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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

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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.

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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

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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 !

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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

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    Re: Moving table (fixed gantry) with two ballscrew

    works - other threads do not work. Peter

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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'').

  13. #13
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Ivan - I think a good machine design should be such that when the spindle is mounted with an average length tool the bottom of the tool should be very close to the bottom of the gantry. Thats the top height. Otherwise the designer is wasting that space. So image 4 is good. Can you juggle bearings and rail so they are more efficient in this position? So they then have to be able to go down. Some systems have to cope with wells in the bed or an bench apron so they have to go down a long way as well. Peter

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    Re: Moving table (fixed gantry) with two ballscrew

    I haven't contributed to this thread,but I have been watching it develop.Its great that Pete has contributed his calculations as lots of builders here will benefit from the example and be able to apply the same process to their own projects.I agree with him that a forum is a great way for people with a shared interest from different parts of the world to contribute to progress on machine building and to build a store of knowledge.

    My real point is to say how pleased I am to see two clamps holding the spindle.It bothers me a bit when I see otherwise promising machines using only a single clamp and a quick reference to Pete's figures illustrates the kind of forces the machine might have to withstand.This looks like a machine that will be capable of quality work.

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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Ivan - I didn't mention disadvantages of moving a spindle. You will have to make the fit of the spindle to the Z plate very good. Otherwise everytime you move the spindle you will have to re tram it. If you think its a quick undo some bolts refit and move along that's not going to happen unless you engineer a very precision arrangement. Peter

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    Re: Moving table (fixed gantry) with two ballscrew

    Quote Originally Posted by peteeng View Post
    ... You will have to make the fit of the spindle to the Z plate very good. Otherwise everytime you move the spindle you will have to re tram it. If you think its a quick undo some bolts refit and move along that's not going to happen unless you engineer a very precision arrangement....
    Thanks Peter !

    Quote Originally Posted by routalot View Post
    ....My real point is to say how pleased I am to see two clamps holding the spindle.It bothers me a bit when I see otherwise promising machines using only a single clamp and a quick reference to Pete's figures illustrates the kind of forces the machine might have to withstand.....
    I use 3 brackets. I just didn't show them all in the previous photos.
    I made simulations (FEA) in Solidworks between 1 whole bracket (120mm wide) and the same bracket but cut in three pieces (3 x 38mm with a total width of 200mm).
    The results showed much smaller deformations with 3 mounting brackets.

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    Re: Moving table (fixed gantry) with two ballscrew

    I continue to design the epoxy base. I post a few pictures. If you see any disadvantages - to correct them.
    For all axis I use double nuts which will be tightened during installation --> anti backlash. Maybe for Z-axis this is unnecessary because of the weight of the motor (no backlash). But but in this case I have 2 ballnut aluminum holders (total 8 screw) - which is good for my heavy Z.
    I have a question about stand feet: 4 or 6 feet ?

    I think 6 legs are hard to adjust. ?ven a few will be support only (no weight on some legs).
    ?aybe the best is 3 legs as on the surface plates (total 5, but 2 are for support only).

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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Ivan - I go with Stand 1, as you say getting load on all those feet will be V hard in Stand 2. Stand No1 I'd use the "main" 3 feet for leveling and the rear foot for support. I'd take advantage of the geometry and make the "wings" wider or as wide as you can go. I'd also make a mount so the machine can be bolted to the floor vs free standing. Your not going to use any cross bracing? Peter

    Also with a frame design like you have the top plates will "move" when welded and it will be difficult to keep them in their intended position unless a plate template is bolted to the top or beams tacked across it. Will it be stress relieved? Think about a heavy channel top frame vs what you have. This will be easier to get flat and stay in place? Most frame designs have a top rail and a bottom rail so it stays in shape better thru welding and SR.

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    Re: Moving table (fixed gantry) with two ballscrew

    Thanks Peter !
    I wanted the weight of the epoxy to be within 150 kg, so that we could lift it from two people. The first variant is 153 kg.
    Now is 180 kg. I hope we can move it and lift it manually (I don't have a crane). I also revised the steel frame on your recommendations.

    What think about adjustable feet between steel frame and epoxy base (free, without bolted connections) ?
    There will be only four limiters on all sides.
    Attached Thumbnails Attached Thumbnails EB-new.jpg   stand1-new.jpg   feet.jpg  

  20. #20
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    Re: Moving table (fixed gantry) with two ballscrew

    Hi Ivan -There are two concepts with machine bases. 1) That the stand is part of the structure of the machine and needs to be firmly integrated with it 2) That the stand is a stand only. I think your current design falls into the 2nd category. I like it. The stand is sturdy and heavy, the machine base is stiff. I'd use larger diameter screws for the adjusters. They look a bit small. Probably will be fine but image does count sometimes.

    EG casting - you can make the members lighter by placing timber or foam "voids" inside. Like a tube the inside of the member does not do much so by creating a "void" you lose little stiffness but loss some weight.

    What filler are you using for the EG? Peter

    https://cncrouterkits.com.au/2019/11/22/for-sale/ maybe some CF blocks would be good?

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