

    Extending this reasoning to concrete leads me to think that if you where succesfull in totally preventing cracking you would greatly reduce the vibration damping because the whole mass can now ring like a bell.

  11. #11
    Join Date
    Nov 2005
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    13
    Concrete bases have been used over here in the UK for a few years now - Colchester use them on the bases of their more expensive CNC lathes like the Tornado (may be branded Storm in the US, via Clausing?). They call their technology "DuoStable"

    Here's the blurb from their website:

    The T10 series feature Colchester's unique and patented duo-stable construction supplied with its own foundation enhancing both thermal and dynamic stability.

    Duo-stable is an advanced engineering construction making use of an inert concrete composite encapsulated in a heavy duty fabrication and reinforced with a network of ribs. This creates a machine that is not only heavier but also dramatically more thermally and dynamically stable than traditional cast iron.


    Sounds like your idea has some merit - I wonder what they mean by "concrete composite" - maybe some kind of fibre reinforcement? Anyone know?

    cheers

    Alastair

  12. #12
    Join Date
    Aug 2003
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    11
    if using concrete i would use silica fume and steel fibres
    do a google search

  13. #13
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    Feb 2006
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    Hey Tristar, I have already built my mill and am planning on filling the column with a grout substance that is used on the bases of steel columns. They actually use it as a water proofer or like a caulking. The stuff (forgive me as I,m still trying to get the name of the stuff) is as hard as steel when it cures. I had to remove some one time, and at first I tried a very heavy sledge hammer only to have it bounce off the stuff, I eventually had to grind it off. Theres also a marine grade polymer concrete that I,m looking at. I,ll get the names here in a few days for ya.

  14. #14
    Join Date
    Mar 2003
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    4826
    I wonder would a viscous fluid have any damping capacity? Supposing you could cap the ends of all the tube members and fill them with oil and sand or something. Maybe even pressurize the thing a bit with a portable hand pump like a grease gun.

    If it doesn't work, don't call me and ask what to do with the mess afterwards
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  15. #15
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    Dec 2005
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    While at Hardinge many years ago, they were in the process of filling their tool room lathes or some other super precision machine tool with a VARIANT of concrete.

    It was a very fine aggregate compound that both stiffened and "mass dampened" the fabricated steel and/or iron beds for the particular device being stiffened/dampened.

    The stuff was NOT concrete. It was a polymerized stuff that they had specially formulated. It was special in that was thermally and dimensionally stable and it did not cause problems with size change or temp changes like massive thicknesses of concrete will/can/might do.

    The closest stuff I've seen to it is a stuff called "hard block". This is stuff that drag and some early tractor pull racers used to fill iron blocks with so they can bore them to huge sizes and have paper thin walls and still not have water in them and hope that they migh live.

    As far as the damping frequency is concerned, it is essentialy a funtion of the following:

    Square root of (K/M)

    Where K is the spring rate of the item and M is the mass.

    Thus, the stiffer you can make it WITHOUT making it heavier, the higher the natural frequency of excitation.

    Thus if you double the stiffness and doubled the mass in the process, you didn't do yourself any good, damping wise. However, if you doubled the weight and quintupled the stiffness, you done good.

  16. #16
    Join Date
    Aug 2006
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    78
    Hi Larry,

    Another thought on your mill project in addition to concrete filler. When you look at most mill drills such as the RF-45, you notice that the head assembly mounts to the column in such a way that the center point of the head mount is about 22" above the Y axis base when working at vise height. ( 5 - 7" ). This design puts a lot of metal between the machine parts and contributes to the instability of the machine. If you desigh the head/spindle assembly to mount to the column at a center line much closer to the actual collet face, The amount of column travel used for a particular parts height can be greatly reduced. This will enhance the stifness of any machine by several orders of magnitude. I think that flex increases by the square of the distance so a small improvement adds up to a lot of stiffness.

  17. #17
    Join Date
    Jun 2006
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    2512
    Its actually by the cube of the length. So if you increase the span by 25%, say from 8" to 10", the section will have only half the stiffness.

    Regards
    Phil

    Quote Originally Posted by dcprecision View Post
    Hi Larry,

    I think that flex increases by the square of the distance so a small improvement adds up to a lot of stiffness.

  18. #18
    Join Date
    Aug 2006
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    10

    Zanite

    This is something I looked at a few years ago - http://www.zanite.com/index.html . At that time they would sell a 400lb pallet of the materials, ready to be mixed and poured.

    Allan

  19. #19
    Join Date
    Mar 2004
    Posts
    1661
    I would say gor for it! I've been running a production machine with a concrete filled base and that's a sturdy piece.
    There's a company over here known for there extremely well working brake systems and they run a couple of home made lathes where the base is made as a steel frame filled with tarmac. Works pretty well as I see it.

    I use a light weight concrete in my frame, it works very good.

    Regards,
    Sven

  20. #20
    Join Date
    Jul 2005
    Posts
    969
    humm that could be an idea... a bit messy but what about sand and tar but just enough tar to hold the sand compacted together and fill the air gap between the grain

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