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IndustryArena Forum > Mechanical Engineering > Epoxy Granite > Large EG Milling Machine Design
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  1. #1
    Join Date
    Jan 2017
    Posts
    13

    Lightbulb Large EG Milling Machine Design

    I have been looking into building my own CNC mill for some time now. While doing research for that, I was lead to this forum!
    I have read through several of the build post on this form and have finally started the design process for my own!
    Currently I am still very early in the design phase, however I would like to share my progress and get some insight and design tips from the community here!

    Here are some of my goals for this design.
    • Ability to mill hard materials such as 303/304 Stainless.
    • .001 Accuracy or better.
    • Working area grater then 500mmx500mm


    If I can create a design that is seems feasible then construction and testing of the machine will commence as my budget allows.

    The base and frame of the machine has been designed as follows:

    The external dimensions of the base are 1500mm wide x 1250mm deep x 300mm thick.
    The external dimensions of the bridge are 1500mm wide x 400mm thick x 1400mm tall. The columns that support that bridge are 250mm wide.
    The distance between the bridge and the base is 700mm.
    Steel will be used to create a base for the linear rails on the X and Y axis. 2 base plates will be drilled and tapped, milled to a uniform size, and set true to each other for each axis, thus providing a base for each linear rail on the X and Y axis.

    Here is a snapshot of my design so far:


    My Linear motion design is currently made of:
    • 20mm Linear rails and pillow blocks on all 3 axis.
    • 20mm ball screws with a 5mm pitch on all 3 axis.
    • NEMA 34 stepper motors on all 3 axis.


    I am looking for feedback on the design. If you notice any major flaw or errors please comment and I would love to discuss!
    I have created this page that I will try and keep up to date will all the details, design changes, images, and eventually CAD files:
    CNC Design - wiki.caseybarsness.com

  2. #2
    Join Date
    Jun 2010
    Posts
    4252

    Re: Large EG Milling Machine Design

    Nice idea.
    Commercially, you are talking about a temperature-stabilised machine costing well in excess of $100, probably in excess of $200k, and maybe in excess of $400k. Plus the assocated building costs.
    Still, nice idea.

    Cheers
    Roger
    PS: your 20 mm rails are too small. Your 20 mm ball screwsare probably also too small.

  3. #3
    Join Date
    Aug 2004
    Posts
    780

    Re: Large EG Milling Machine Design

    Do you wish to do 0.001 mm accuracy ??

    This is perfectly possible, and the design is very good for it.
    The 20 mm rails are too small, a bit, as You need great rigidíty to get to 0.001mm.
    Otherwise everything deflects under load.

    Likewise, You need the high-accuracy UP or ultra precision matched rails and screws.
    And ground screws of C0 type, perhaps C1.

    I have 0.2 micron theoretical resolution on lathe and similar on mill, both with ac brushless servos, with some experience chasing this single micron resolution in details, so...
    I use 32 mm screws, and 35 mm rails, and machine is 2300 kg in steel.

    My machine is a VMC of similar design, a dual-column machine, and of similar size, 2.5 m wide, with 1600 mm table width.

    You probably under-estimate the needed work by 10-20x, I suspect.

    I have no hope of 1 micron accuracy, but +/- 1-2 micron local resolution and repeatability.
    DMG Mori Seiki DCG machine (google) do 0.7 micron circular interpolation in milling.
    It is definitely doable.
    Manual machines have done 1 micron accuracy, with jig borers, for 40+ years.

    You need about 100x more "strength" (rigidity really) than You probably think, to get to 1 micron levels.
    A 32/5 TBI ballscrew I import and sell, has about 54 kg of force, called kgf, per 1 micron deflection.
    A 20 mm screw is about half or less.


    E.
    My z axis is 50 metric tons "strength" with 12x35 mm carriages on 6 rails.
    Load as in push force is only 50-100 kgf, max 1000 kgf.
    You dont need strength, you need rigidity, and rigidity comes from big components.

    I am not using UP ultra precision rails, but double rails ie overconstrained on z axis.
    Not quite as accurate, but good, and very very rigid due to rail spacing vs linear carriage size.

    As in, double vertical rails on z axis separated by 150 mm, make the total C-C distance between support points 100 mm (carriage is about 100 mm wide, Hiwin HGW-C0).
    And at 400 mm C-C height, for 4 linear-rail-carriages per side, the total system is perhaps 50x more rigid than a single rail system.
    Importantly, it is also about 3-5x more accurate due to geometric averaging.

    Suggestion.
    Make steel core with crosswise reinforcements.
    Make steel shell.
    Pour the EG inside the steel.

    EG and granite are quite flexible.
    But EG is very resistant to compression.
    An external steel skin will force a lot of the load to become compressive, and will make it a lot more rigid overall.

    Get an epoxy that does not shrink, if it expands a bit this is good as it will preload the structure.
    Then shim/grind/sand/scrape as needed for final linear guide adjustments.

  4. #4
    Join Date
    Jun 2010
    Posts
    4252

    Re: Large EG Milling Machine Design

    Hi Hanermo

    How's your machine going? When do we see some photos?

    Cheers
    Roger

  5. #5
    Join Date
    Aug 2004
    Posts
    780

    Re: Large EG Milling Machine Design

    Rc...
    possibly, never.

    Public-photo-stuff is not my current area for business.
    This might change, overnight.
    Never know.

    I would prefer to do pics, but $$$ is more important.
    Unlike many, I have 100k€++ invested, so my pov and interests are somewhat-divergent in some ways.

    I try to help and advise, on experience and physics and lessons learned.

    Do You have a problem with that ?

    Do You think I should not advise ?
    Do You think all or most of my advice is wrong ?
    Examples. Specifics please. Pointing to preponderance of evidence.

    Do You think I should not share some experiences from the most-experienced machine tool builders in the world who trained/helped us ?
    (The guys who designed and built the current Haas STxxx series lathes. Others. Fwiw..)

    Do You think I should betray, defraud by publication (not legally, but morally) the clients (200+) and contacts (2000+) who trusted me and us, not to detail their business practices in public ?

    Be real. Please.

    Fwiw..
    My machine is going slower than it should.
    The first column is now drilled and tapped, semi finished, for the last 3rd rail, needs final assy and test, before putting in place.
    Multiple reasons.
    Looks like about 0.02 mm error max or less, per 3rd rail, tracking, free-form.

  6. #6
    Join Date
    Jun 2010
    Posts
    4252

    Re: Large EG Milling Machine Design

    Hi Hanermo

    A different PoV:
    Client details - strictly Non Disclosure Agreement. I have the same sorts of agreements as a research scientist.
    Sharing general experiences - much appreciated, but it does not mean you need to give away any secret information. This is engineering.
    Photos of what you are building - always of interest to inspire others. I don't see that any loss of confidentiality would happen.
    And the publicity might be of some benefit to you as well.

    My 2c
    Cheers
    Roger

  7. #7
    Join Date
    Jan 2017
    Posts
    13

    Re: Large EG Milling Machine Design

    Thank you for the replies!
    I am sorry about the confusion with the accuracy component. I am looking for .001in accuracy, or 0.0254mm. I forgot to put units on it and keep falling back to my old habits from the machine shop I used to work in.
    Will the 20mm rails and screws still be to small to achieve this?


    Casey

  8. #8
    Join Date
    Jun 2010
    Posts
    4252

    Re: Large EG Milling Machine Design

    Ah - 25 micron.
    Well, the rails might be OK if well secured. Maybe. I would prefer 25 mm.
    My own feeling is that the 20 mm ball screws are too thin for that length, and would prefer 25 mm.
    I think I am using 20 mm for a span of <300 mm on X, and the same for shorter spans on Y & Z.

    Cheers
    Roger

  9. #9
    Join Date
    Jul 2009
    Posts
    419

    Re: Large EG Milling Machine Design

    What I think that would be worth changing is the setup of the Z-axis.

    In your design the carts for the z-axis go up and down.
    Overall stiffness would be better if the rails go up and down and the carts for the z-axis en gantry axis are fitted (more or less) back to back.
    Sven
    http://www.puresven.com/?q=building-cnc-router

  10. #10
    Join Date
    Jun 2010
    Posts
    4252

    Re: Large EG Milling Machine Design

    Not sure I see what that means, but if I do I don't agree.

    Cheers
    Roger

  11. #11
    Join Date
    Mar 2003
    Posts
    35538

    Re: Large EG Milling Machine Design

    Will the 20mm rails and screws still be to small to achieve this?
    I think the preload on the bearings has something to do with this. If you get them with a heavy preload, they should be more rigid than standard or light preload.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

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

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