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

    Hi All,

    First excuse me for my bad English.

    I almost completed the design of my cnc router with moving gantry (x:1260mm, y:620mm, z:220mm).
    Everything without the base frame was purchased a long time ago:
    - 3kw ER20 water cooling permanent power spindle motor for metal work 110mm (RTM110-24Z-3.0)
    - 25mm original HIWIN linear rails and HGH25CA carriages
    - SFU2005 ballscrew for all axis with separate double nuts (4 pieces).
    - 3 Phase 220V Nema34 3A 7Nm LC86H3114 Closed loop Servo Stepper motor Driver kit (4 pieces)
    - heavy aluminum extrusion for Y-axis (ITEM 200x100mm)....
    etc.


    ...But now I decided to fundamentally change the concept and move to fixed gantry style.
    I'll probably will build an EP base stand (my 200x100 extrusion will remain unused..)


    Almost all designs use one ballscrew for moving table. But I still have one left unused ballscrew+motor+controller for X-axis.
    What do you think about moving table with two ballscrew ?
    My moving table is about 1000x600x25mm (it is heavy). I will also add reinforcing beams below. Also ... + heavy vise and heavy detail.
    So my question is: Will the second ballscrew help (higher speed, less motor load) or is it pointless to add one ?

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

    My understanding is dual screws are used on a moving gantry to reduce the possiblity of racking.
    With the rails you're using and a central screw --racking of the moving table doesn't look likely so no need for dual screws in my opinion.

    Is that now enough power for that axis ? Others would calculate that better than I can. The problem with changing a design midway, we've all done it.
    Anyone who says "It only goes together one way" has no imagination.

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

    Hi Ivan - There are two reasons to use two screws 1) because you need more force or power then one provides and 2) as cyclestart says says to prevent racking in a system that is not stiff enough not to rack eg a moving gantry system. It's not possible to make a wide structure stiff enough to be driven from one side. And a wide gantry is probably not stiff enough to be driven from the middle if its a router or mill. Some designs drive the gantry from under the table via a cross beam or saddle. A laser or extruder probably would be OK as the tool has no load. In your case you need to calculate the loads and accelerations required to decide but you probably don't need 2 drives. Cheers Peter

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

    Quote Originally Posted by cyclestart View Post
    .. Is that now enough power for that axis ? ...
    I don't know.
    For moving table instead of aluminum (30 kg), I have and can use a 30mm iron plate for more rigidity. It weighs 140 kg. Plus vise about 40kg + 80 kg maximum weight of the workpiece = 260 kg.

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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Peter,
    Glad to hear you again. I follow your posts about EG.

    There is no way I can calculate what I need. I just have free motor+controller+ballscrew.

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

    Hi Ivan - tell me the motor spec, balls screw spec and weight you want to move and I'll calculate it for you. Peter

    Here's some Tetrium I just made. Stiffer than EG about 30GPa and about 2300kg/m3. Easily cut on my little router that chucks a wobbly at aluminium and I'm gearing up to make slabs. Plus a bonus, a timber stand I made yesterday for a friend. Peter

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

    Hi Ivan - Here's the calcs for your drive. It's not a whimpy system and will not require 2 drives!! It's a very powerful motor and can accelerate 300kg very easily. Haven't checked the math you can do that or others. You can use same maths for Z axis but in this case you are lifting and dropping a weight so neglect friction and consider the weight as an inertial load only. Cheers Peter

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

    Wow Peter...!!!
    I feel uncomfortable. You have spent so much time calculating and writing for me.
    I do not know how to thank you. You are an exceptional person who helps many of the members here !
    Thank you very very much for your help and I wish you good luck !

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

    Hello Ivan - Do not be uncomfortable. This stuff is what the forum is for. Its 15mins work for me no issue Keep Making - Peter

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    Re: Moving table (fixed gantry) with two ballscrew

    Thanks again my friend

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

    works - other threads do not work. Peter

  12. #12
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    Moving spindle mount (on Z axis)

    Ok. New question.

    What do you think of a moving spindle (on Z axis) ?
    If a higher workpiece is required and insufficient Z travel, the spindle mounted higher on the z axis.
    Any disadvantages?


    My linear rails are 360mm (14'').
    Z travel - 153mm (6'').
    Gantry clearance 280 mm (11'').

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