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  1. #51
    Quote Originally Posted by Ross77 View Post
    Ok, I'm a bit confused. You keep saying fixed gantry but then you show a bed with rails on the side.
    Sorry for being double meaning.
    I have started the thread with the moving gantry principle and as the design and details developed according to the useful comment from the site two days ago especially after having Dean's #41 post I have changed my mind .
    My intention was not to leave my earlier ne'er-do-well concept and correct it.
    This is the case when the design concept turn to other way so I put this moving gantry into a drawer now and finish this chapter.
    I will start a new thread with it when it is actual again.
    So I am giving thanks everybody and I am at full pelt with the fix gantry from now on.

  2. #52
    Nevertheless keep this thread alive, cause that's one of the most interesting things, somebody starts with one idea that evolves in something quite different sometimes.
    project 1 , 2, Dust Shoe ...

  3. #53
    Chapter #2 Fixed gantry

    I am back again after aging some thoughts however I could not go too far. I cannot find too much examples for the ideal fixed gantry frame.
    Probably I am making the typical mistake and try to reform something that is good as it is.
    My viewpoints are:
    using hot rolled profile -do not know but think it is better vibration damper than cold formed tubes (SQ or RECT)
    using bigger cross sections- to eliminate the bracing and welding as far as possible
    setup with 1700 mm total length-just slightly longer than moving gantry

    My concern are:

    -bolted connection as figure shows-is it is good enough- instead of welding?

    -The spindle overhang is quite big -though is fixed in one direction . Is there a better setup to eliminate?

    -I would balance the moving bridge on both side with pneumatic cylinders or gas spring that I red here and like this option.
    As the spindle moves in Y direction the Center of Gravity moves with it -that seems not too beneficial for the balancing

    Figure below shows mainly principles first to make a good machine setup



    Thank you for your comments and advice in advance.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by vargai; 15-12-2014 at 12:07 PM.

  4. #54
    Quote Originally Posted by vargai View Post
    My concern are:
    -The spindle overhang is quite big -though is fixed in one direction . Is there a better setup to eliminate?
    Double gantry or move the uprights to the end of the gantry, just some thoughts but I realise it might make the machine wider.

    Click image for larger version. 

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    Spelling mistakes are not intentional, I only seem to see them some time after I've posted

  5. #55
    Quote Originally Posted by EddyCurrent View Post
    Double gantry or move the uprights to the end of the gantry, just some thoughts but I realise it might make the machine wider.
    Neither do I would make it wider.
    The right figure with protruding plates or so with overlap the post could be good.
    I do not assume real rigidity loos at Y axis when the rail ends are bolted only onto a strong plate.
    I will make some detailing.
    Thanks
    Last edited by vargai; 15-12-2014 at 02:00 PM.

  6. #56
    To get the best rigidity you need to keep the spindle inline with the support bearings so no overhang. You could rotate the rails so they are mounted end on and in the middle of the beam.

    The vertical moving gantry beam looks a bit slender with regards to the end bearing spacing. I would guess its 200mm deep and 1000mm wide? Thats is not a good ratio for racking and I would make the most of the gantry height and increase the bearings to maximum so the moving beam is like 'H' shape.

    That would naturally reduce the racking and take the pressure off the screws and rams.

    Just how I would do it anyway :0)

  7. #57
    Quote Originally Posted by Ross77 View Post
    To get the best rigidity you need to keep the spindle inline with the support bearings so no overhang.
    Hope I understand well. I made a top view sketch (left figure) -I think the spindle can get much closer to the posts.
    The higher H gantry beam is agreed.
    Ideally it should be balanced like a VMC in the right figure- manufacturer apply 500-600 mm and more overhang without problem.
    Theoretically a reversed moving balance can be done but it increases the mass on Y axis motor
    There are tasks left.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by vargai; 16-12-2014 at 09:50 AM.

  8. #58
    The spindle on a vmc has to be like that so the table can move. But the column is fixed and only the z axis moves. Don't get your designs mixed up, you have the opportunity to build some thing specific to your requirements so best look to optimise it.

    If you want to get technical then you don't have to put the cutter directly in line with the bearing but you need to balance the y axis and put its centre of gravity in line with the rails as that will produce the smallest moment loading in the bearings and hence deflection.

    I wasn't meaning to use a H beam but rather trying to explain the position of the beam relative to the vertical bearings, ie keep the beam 200mm deep but increase the bearing spacing To 400-500mm. Might need to draw something up.

  9. #59
    Quote Originally Posted by Ross77 View Post
    Might need to draw something up.
    Tried to catch the principle based on sketches but this is more complex and one detail can change the others.
    Here are some slightly more detailed plans -the C/G is near the C/L of the horizontal ball screw so hope this setup works.

    Quote Originally Posted by Ross77;
    That would naturally reduce the racking and take the pressure off the screws and rams.
    I have increased the bearing spacing-350 mm now.I do not think it is worse than a moving gantry since it has double Z screw. Forces on screws mainly come from the vertical feed force -assuming it about 200 N(??) while I would like to reach 1000 N machining force in the horizontal plane.
    If something might be considered to apply one more shorter Z rail and +1 each carriage on both side-I do not know if it worth
    Drilling only means center drilling in this machine and max 6-8 mm holes. The machining that requires Z force will be arranged in the middle zone of the table where the load is almost symmetrical on screws

    Regarding to the profile I started to change them to bend opened ones from 6-8 mm thickness. All of them havv to be designed according to their function

    By the way when I am taking about balancing I am thinking of the Z motor too. I want it to get relieved form beam mass. It cannot be balanced perfectly since the spindle is moving but I want to remove the dead load expect the spindle at least.

    This vertical in-between railing makes me worry a little.
    The implementation will not be too easy. At the moment I am planing to put together the Z frame from three bolted parts



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    Last edited by vargai; 18-12-2014 at 11:27 PM.

  10. #60
    Looks good. I would probably extend the gantry uprights down to the floor so the more area would be available for welding and increase rigidity. the vertical rails could then drop down and increase the bearing spacing a bit more.

    I think golden ratio is 2:1 so if the y axis is 800mm wide then you need 400mm bearing spacing. as you pointed out one change can affect another and you will need to look at flex in the mounting plates and possible bracing,


    Do you think the gantry uprights need bracing as well? looks quite tall

    It may have already been mentioned but how to plan to keep all the surfaces perfectly aligned whilst welding and given the surface tolerance on steel c section?

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